Preparation and performance study of Ag/Ag3PO4-g-C3N4/BiOI composite photocatalystsABSTRACT
Graphite-phase carbon nitride (gC₃N₄) is a two dimensional semiconductor material known for its tunable surface acidity/basicity, structural stability, low cost, and facile synthesis. The quaternary composite catalyst covers a broader visible light spectrum, significantly enhancing light utilization efficiency. This leads to more efficient generation of photo generated electron–hole pairs, thereby supplying abundant active species for degradation reactions.” By preparing g-C3N4/BiOI as a precursor and then fabricating the Ag/Ag3PO4-g-C3N4/BiOI quaternary photocatalyst, the photocatalytic performance was evaluated by degrading two typical water pollutants—rhodamine B (RhB) and ciprofloxacin (CIP)—under visible light. The structure and properties of the samples were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and elemental distribution analysis, and the possible catalytic degradation mechanism was further analyzed. Among the gC₃N₄/BiOI precursors, CB3 showed the best degradation performance, achieving efficiencies of 93.3% for RhB (20 mg/L) and 83.3% for CIP (10 mg/L). Subsequently, Ag3PO4 was composited onto the CB3 precursor via the sedimentation method, and the quaternary composite was treated with photoreduction. When the photoreduction time was 20 min, the resulting 20 min Ag/Ag3PO4-CB3 showed the best catalytic activity: its degradation efficiencies for 20 mg/L RhB and 10 mg/L CIP were further enhanced to 97.0% and 90.2%, respectively. Cyclic degradation experiments further confirmed the excellent stability of the catalysts. The enhanced performance of the quaternary photocatalyst is attributed to the formation of heterojunctions (p-n junction between g-C3N4/BiOI and Z-scheme heterojunction between Ag3PO4 and g-C3N4) and the localized surface plasmon resonance (LSPR) effect of AgO. These structures efficiently promote the separation of photogenerated electron-hole pairs, extend the lifetime of photogenerated carriers, and reduce photocorrosion, thereby significantly improving the visible-light photocatalytic activity and stability.
Keywords:
Ag/Ag3PO4-g-C3N4/BiOI composite photocatalyst; Degradation; Environmental remediation; Visible-light photocatalysis
1. INTRODUCTION
Environmental pollution causes substantial direct and indirect harm to the ecosystem, with water pollution being particularly critical due to its close association with human life and serious impacts on public health [1]. Synthetic organic dyes (e.g., methylene blue, bromophenol blue, methyl orange) and their mixtures are typical refractory pollutants in industrial wastewater—they not only cause visible coloration of water bodies to block light penetration (affecting aquatic ecosystems) but also contain toxic aromatic structures that pose long-term risks to human health even at low concentrations [2]. Besides dye pollution, antibiotic pollution (e.g., amoxicillin, tetracycline, cephalexin, ciprofloxacin) has become another critical water environmental problem—antibiotics are widely used in medicine and animal husbandry, and their residual low concentrations in water bodies can induce bacterial resistance, posing a potential threat to public health [3]. Conventional sewage treatment methods struggle to achieve complete degradation of organic pollutants in wastewater. Photocatalytic degradation, a clean technology that utilizes solar energy to break down pollutants in water, holds broad application prospects for water pollution management, purification, and environmental remediation [4]. Graphite-phase carbon nitride (g-C3N4) is a two-dimensional semiconductor material with special surface acid-base properties, structural stability, low cost, and simple preparation process [5]. However, its forbidden band width is high, and the compounding rate of photogenerated electron-hole pairs is not low., which limits the scenario of using g-C3N4 alone, the quaternary composite catalyst covers a broader visible light spectrum, significantly enhancing light utilization efficiency. This directly manifests as “exciting more photo-generated electron-hole pairs, thereby providing ample active species for degradation reactions.” Therefore, research on g-C3N4 has primarily focused on the development of composite photocatalysts [6]. To overcome the limitations of pure BiOI and g-C3N4, ZHANG et al. [7] prepared g-C3N4 nanosheets (CN-E) by using microwave-assisted rapid freezing at different times of thermal exfoliation, and then synthesized BiOI/CN-E nanocomposites by using the microwave-assisted method of in-situ growth of BiOI on CN-E, BiOI/CN-E had a better effect on the Cr (VI) removal efficiency of BiOI/CN-E was improved compared with both of them alone. Silver phosphate (Ag3PO4) shows strong photocatalytic decomposition of organic dyes under visible light irradiation, is a p-type semiconductor material with strong photocatalytic ability and high quantum yield, and has high photo-oxidation ability for degradation of organic pollutants under sunlight [8].
CUI et al. [9] prepared Ag3PO4/BiOI composites via a liquid-phase deposition method at room temperature, achieving uniform distribution of Ag3PO4 nanoparticles on the surface of BiOI nanosheets.. It has been found that Ag0-loaded Ag3PO4 has better stability and photocatalytic activity than Ag3PO4 itself, i.e., Ag0/Ag3PO4 has the advantage of Schottky-bound localized surface plasmon resonance effects [10]. CHEN et al. [11] prepared a g-C3N4/Ag3PO4 composite Z-scheme heterojunction photocatalyst by in situ deposition and applied it to the degradation of ofloxacin (OFX) for the first time. When the mass ratio of g-C3N4 to Ag3PO4 was optimized to 1:10 (denoted as AC-10), the composite achieved optimal OFX degradation, with an efficiency of 71.9% within 10 minutes.
In this work, a quaternary Ag/Ag3PO4-g-C3N4/BiOI composite photocatalyst was fabricated through a multi-step process involving thermal polycondensation, hydrothermal synthesis, and deposition-photoreduction. The photocatalytic degradation mechanism was investigated. Among the synthesized precursors, the sample CB3 was identified as the most active based on its degradation efficiency for RhB (20 mg/L) and CIP (10 mg/L). Subsequently, Ag3PO4 was deposited onto the optimal CB3 precursor to construct the Ag/Ag3PO4-CB3 composite. The photocatalytic performance was evaluated by monitoring the degradation rates of RhB (20 mg/L) and CIP (10 mg/L) over different irradiation times. The composite subjected to 20 minutes of photoreduction exhibited the optimal degradation activity.
2. MATERIALS AND METHODS
2.1. Experimental apparatus and reagents
The reagents and apparatus used in the experiment are shown in Tables S1 and S2.
2.2. Preparation of photocatalysts
Thermal polycondensation method for the preparation of g-C3N4: melamine into the alumina crucible with a lid, placed in a muffle furnace, the heating rate of 3 °C/min, so that the uniform temperature up to 550 °C, keep 550 °C under the condition of calcination for 4 h, the end of the natural cooling, cooling down, after the preparation of g-C3N4 will be prepared with a mortar and pestle clockwise grinding into a powder for spare parts.
Preparation of g-C3N4/BiOI by hydrothermal method: 0.0169 g-C3N4 and 0.2004 g BiOI were weighed into 60 mL of ethanol by an electronic balance, and then transferred to a 30 mL of PTFE tank after ultrasonic treatment for 1 h (Figure 1). It was put into a reactor for solvent-heated reaction at 150°C, kept at the end of the reaction for 8 h, and then washed with ethanol and deionized water alternately for 2 times each, centrifuged and placed in a blast drying oven at 60 °C for 10 h, grinding to obtain a g-C3N4-BiOI precursor composite photocatalyst with a substance amount ratio of 0.3, and continuing the above steps, g-C3N4-BiOI precursors with substance amount ratios of 0.3, 0.4, 0.5, 0.6, and 0.7 were prepared, respectively, and were noted as CB1, CB2, CB3, CB4, and CB5.
Preparation of four-membered composite photocatalyst Ag/Ag3PO4-g-C3N4/BiOI by deposition-light reduction method: 0.9229 g of CB3 was dispersed in 60 mL of H2O, and ultrasonication was assisted to make a homogeneous mixture, 0.2718 g of AgNO3 was added, and the ultrasonication was continued to make the CB3 sufficiently adsorb Ag+, and 20 mL of Na2HPO4 solution (0.025 mol/L) was added dropwise, and stirring was continued throughout the process. mol/L) with continuous stirring throughout the process, centrifuged the precipitate at the end of the reaction, added anhydrous ethanol, placed under a xenon lamp light source, illuminated for 10 min, 20 min, and 30 min, respectively, centrifuged and dried, and then milled and collected the samples recorded as 10 minAg/Ag3PO4-CB3, 20 minAg/Ag3PO4-CB3, and 30 minAg/Ag3PO4-CB3.
3. CHARACTERIZATION
3.1. X-ray diffraction (XRD) analysis
X-ray diffraction was used to detect the physical phase of the catalysts, and Figure 2 shows the XRD pattern of the precursor g-C3N4/BiOI photocatalysts.According to the standard card for the tetragonal crystalline system g-C3N4 (JCPDS No. 87-1526), the characteristic diffraction peak at 9.44° corresponds to the (100) crystallographic plane of g-C3N4, and the characteristic diffraction peak at 27.5° corresponds to the (002) crystal plane of g-C3N4 [12]. From Figure 2, it can be seen that the characteristic diffraction peaks corresponding to the same crystal plane from CB1 to CB5 change, i.e., the characteristic diffraction peak of the composite photocatalysts shifts rightward from 29.5° to 29.7° as the doping amount of g-C3N4 increases, approaching 27.5°, which is close to the standard characteristic peak of g-C3N4, and this is consistent with the literature [13], which preliminarily proves that the composite of g-C3N4 and BiOI is successful. The diffraction peaks on the figure at 22.2°, 29.5°, 31.6°, 45.4° and 51.4° correspond to (101), (102), (110), (200) and (114) crystal planes, respectively, which are consistent with the pure BiOI and all of them match with the standard card (JCPDS No. 10-0445).
3.2. Elemental distribution and EDS spectral analysis
EDS analysis of CB3, which has the best degradation performance, is shown in Figure S1, and in conjunction with Figure 3. The elemental composition of the microspheres comprises five elements: Bi, O, I, C, and N. To further investigate the structure of the g-C3N4/BiOI composite, elemental mapping (EDS) was performed on the precursor CB3. The results show a uniform distribution of C and N elements throughout the BiOI matrix [14]. The heterojunction formed at the interface between g-C3N4 and BiOI facilitates electron (e−) injection into BiOI. Meanwhile, because the valence band (VB) of g-C3N4 is lower than that of BiOI, holes (h+) migrate toward g-C3N4. This charge separation creates a built-in electric field [15], which promotes the efficient separation of photogenerated electron-hole pairs, prolongs carrier lifetimes, and thereby enhances catalytic efficiency.
3.3. Infrared spectral analysis
In Figure S2, the absorption peak at 3470 cm–1 is attributed to the telescopic vibration absorption peak of the O-H bond of the residual H2O, the broad peak at 3200 cm–1 corresponds to the telescopic vibration peak of the N-H bond of g-C3N4, the absorption peaks within 1650–1200 cm–1 are all attributed to the telescopic vibration absorption peaks of the C-N-HC and C=N in the triazine heterocyclic skeleton of g-C3N4, and the absorption peak at 806 cm–1 is the characteristic peak of the fingerprint region of g-C3N4 belonging to the typical triazine ring structure, and the above characteristic peaks indicate the existence of the g-C3N4 structure in the composite photocatalyst [16].
Meanwhile, the absorption peak at 772 cm–1 is attributed to the symmetric telescopic vibration peak of Bi-O bond, and the absorption peak at 497 cm–1 is the telescopic vibration peak of Bi-O bond, and the above characteristic peaks indicate the presence of BiOI structure in the composite photocatalysts.
3.4. X-ray spectroscopy analysis
The survey XPS spectrum in Figure 4a confirms that the composite photocatalyst contains C, N, Bi, O, and I. The high-resolution Bi 4f spectrum (Figure 4b) exhibits two strong peaks at 159.0 eV and 164.2 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively. This confirms the presence of Bi3+, consistent with the formation of BiOI. Similarly, the I 3d spectrum (Figure 4c) shows two peaks at 618.9 eV and 630.4 eV, assigned to I 3d5/2 and I 3d3/2, respectively. This signifies the presence of I−, further corroborating the identity of BiOI in the composite.
The O 1s spectrum (Figure 4d) shows three peaks. The peak at 529.8 eV is attributed to the Bi–O bonds in BiOI. Another peak at 531.5 eV corresponds to I–O bonds on the BiOI surface. The final peak at 532.8 eV likely originates from adsorbed oxygen-containing species, such as H2O or CO2, on the sample surface [17]. In the C 1s spectrum (Figure 4e), the peaks at 284.8 eV, 286.4 eV, and 288.6 eV are assigned to sp2-hybridized carbon (C–C/C=C), carbon in the C–N=C structure, and carbon in the N–(C)3 bond, respectively [18]. The N 1s spectrum (Figure 4f) displays two peaks at 399.1 eV and 400.8 eV, corresponding to sp2-hybridized aromatic nitrogen (C–N=C) in the triazine rings and to sp3-hybridized nitrogen in N–(C)3 groups, respectively [19]. The characteristic signals from both C 1s and N 1s spectra confirm the presence of g-C3N4 in the composite photocatalyst.
3.5. UV diffuse reflectance analysis
Figure S3 shows the UV-visible diffuse reflectance absorption spectra of pure BiOI, g-C3N4, CB1, CB2, CB3, CB4 and CB5, respectively.Pure g-C3N4 exhibits an absorption edge near 400 nm, confining its absorption to the UV region. In contrast, all g-C3N4/BiOI composite samples (CB1-CB5) show significantly red-shifted absorption edges beyond 420 nm, extending well into the visible region. This, along with their broader absorption profiles, indicates enhanced visible-light harvesting capability, which improves the utilization of visible light. The band gap energies (Eg) of the photocatalysts, calculated from the spectra using the Tauc plot method [20], are listed in Table 1. Pure g-C3N4 has a wide band gap of 3.05 eV. This large Eg limits light absorption, results in poor charge carrier mobility, and requires electrons to overcome a high energy barrier to reach the conduction band [21]. In contrast, forming a heterojunction between g-C3N4 and BiOI reduces the band gap of the composite materials to a range of 1.7–1.9 eV. This optimal narrowing of the band gap broadens the spectral response into the visible region without compromising the activity of the constituents, thereby enhancing the visible-light photocatalytic performance.
3.6. Photoluminescence spectroscopy
The recombination of photogenerated electron-hole pairs was investigated by photoluminescence (PL) spectroscopy. As shown in Figure S4, pure g-C3N4 exhibits a strong emission peak around 450 nm. Among the composite g-C3N4/BiOI precursors, CB3 shows the weakest PL intensity at a similar position. Since a lower PL intensity corresponds to a lower charge carrier recombination rate [22], this indicates that the recombination of photo-generated electron-hole pairs is most effectively suppressed in CB3. Given that efficient charge separation is crucial for enhancing catalytic activity, this result further confirms that CB3 possesses the highest photocatalytic activity among the composite precursors.
3.7. Specific surface area testing and analysis
The specific surface areas and pore structures of the catalysts were analyzed using nitrogen adsorption-desorption measurements. As shown in Figure 5, all samples exhibit type IV adsorption isotherms with H3-type hysteresis loops, which are characteristic of slit-shaped pores in mesoporous/macroporous materials composed of plate-like particle aggregates [23]. The H3 loop, where the desorption branch is typically steep at a relative pressure (P/P0) around 0.42, is associated with the capillary evaporation from pore networks in non-rigid, lamellar aggregates [24]. These isotherm features align well with the layered structure of the materials investigated in this study.
Nitrogen adsorption desorption curve and pore size distribution of precursor g-C3N4/BiOI (illustrated).
The specific surface area, pore volume, and average pore size of the catalyst samples are summarized in Table 2. Compared to pure BiOI, the composite catalysts exhibit increased specific surface areas, which provides more active sites and facilitates the contact and transfer of reactants [25]. At the same time, too large specific surface area is less favorable to the cycling stability of the material, and the specific surface area of the precursor catalyst after composite g-C3N4 is reduced compared to pure g-C3N4.Among them, CB3 shows the greatest increase in specific surface area and the largest average pore size. This correlates with the most significant reduction in the recombination rate of photogenerated charge carriers, which coincides with its optimal catalytic activity.
3.8. Photocatalytic degradation performance and reproducibility analysis
The photocatalytic performance was evaluated by degrading two model pollutants under simulated aqueous conditions: rhodamine B (RhB, 20 mg/L) and ciprofloxacin (CIP, 10 mg/L).
Figure S5a shows the absorbance changes during the photodegradation of RhB (20 mg/L). After 30 min of dark adsorption to reach equilibrium, all g-C3N4/BiOI composite photocatalysts exhibited enhanced degradation efficiency compared to pure BiOI under light irradiation. The order of degradation efficiency is: CB3 > CB4 > CB2 > CB5 > CB1 > g-C3N4 > BiOI. The specific degradation rates are listed in Table S1. The composite ratio of g-C3N4 to BiOI decreased the degradation performance from 6:10, and the possible reason was analyzed that the addition of too much g-C3N4 would reduce the absorption and utilization of visible light by the composite catalysts, which would decrease the catalytic activity. The degradation capability for antibiotic pollutants was further evaluated using CIP (10 mg/L) under visible light. As also shown in Figure S5a, the photocatalytic degradation efficiency of CIP followed the order: CB3 > CB4 > CB5 > CB2 > CB1 > BiOI > g-C3N4. For g-C3N4, the composite samples obtained after the addition of BiOI were able to utilize a larger range of light and the catalytic activity was improved.
The photodegradation kinetics were analyzed using pseudo-first-order models. The fitting lines are shown in Figure S5b and Figure 5b, with the corresponding correlation coefficients (R2) and rate constants (k) listed in Table S3. For CB3, the fitted lines for both pollutants have the largest slopes, and the high R2 values (>0.95) confirm the reliability of the kinetic model. The calculated rate constants k (0.0438 min−1 for RhB and 0.0311 min−1 for CIP) are the highest among all precursor g-C3N4/BiOI composites. This result, consistent with the degradation profiles, confirms that CB3 possesses the best photocatalytic performance for both RhB and CIP degradation. Therefore, CB3 was selected as the precursor for synthesizing the quaternary composite.
The cycling stability of CB3 was evaluated by its repeated use in degrading RhB (20 mg/L) and CIP (10 mg/L) under visible light, as shown in Figure S6a and S6b, respectively. The catalyst maintained stable degradation rates over five consecutive cycles. After 1 hour of visible-light irradiation in each cycle, the degradation efficiency remained at 85.6% for RhB and 79.1% for CIP, with no significant loss of activity observed upon catalyst recovery and reuse. For the precursor g-C3N4/BiOI, the degradation performance of CB3 was still improved. It indicates that the CB3 composite photocatalyst has excellent catalytic stability (Figure 6).
Photocatalytic degradation of precursor g-C3N4/BiOI in CIP solution (20 mg/L) under visible light irradiation (a), first-order kinetic fitting curve (b).
3.9. Preparation and degradation properties of Ag/Ag3PO4-g-C3N4/BiOI composite photocatalysts X-ray diffraction analysis
The XRD pattern of the Ag/Ag3PO4-CB3 composite photocatalyst (prepared using CB3 as a precursor) is shown in Figure 7. The characteristic peaks at 20.86°, 29.6°, 33.2°, 46.4°, 55.1°, and 61.9° are indexed to the (110), (200), (210), (310), (320), and (400) crystal planes of Ag3PO4, respectively. Additionally, a strong peak appears at 38.1° in the XRD pattern of the photo-reduced sample. By comparison with the standard card for cubic metallic silver (JCPDS No. 04-0783), this peak is assigned to the (111) plane of elemental Ag [26]. This confirms the effective photodeposition reduction of Ag3PO4 to metallic Ag [26]. According to the standard card of tetragonal crystal system BiOI (JCPDS No. 10-0445), 2θ at 22.2°, 29.0°, 31.6°, 45.4°, and 51.4°corresponds to the characteristic diffraction peaks at the (101), (102), (110), (200), and (114) crystal planes of tetragonal crystal system BiOI, respectively.
3.10. Scanning electron microscope analysis
Figures 8a, 8b, and 8c present the SEM images of the Ag/Ag3PO4-CB3 composites obtained after different photoreduction durations: 10 min, 20 min, and 30 min, respectively.SEM analysis reveals that the synthesized Ag3PO4 exhibits a three-dimensional rod-like morphology. Both the precursor CB3 (g-C3N4/BiOI) and the in-situ reduced Ag nanoparticles are uniformly distributed on the Ag3PO4 surface. This homogeneous distribution alleviates the agglomeration of g-C3N4/BiOI and facilitates electron transfer. As the photoreduction time increased from 10 to 30 minutes, the loading density of Ag nanoparticles on the Ag3PO4 surface progressively increased. This enhanced Ag loading extends the light absorption of the composite into the visible region [27]. Therefore, the preparation of Ag/Ag3PO4 complexes from Ag3PO4 by photoreduction method can not only further improve the light utilization absorption range, but also obtain the noble metal silver particles, so that the Ag plasmon resonance effect can be fully utilized.
SEM patterns of photocatalyst: 10minAg/Ag3PO4-CB3 (a), 20minAg/Ag3PO4-CB3 (b), 30minAg/Ag3PO4-CB3 (c).
3.11. Elemental distribution analysis and EDS spectral analysis
In order to further determine the successful composite of the tetrameric photocatalyst, EDS analysis of 20minAg/Ag3PO4-CB3 with the best degradation performance was carried out, and the samples were selected for scanning, and it can be seen from Figure S7 (b) that the composition of the complex consists of eight elemental compositions of Bi, O , I , C, N, P, O, and Ag. In order to better study the structure of the tetrameric composites, surface-scan analysis of elemental distributions of the 20minAg/ Ag3PO4-CB3 was subjected to elemental distribution surface sweep analysis, and Figures S7 (c), (d), (e), (f), (g), (h), and (i) show the distribution of Bi, I, O, C, N, Ag, and P atoms, respectively, and it can be seen that the individual elements are uniformly distributed in the quaternary complexes, which proves that Ag/Ag3PO4-CB3 was successfully prepared.
3.12. Infrared mapping analysis
The FTIR spectra of Ag/Ag3PO4-CB3 photocatalyst are shown in Figure S8. The two characteristic peaks located at 560 cm–1 and 1012 cm–1 in the figure are the O-P-O stretching vibration and the P-O bond stretching vibration, which proves the presence of PO43- [28] the characteristic peak at 813 cm–1 is attributed to the triazine ring structure of g-C3N4, and that at 1240 cm–1 is attributed to the C=N double bond. The characteristic peak at 1638 cm–1 is attributed to the C-N stretching vibration, and the broad peaks at 3000–3500 cm–1 are originated from the N-H as well as the O-H stretching vibration in the residual H2O. The characteristic peak at 497 cm–1 is attributed to the stretching vibration of the Bi-O bond in the BiOI, and the characteristic peak of the absorption near 772 cm–1 is attributed to the symmetric stretching vibration of the Bi-O bond in the tetradentate composite The absorption peaks of the semiconductor include all the characteristic peaks of Ag3PO4, BiOI and g-C3N4, further indicating that the Ag/Ag3PO4-CB3 photocatalyst has been successfully prepared.
3.13. X-ray spectroscopy analysis
Figure 9a shows the full-spectrum scan of 20minAg/Ag3PO4-CB3, and the peak positions of O 1s, Ag 3d and P 2p in Ag3PO4 and Bi 4f, I 3d and O 1s in BiOI as well as N 1s and C 1s in g-C3N4 can be clearly seen, which confirms the previous characterization results of the FT-TR, and indicates that the successful preparation of 20 min Ag/Ag3PO4-CB3 four-member composite photocatalyst. In Figure 9(b), there are four fitted peaks for Ag 3d, in which the characteristic peaks at 373.8 eV and 367.6 eV correspond to Ag+ 3d 3/2 and Ag+3d 5/2, respectively, while the characteristic peaks at 373.9 eV and 367.9 eV belong to Ag0 3d 3/2 and Ag0 3d 5/2, which proves that Ag+ and Ag0 exist in the composite at the same time [29] i.e., Ag3PO4 has been successfully composited and Ag has been successfully reduced in the quaternary complex. Figure 9(c) shows a high-resolution XPS image of P 2p, in which the characteristic peak with a binding energy of 132.6 eV is attributed to PO43- [30].
XPS patterns of 20minAg/Ag3PO4-CB3: Full spectrum (a), Ag 3d (b), P 2p (c), O 1s (d), C 1s (e), N 1s (f), Bi 4f (g), I 3d (h).
The XPS spectrum of O 1s is shown in Figure 9d fitted with four peaks, the peak at 529.6 eV is attributed to Bi-O, the peak at 530.6 eV is attributed to P-O, the peak at 531.5 eV is attributed to I-O on the surface of BiOI, and the peak at 532.8 eV is attributed to O in H2O or CO2 at the surface of the sample. Figure 9e is a high-resolution spectrum of the C 1s, the peaks located in the 284.8 eV, 286.4 eV and 288.6 eV diffraction peaks correspond to C-C bonds of standard carbon with calibrated binding energies, sp2 hybridized C atoms in the C=N-C structure, and C atoms of N-(C)3 bonds, respectively, and as can be seen in Figure 9f, the peaks at 399.1 eV and 400.8 eV correspond to the sp2 hybridized aromatic nitrogen C=N-C and sp3 hybridized tertiary nitrogen including both N-(C)3 and C-N(-H)-C portions of the nitrogen in the triazine ring, respectively, and the above basic structure of the four-membered photocatalysts containing g-C3N4 is in agreement with the above. Figure 9(g), Bi 4f 7/2 and Bi 4f 5/2 correspond to two strong peaks at 159.3 eV and 164.6 eV, respectively, indicating that the elemental Bi in the samples exists in the form of Bi3+, and Figure 9h shows I 3d, with 618.9 eV and 630.4 eV attributed to I 3d 5/2 and I 3d 3/2, respectively, which exists in the form of I-, in agreement with the above BiOI is consistent (Figure 10).
3.14. UV diffuse reflectance analysis
The optical absorption edges of the Ag/Ag3PO4-CB3 composites all extended to approximately 600 nm (see Figure S9 and Table S4 for details). Notably, the sample prepared with a 20-minute photoreduction (20 min Ag/Ag3PO4-CB3) exhibited the most red-shifted absorption edge at 612 nm. A broader absorption range enables the excitation of more photogenerated electron-hole pairs, which in turn enhances the catalytic activity.
3.15. Photoluminescence spectroscopy analysis
As shown in Figure S10, all three quaternary composite photocatalysts exhibit PL emission peaks around 470 nm with similar spectral profiles. The sample 20 min Ag/Ag3PO4-CB3 shows the lowest peak intensity, indicating the most effective suppression of electron-hole recombination among them. This minimal recombination rate is a key factor contributing to its superior photocatalytic degradation performance.
3.16. Specific surface area testing and analysis
In this experiment, the specific surface area and pore size distribution of the four-membered composite photocatalysts were tested by nitrogen adsorption-desorption method, as shown in Figure S11, and the types of adsorption isotherms of all catalyst samples were categorized as type IV isotherms, and inconspicuous H3-type hysteresis loops. The specific values of specific surface area, pore volume and average pore size of the samples are shown in Table 3, and 20 minAg/Ag3PO4-CB3 has the largest specific surface area, which is favorable for photocatalytic activity.
3.17. Photocatalytic degradation performance and reproducibility analysis
The photocatalytic performance of the quaternary Ag/Ag3PO4-CB3 composite was evaluated by degrading two model pollutants: rhodamine B (RhB, 20 mg/L) and ciprofloxacin (CIP, 10 mg/L). For RhB degradation (Figure S12a), the Ag/Ag3PO4-CB3 composite achieved a degradation efficiency of 97.0% under visible light (after 30 min of dark adsorption), a significant improvement over the 93.3% efficiency of the precursor CB3 (specific data are listed in Table 4). This result confirms that the Ag/Ag3PO4 modification enhances the catalytic performance compared to pure Ag3PO4, consistent with prior studies [31]. The degradation of CIP (10 mg/L) was also investigated (Figure S12b). Among the composites, the 20 min Ag/Ag3PO4-CB3 sample exhibited the highest efficiency, reaching 90.2%, which represents a notable enhancement from the 83.3% achieved by the precursor CB3.
The photodegradation kinetics were further analyzed using the pseudo-first-order model. The fitting lines, correlation coefficients (R2), and rate constants (k) are presented in Figure S13 and Table 4. For the degradation of both pollutants, the 20 min Ag/Ag3PO4-CB3 composite exhibited the largest slope (i.e., the highest rate constant k = 0.0578 min−1) and an R2 value > 0.95, confirming a good fit. These fitted slopes (or k values) were higher than those for the 10 min and 30 min counterparts. This kinetic analysis aligns with the degradation efficiency results, demonstrating that the 20 min Ag/Ag3PO4-CB3 composite possesses the best photocatalytic performance for both RhB and CIP.
The cycling stability of the optimal 20 min Ag/Ag3PO4-CB3 composite was evaluated for degrading both RhB (20 mg/L) and CIP (10 mg/L) under visible light, as shown in Figure S14a and S14b, respectively. Over five consecutive cycles, the composite maintained high and stable degradation efficiency. After 1 hour of visible-light irradiation per cycle, the degradation rates remained at 91.1% for RhB and 85.2% for CIP, with no significant activity loss observed upon catalyst recovery and reuse. For the precursor CB3, the degradation performance of the 20 minAg/Ag3PO4-CB3 composite photocatalysts still improved, indicating that the 20 minAg/Ag3PO4-CB3 composite photocatalysts have excellent catalytic stability.
The photocatalytic performance of the quaternary composite is further enhanced beyond that of the precursor g-C3N4/BiOI heterojunction. This enhancement stems from multiple synergistic effects. Firstly, the formation of an additional heterojunction between Ag3PO4 and g-C3N4 plays a key role. Radical quenching experiments [32] identified superoxide radicals (•O2−) as the primary active species in the g-C3N4/Ag3PO4 system.Secondly, the metallic silver (Ag0) nanoparticles, derived from the partial reduction of Ag3PO4, serve a dual function. They act as an electron reservoir and a charge-transfer bridge. More importantly, these Ag0 nanoparticles exhibit a strong surface plasmon resonance (SPR) effect under visible light. The SPR effect not only significantly enhances the overall light absorption but also creates a strong localized electromagnetic field. This field promotes the excitation of both g-C3N4 and Ag3PO4, leading to the generation of more electron-hole pairs. Additionally, a Schottky heterojunction formed at the Ag0/semiconductor interface facilitates the efficient transfer of photogenerated electrons to Ag0. Therefore, the advantage of reducing Ag3PO4 to obtain Ag/Ag3PO4 and then compounding it with the precursor g-C3N4-BiOI lies in the fact that both the highly oxidizing holes in the valence band of Ag3PO4 and the highly reducing electrons in the valence band of g-C3N4 are retained, and the excess electrons of Ag3PO4 are transferred to Ag0 in time to reduce photocorrosion, which continues to enhance the photocatalytic performance by utilizing the plasma effect of Ag0. The plasma effect of Ag0 continues to be utilized to enhance the photocatalytic performance (Figure 11).
4. CONCLUSION
The g-C3N4-BiOI precursor can form a heterojunction at the phase interface, which creates an internal electric field in the prepared nanosheet composites, which is favorable for the interfacial transfer of light-induced charge, so that the photogenerated electron-hole pairs can be separated efficiently, and the lifetimes of photogenerated carriers can be extended. The best degradation performance was achieved by CB3, with degradation efficiencies of 93.3% and 83.3% for RhB (20 mg/L) and CIP (10 mg/L), respectively.
Composite Ag3PO4 in the precursor CB3, between Ag3PO4 and g-C3N4 to form a Z-type structure, continue to use the photo-reduction method to reduce part of Ag+ to Ag0, the electron storage capacity of silver clusters Ag0 can be used as a charge transfer bridge. g-C3N4 and Ag3PO4 in the resulting strong local electric field under the action of the excitation of more e- and h +, at the same time g-C3N4 and At the same time, the excited e- on Ag3PO4 can also be transferred to Ag0 using Schottky heterojunction, which not only retains the high oxidizing holes in the valence band of Ag3PO4 and the high reducing electrons on the valence band of g-C3N4, but also reduces photocorrosion by transferring the excess electrons from Ag3PO4 to Ag0 in time. The best degradation performance of 20 minAg/Ag3PO4-CB3 reached 97.0% and 90.2% for rhodamine solution (RhB, 20 mg/L) and ciprofloxacin solution (CIP, 10 mg/L), respectively.
5. ACKNOWLEDGMENTS
The authors would like to thank the Qinghai University for Nationalities for the financial support of the Science and Technology Scientific Research Program (No. 2024XJMA07).
SUPPLEMENTARY MATERIAL
The following online material is available for this article.
Figure S1 - EDS patterns of precursor CB3: (a), (b); Surface scanning images of elements corresponding to precursor CB3:C (c), N (d), Bi (e), I (f), O (g).
Figure S2 - FT-IR pattern of precursor g-C3N4/BiOI.
Figure S3 - UV Visible Diffuse Reflection Absorption Spectrogram of Precursor g-C3N4/BiOI (a), Catalyst Band Gap Energy (b).
Figure S4 - Photoluminescence Spectra of precursor g-C3N4/BiOI.
Figure S5 - Photocatalytic degradation of precursor g-C3N4 BiOI in RhB solution (20 mg/L) under visible light irradiation(a), first-order kinetic fitting curve (b).
Figure S6 - Stability test of precursor g-C3N4/BiOI degradation of RhB (20 mg/L)(a), stability test of precursor g-C3N4/BiOI degradation of CIP(10 mg/L) (b).
Figure S7 - EDS diagrams of 20minAg/Ag3PO4-CB3 photocatalyst: (a), (b); Surface scanning images of elements corresponding to 20minAg/Ag3PO4-CB3 photocatalyst: Bi (c), I (d), O (e), C (f), N (g), Ag (h), P (i).
Figure S8 - FT-IR pattern of Ag/Ag3PO4-CB3 photocatalyst.
Figure S9 - UV Visible Diffuse Reflection Absorption Spectrogram of Ag Ag3PO4-CB3 photocatalyst (a), Catalyst Band Gap Energy (b).
Figure S10 - Photoluminescence Spectra of Ag/Ag3PO4-CB3 Photocatalyst.
Figure S11 - Nitrogen adsorption desorption curve and pore size distribution of Ag/Ag3PO4-CB3.
Figure S12 - Degradation of RhB solution by Ag/Ag3PO4-CB3 under visible light irradiation (20 mg/L) (a), first-order kinetic fitting curve (b).
Figure S13 - Photocatalytic degradation of Ag/Ag3PO4-CB3 photocatalyst in CIP solution (10mg/L) under visible light irradiation (a), first-order kinetic fitting curve (b).
Figure S14 - Stability test of 20minAg/Ag3PO4-CB3 photocatalyst degradation of RhB (20mg/L) (a), stability test of 20minAg/Ag3PO4-CB3 photocatalyst degradation of CIP(10mg/L) (b).
Table S1 - Experimental reagents.
Table S2 - Experimental apparatus and equipment.
Table S3 - Degradation rate and first-order kinetic constant of precursor g-C3N4/BiOI.
Table S4 - Absorption sideband and bandgap energy of Ag/Ag3PO4-CB3 photocatalyst.
DATA AVAILABILITY
This study is an original experimental research and all data used are obtained from real experiments.
The material and data in the article are copyright free.
6. BIBLIOGRAPHY
-
[1] HU, H., CAO, H., ZHANG, L., et al, “Effects of heterogeneous environmental regulation on the control of water pollution discharge”, Desalination and Water Treatment, v. 205, pp. 208–213, 2020. doi: https://doi.org/10.5004/dwt.2020.26349.
» https://doi.org/10.5004/dwt.2020.26349 -
[2] NEZAMZADEH-EJHIEH, A., ZABIHI-MOBARAKEH, H., “Heterogeneous photodecolorization of mixture of methylene blue and bromophenol blue using CuO-nano-clinoptilolite”, Journal of Industrial and Engineering Chemistry, v. 20, n. 4, pp. 1421–1431, 2014. doi: https://doi.org/10.1016/j.jiec.2013.07.027.
» https://doi.org/10.1016/j.jiec.2013.07.027 -
[3] NOSUHI, M., NEZAMZADEH-EJHIEH, A., “Comprehensive study on the electrocatalytic effect of copper–doped nano-clinoptilolite towards amoxicillin at the modified carbon paste electrode-solution interface”, Journal of Colloid and Interface Science, v. 497, pp. 66–72, 2017. doi: https://doi.org/10.1016/j.jcis.2017.02.055. PMid:28268183.
» https://doi.org/10.1016/j.jcis.2017.02.055 -
[4] BHATKHANDE, D.S., PANGARKAR, V.G., BEENACKERS, A.A.C.M., “Photocatalytic degradation for environmental applications: a review”, Journal of Chemical Technology and Biotechnology, v. 77, n. 1, pp. 102–116, 2002. doi: https://doi.org/10.1002/jctb.532.
» https://doi.org/10.1002/jctb.532 -
[5] XIA, X., YIN, H., ZHANG, Y., et al, “Boron-doped g-CN monolayer as a promising anode for Na/K-ion batteries”, Surfaces and Interfaces, v. 36, pp. 102479, 2023. doi: https://doi.org/10.1016/j.surfin.2022.102479.
» https://doi.org/10.1016/j.surfin.2022.102479 -
[6] BHUNIA, K., CHANDRA, M., KHILARI, S., et al, “Bimetallic PtAu alloy nanoparticles-integrated g-C3N4 hybrid as an efficient photocatalyst for water-to-hydrogen conversion”, ACS Applied Materials & Interfaces, v. 11, n. 1, pp. 478–488, 2019. doi: https://doi.org/10.1021/acsami.8b12183. PMid:30525406.
» https://doi.org/10.1021/acsami.8b12183 -
[7] ZHANG, H., YANG, J., GUO, L., et al, “Microwave-aided synthesis of BiOI/g-C3N4 composites and their enhanced catalytic activities for Cr (VI) removal”, Chemical Physics Letters, v. 762, pp. 138143, 2021. doi: https://doi.org/10.1016/j.cplett.2020.138143.
» https://doi.org/10.1016/j.cplett.2020.138143 -
[8] WANG, J., CAI, Y., DU, H., et al, “Improve the structure through pH-control to improve the photocatalytic performance of cubic silver phosphate photocatalyst”, Journal of Dispersion Science and Technology, v. 43, n. 9, pp. 1399–1404, 2022. doi: https://doi.org/10.1080/01932691.2020.1869030.
» https://doi.org/10.1080/01932691.2020.1869030 -
[9] CUI, Z., SI, M., ZHENG, Z., et al, “Preparation and characterization of Ag3PO4/BiOI composites with enhanced visible light driven photocatalytic performance”, Catalysis Communications, v. 42, pp. 121–124, 2013. doi: https://doi.org/10.1016/j.catcom.2013.08.011.
» https://doi.org/10.1016/j.catcom.2013.08.011 -
[10] ZHANG, W., ZHOU, L., SHI, J., et al, “Synthesis of Ag3PO4/G-C3N4 composite with enhanced photocatalytic performance for the photodegradation of diclofenac under visible light irradiation”, Catalysts, v. 8, n. 2, pp. 45, 2018. doi: https://doi.org/10.3390/catal8020045.
» https://doi.org/10.3390/catal8020045 -
[11] CHEN, R., DING, S., FU, N., et al, “Preparation of a g-C3N4/Ag3PO4 composite Z-type photocatalyst and photocatalytic degradation of Ofloxacin: degradation performance, reaction mechanism, degradation pathway and toxicity evaluation”, Journal of Environmental Chemical Engineering, v. 11, n. 2, pp. 109440, 2023. doi: https://doi.org/10.1016/j.jece.2023.109440.
» https://doi.org/10.1016/j.jece.2023.109440 -
[12] ZHANG, Z., GONG, L., ZHAN, X., et al, “Complete photodegradation of tetracycline induced by surface microenvironment of graphitic carbon nitride/silver phosphate”, Journal of Environmental Chemical Engineering, v. 12, n. 3, pp. 112583, 2024. doi: https://doi.org/10.1016/j.jece.2024.112583.
» https://doi.org/10.1016/j.jece.2024.112583 -
[13] AN, H., LIN, B., XUE, C., et al, “Formation of BiOI/g-C3N4 nanosheet composites with high visible-light-driven photocatalytic activity”, Chinese Journal of Catalysis, v. 39, n. 4, pp. 654–663, 2018. doi: https://doi.org/10.1016/S1872-2067(17)62927-9.
» https://doi.org/10.1016/S1872-2067(17)62927-9 -
[14] LIANG, J., LI, X., ZUO, J., et al, “Hybrid 0D/2D heterostructures: in-situ growth of 0D g-C3N4 on 2D BiOI for efficient photocatalyst”, Advanced Composites and Hybrid Materials, v. 4, n. 4, pp. 1122–1136, 2021. doi: https://doi.org/10.1007/s42114-021-00341-x.
» https://doi.org/10.1007/s42114-021-00341-x -
[15] HUANG, H., LIU, C., OU, H., et al, “Self-sacrifice transformation for fabrication of type-I and type-II heterojunctions in hierarchical BixOyIz/g-C3N4 for efficient visible-light photocatalysis”, Applied Surface Science, v. 470, pp. 1101–1110, 2019. doi: https://doi.org/10.1016/j.apsusc.2018.11.193.
» https://doi.org/10.1016/j.apsusc.2018.11.193 -
[16] TABASUM, S., RANI, S., SHARMA, A., et al, “Efficient photocatalytic degradation of chlorpyrifos pesticide from aquatic agricultural waste using g-C3N4 decorated graphene oxide/V2O5 nanocomposite”, Topics in Catalysis, v. 67, n. 9–12, pp. 725–736, 2024. doi: https://doi.org/10.1007/s11244-023-01865-w.
» https://doi.org/10.1007/s11244-023-01865-w -
[17] CHEN, C.C., FU, J.Y., CHANG, J.L., et al, “Bismuth oxyfluoride/bismuth oxyiodide nanocomposites enhance visible-light-driven photocatalytic activity”, Journal of Colloid and Interface Science, v. 532, pp. 375–386, 2018. doi: https://doi.org/10.1016/j.jcis.2018.07.130. PMid:30096531.
» https://doi.org/10.1016/j.jcis.2018.07.130 -
[18] ZHANG, L., LIAO, J., LI, Y., et al, “Cu single atoms embedded on hollow g-C3N4 nanospheres with enhanced charge transfer and separation for efficient photocatalysis”, Chinese Chemical Letters, v. 35, n. 2, pp. 108568, 2024. doi: https://doi.org/10.1016/j.cclet.2023.108568.
» https://doi.org/10.1016/j.cclet.2023.108568 -
[19] LIANG, Y.F., LU, J.R., TIAN, S.K., et al, “Pt nanoclusters modified porous g-C3N4 nanosheets to significantly enhance hydrogen production by photocatalytic water reforming of methanol”, Chinese Journal of Chemical Engineering, v. 66, pp. 40–50, 2024. doi: https://doi.org/10.1016/j.cjche.2023.11.005.
» https://doi.org/10.1016/j.cjche.2023.11.005 -
[20] NOBRE, F.X., MUNIZ, R., DO NASCIMENTO, E.R., et al, “Hydrothermal temperature dependence of CaWO4 nanoparticles: structural, optical, morphology and photocatalytic activity”, Journal of Materials Science Materials in Electronics, v. 32, n. 8, pp. 9776–9794, 2021. doi: https://doi.org/10.1007/s10854-021-05638-7.
» https://doi.org/10.1007/s10854-021-05638-7 -
[21] ASHFAQ, M.H., IMRAN, M., HAIDER, A., et al, “Antimicrobial potential and rhodamine B dye degradation using graphitic carbon nitride and polyvinylpyrrolidone doped bismuth tungstate supported with in silico molecular docking studies”, Scientific Reports, v. 13, n. 1, pp. 17847, 2023. doi: https://doi.org/10.1038/s41598-023-44799-9. PMid:37857696.
» https://doi.org/10.1038/s41598-023-44799-9 -
[22] WANG, S., GUAN, Y., WANG, L., et al, “Fabrication of a novel bifunctional material of BiOI/Ag3VO4 with high adsorption–photocatalysis for efficient treatment of dye wastewater”, Applied Catalysis B: Environment and Energy, v. 168–169, pp. 448–457, 2015. doi: https://doi.org/10.1016/j.apcatb.2014.12.047.
» https://doi.org/10.1016/j.apcatb.2014.12.047 -
[23] ZHONG, L., LI, W., LI, G., et al, “Alkali-assisted solvothermal synthesis of I-deficient nanoplatelet bioi microflowers with enhanced visible-light photocatalytic performance for degradation of organic pollutants”, ACS Applied Nano Materials, v. 6, n. 10, pp. 8250–8259, 2023. doi: https://doi.org/10.1021/acsanm.3c00533.
» https://doi.org/10.1021/acsanm.3c00533 -
[24] SILVA, T.S., FERNANDES, E.P., VITHANAGE, M., et al, “A facile synthesis of MgAl/layered double hydroxides from aluminum wastes”, Materials Letters, v. 324, pp. 132624, 2022. doi: https://doi.org/10.1016/j.matlet.2022.132624.
» https://doi.org/10.1016/j.matlet.2022.132624 -
[25] ZHANG, S., LEI, C., SONG, L., “Chiral induction enhanced photocatalytic degradation of methyl orange by Ag@ Ag3PO4 and the reaction mechanism”, Ceramics International, v. 49, n. 16, pp. 26548–26557, 2023. doi: https://doi.org/10.1016/j.ceramint.2023.05.189.
» https://doi.org/10.1016/j.ceramint.2023.05.189 -
[26] DU, C., SONG, J., TAN, S., et al, “Facile synthesis of Z-scheme ZnO/Ag/Ag3PO4 composite photocatalysts with enhanced performance for the degradation of ciprofloxacin”, Materials Chemistry and Physics, v. 260, pp. 124136, 2021. doi: https://doi.org/10.1016/j.matchemphys.2020.124136.
» https://doi.org/10.1016/j.matchemphys.2020.124136 -
[27] SIVARANJANI, P.R., SUBHIKSHA, V., OKLA, M.K., et al, “Construction of pnp nano heterojunction through coupling La2O3,(BiO)2CO3 and Ag3PO4 for effective photocatalytic degradation of doxycycline: Insights into mechanism, pathway and intermediate toxicity evaluation”, Environmental Pollution, v. 345, pp. 123521, 2024. doi: https://doi.org/10.1016/j.envpol.2024.123521. PMid:38331239.
» https://doi.org/10.1016/j.envpol.2024.123521 -
[28] WAN, L., YU, L., HOU, J., et al, “Synthesis and excellent antibacterial activity of Ag2O-loaded carboxymethyl starch nanocomposites”, Journal of Ocean University of China, v. 22, n. 3, pp. 728–734, 2023. doi: https://doi.org/10.1007/s11802-023-5349-3.
» https://doi.org/10.1007/s11802-023-5349-3 - [29] YANG, T., DENG, P., WANG, L., et al, “Simultaneous photocatalytic oxygen production and hexavalent chromium reduction in Ag3PO4/C3N4 S-scheme heterojunction”, Chinese Journal of Structural Chemistry, v. 41, n. 6, pp. 2206023–2206030, 2022.
-
[30] LU, M., DONG, J., HU, M., et al, “Perovskite LaMnO3 composite graphene carbon nitride g-C3N4 improves the photocatalytic performance of tetracycline degradation”, Water, v. 15, n. 8, pp. 1627, 2023. doi: https://doi.org/10.3390/w15081627.
» https://doi.org/10.3390/w15081627 -
[31] HUANG, Z., LIU, J., ZONG, S., et al, “Fabrication of graphitic carbon Nitride/Nonstoichiometric molybdenum oxide nanorod composite with the nonmetal plasma enhanced photocatalytic hydrogen evolution activity”, Journal of Colloid and Interface Science, v. 606, n. Pt 1, pp. 848–859, 2022. doi: https://doi.org/10.1016/j.jcis.2021.08.073. PMid:34425272.
» https://doi.org/10.1016/j.jcis.2021.08.073 -
[32] LI, K., CHEN, M., CHEN, L., et al, “Synthesis and application of a Fe3O4/Ag3PO4/g-C3N4 magnetic composite photocatalyst for sulfonamide antibiotics degradation”, Sustainability, v. 15, n. 17, pp. 13279, 2023. doi: https://doi.org/10.3390/su151713279.
» https://doi.org/10.3390/su151713279
