English

• 0.   Introduction

  Photocatalysis is an effective solution for energy and environmental problems as it can degrade organic pollutants. Many new photocatalytic materials have been developed and reported, including graphite-like carbon nitride (g-C₃N₄), which has gained significant attention^[1-2]. This material is a promising photocatalyst due to its low cost, wide visible light response range, and high stability^[3-4]. However, the photocatalytic performance of g-C₃N₄ is hindered by the efficiency of charge‑fast recombination and the limited range of light absorption. To efficiently develop and utilize g-C₃N₄, modification studies on g-C₃N₄ are necessary^[5-6]. Recent research has shown that the properties of photocatalysts, such as photoelectric properties and charge separation, greatly affect their photocatalytic activity. Therefore, the modification of g-C₃N₄ has focused on improving these properties, including optical properties, charge separation, and surface charge separation^[7]. For instance, the catalyst′s wide absorption region is crucial for enhancing photoactivity. Combining g-C₃N₄ with other semiconductors expands its light absorption range and enhances its catalytic activity. The composite photocatalysts based on g‑C₃N₄ show satisfactory photoresponsive properties, which effectively improve the photocatalytic performance. Improving all the properties of g‑C₃N₄ with a single modification is difficult. To obtain g‑C₃N₄‑based photocatalysts with excellent performance, it is essential to dope and compose multiple substances. Researchers have developed new photocatalysts using this strategy, such as Ag₃PO₄/Ag/g‑C₃N₄^[8], ZnO/g‑C₃N₄^[9], WO₃/g‑C₃N₄^[10], and CoFe_1.9Y_0.1O₄/g‑C₃N₄^[11]. However, their efficiency requires further improvement.

  Metal nanoparticles, such as Au, Ag, and Cu, exhibit a surface plasmon resonance effect that broadens the catalyst′s absorption range in the visible light spectrum. When silver is introduced onto the g-C₃N₄ surface, it creates a metal-semiconductor heterojunction, enhancing the efficiency of charge transfer and minimizing the recombination of photogenerated electron-hole pairs. This, in turn, leads to a marked enhancement in the photocatalytic performance of g-C₃N₄.

  MoS₂ is a semiconductor photocatalyst with a narrow band gap (E_g) ranging from 1.2 to 2.0 eV, making it highly sensitive to visible light. The staggered band alignment between g-C₃N₄ and MoS₂ enhances the separation of photogenerated carriers, thereby improving photocatalytic efficiency. Spherical MoS₂, which consists of multiple nanosheets stacked together, offers a higher number of active sites, as these sites are primarily concentrated at the edges of the nanosheets.

  The study of metal nanoparticles is necessary due to their significant impact on electron-hole transport and separation. In a study by Ju et al.^[12], noble metals Pt, Au, and Ag have the potential to enhance the photocatalytic activity of g-C₃N₄/MoS₂ composites through density functional theory. However, research on MoS₂/Ag/g-C₃N₄ composites is currently limited to theoretical calculations and predictions. Composite photocatalytic materials consisting of g‑C₃N₄, Ag, and MoS₂ have gained significant attention due to their construction and performance.

  This paper presents the preparation of MoS₂/Ag/g-C₃N₄ composites to enhance the photocatalytic activity for the degradation of organics. The addition of silver significantly prolonged the visible-light response of g-C₃N₄ and improved charge separation. The performance of the photocatalysts was characterized by various means. The article investigates the catalytic degradation mechanism of MoS₂/Ag/g-C₃N₄ to reveal the reason behind the increased photocatalytic activity.

  1.   Experimental

  1.1   Materials and reagents

  The analytically pure reagents were obtained from China Pharmaceutical Group Chemical Reagent Co. These included urea, thiourea, sodium molybdate dihydrate, polyvinylpyrrolidone, ethylene glycol, methanol, rhodamine B (RhB), disodium ethylenediaminetetraacetic acid, p-benzoquinone (BQ), and concentrated hydrochloric acid. Solutions were prepared using deionized water.

  1.2   Preparation of Ag/g-C₃N₄ and MoS₂

  Typically, 25.06 g of urea was placed in a crucible with a lid. The crucible was then placed in an oven, heated at a rate of 5 ℃·min^-1 until the temperature reached 500 ℃, and held for 6 h. After cooling to room temperature, the obtained g-C₃N₄ sample was ground. Then, 2.40 g of the prepared g-C₃N₄ was weighed into a 100 mL glass beaker, 80 mL of methanol was added, and the mixture was ultrasonicated for 1 h to obtain suspension A. A specific amount of 0.24 mol·L^-1 AgNO₃ aqueous solution was added dropwise to suspension A using a constant pressure dropping funnel. The mixture was stirred for 2 h at 70 ℃ while irradiating a 500 W xenon lamp. Finally, the mixture was filtered, washed, and vacuum-dried at a temperature of 70 ℃ for 10 h to yield the desired Ag/g-C₃N₄ sample.

  The 1.26 g of sodium molybdate dihydrate was transferred into a 50 mL beaker containing 20 mL of deionized water, and solution B was obtained after sufficient stirring. In a separate container, 1.37 g of thiourea was weighed and dissolved in 25 mL of anhydrous ethanol. The solution was stirred at a temperature of 60 ℃ on a thermostatic stirrer until the solid was completely dissolved. Using a constant-pressure funnel, solution B was added to the thiourea solution and stirred at a temperature of 60 ℃ for 1 h. Concentrated hydrochloric acid was introduced to adjust the pH to 3. The mixture was then stirred for 1 h, after which it was filtered. The precipitate obtained was washed three times with deionized water and anhydrous ethanol and subsequently dried in a vacuum oven at 60 ℃ for 24 h. Finally, the dried samples were ground to obtain the desired MoS₂ sample.

  1.3   Preparation of MoS₂/Ag/g-C₃N₄

  To prepare dispersion C, 0.50 g of Ag/g-C₃N₄ was dispersed with 20 mL of absolute ethanol in a 50 mL beaker. The mixture was stirred at room temperature for 30 min on a magnetic stirrer and then sonicated for 60 min in an ultrasonic cleaner. In a 50 mL separate beaker, 1.24 g of sodium molybdate dihydrate was dissolved in 20 mL of deionized water to obtain solution D. Finally, the two solutions had to be thoroughly mixed to achieve the desired outcome. 1.37 g thiourea was dissolved in 25 mL of anhydrous ethanol at a temperature of 60 ℃ with magnetic stirring until completely dissolved, forming solution E. Solution E was added to the previously prepared dispersion through a constant-pressure funnel. The mixture was stirred at 60 ℃ for 1 h, after which the pH was adjusted to 3 using concentrated hydrochloric acid, and was continuously stirred for an additional hour. The mixture was then filtered, and the resulting precipitate was washed three times with deionized water and three times with anhydrous ethanol. The washed precipitate was dried in a vacuum oven at 60 ℃ for 24 h. Finally, the dried material was ground to obtain the target sample.

  1.4   Characterizations

  The phase structures of the samples were analyzed using X-ray diffraction (XRD, Bruker D8 Advance), test conditions: Cu Kα (λ=0.154 056 nm), ceramic X-ray tube, a voltage of 40 kV, a current of 40 mA, a scanning range of 5°-80°, and a scanning speed of 5 (°)·min^-1. The morphology and crystalline architecture were examined utilizing a ZEISS Sigma500 Scanning Electron Microscope (SEM, Carl Zeiss AG, Oberkochen, Germany). The morphology of the samples was analyzed using high-resolution transmission electron microscopy (HRTEM, FEI Talos-F200S, accelerating voltage: 200 kV). Elemental composition and valence band (VB) structure were investigated via X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The band gap was measured using UV‑Vis diffuse reflectance spectroscopy (DRS, Shimadzu UV‑3600). The electron-hole recombination efficiency of the samples was analyzed using a photoluminescence (PL) spectrometer (RF-5301PC). The electrochemical properties of the samples were determined using an electrochemical workstation (CHI660E), and free radical active species experiments were performed using electron paramagnetic resonance (EPR, JEOLJES-FA200). Ultraviolet photoelectron spectroscopy (UPS, Japan PHI 5000 Versaprobe Ⅲ) was used to determine the sample work function. The total organic carbon (TOC) CONTENT of the RhB solution was detected using the German Jena Multi3100 TOC analyzer.

  1.5   Photocatalytic degradation

  To improve the accuracy of the catalytic degradation experiments and to achieve an adsorption-desorption equilibrium between the catalyst and the pollutant, adsorption in the dark was conducted for 30 min. 50 mg of the sample was accurately weighed and added to 50 mL 10 mg·L^-1 RhB solution. The mixture was stirred under a 500 W xenon lamp for 10 min, and 5 mL of the sample was centrifuged and filtered every 10 min to determine the absorbance of the supernatant at 550 nm. Additionally, five cyclic degradation experiments were conducted under the aforementioned experimental conditions in the absence of dark adsorption to investigate the cyclic degradation stability of the photocatalytic composites.

  2.   Results and discussion

  2.1   Structures and morphologies

  Fig. 1 displays the XRD patterns of MoS₂, Ag/g-C₃N₄, and MoS₂/Ag/g-C₃N₄. The MoS₂ sample exhibited three distinct diffraction peaks corresponding to the (002), (100), and (110) crystal planes, respectively^[13]. The diffraction pattern of Ag/g-C₃N₄ showed two diffraction peaks at 12.9° and 27.4°, corresponding to the (100) and (002) crystal planes of g-C₃N₄, respectively^[14-15]. No silver diffraction peak was observed in Ag/g-C₃N₄, possibly due to the low diffraction intensity or the low silver content. The XRD patterns of MoS₂/Ag/g-C₃N₄ showed the diffraction peaks of MoS₂ and g-C₃N₄, indicating the successful preparation of the composite photocatalyst.

  Figure 1

  

  Figure 1.  XRD patterns of the samples

  下载: 全尺寸图片 幻灯片

  SEM and TEM images of the composite samples are shown in Fig. 2a and 2b. The larger black particles were MoS₂, and the smaller grey particles loaded on the surface of the graphite phase were silver, which can be seen from the figure, indicating that the target photocatalyst has been successfully composited^[16-18]. Fig. 2c shows the HRTEM image of MoS₂/Ag/g-C₃N₄, from which it can be seen that the crystal plane spacings (d) of silver and MoS₂ nanoparticles were 0.269 2 and 0.618 6 nm respectively, corresponding to the (111) crystal plane of silver and the (002) crystal plane of MoS₂, respectively. In addition, the loading of silver nanoparticles can establish a fast charge transfer channel between MoS₂ and g-C₃N₄, thus enhancing the photocatalytic activity.

  Figure 2

  

  Figure 2.  (a) SEM, (b) TEM, and (c) HRTEM images of MoS₂/Ag/g-C₃N₄

  Inset: Fast Fourier transform phase profiles of MoS₂ (002) and Ag (111) crytal planes in the d-space of the MoS₂/Ag/g-C₃N₄ composites.

  下载: 全尺寸图片 幻灯片

  2.2   XPS analysis

  Fig. 3a shows the XPS survey spectra of the MoS₂/Ag/g-C₃N₄ sample. MoS₂/Ag/g-C₃N₄ was mainly composed of the elements C, N, Ag, Mo, and S. The peaks at 284.44 and 287.85 eV in Fig. 3b correspond to C—C and C—C=N in Ag/g-C₃N₄, respectively. Notably, in composite material MoS₂/Ag/g-C₃N₄, it can be seen that the binding energy of C1s shifted, indicating that the composite of MoS₂ and Ag/g-C₃N₄ was not a simple mixture but rather a stress generated between the two, and the chemical environment of the composite changed. Fig. 3c shows the high-resolution N1s XPS spectra of MoS₂/Ag/g-C₃N₄, with the peak of the three-coordinated N—C₃ located at 399.65 eV, the peak of the surface amino group (—NH₂) located at 400.64 eV, the π-excitation peak of the C—N heterocyclic ring located at 404.59 eV, and the peak of the dicoordinated N species (C=N—C) located at 398.35 eV^[17-18]. The S2p XPS spectra of MoS₂/Ag/g-C₃N₄ are shown in Fig. 3d, and the characteristic peaks at 160.62 and 161.74 eV are attributed to S2p_3/2 and S2p_1/2, respectively, which correspond to S^2- in MoS₂. From the high-resolution Mo3d XPS spectrum of MoS₂/Ag/g-C₃N₄ in Fig. 3e, the characteristic peaks at 227.73 and 230.82 eV are attributed to Mo3d_5/2 and Mo3d_3/2, respectively^[18-19]. As shown in Fig. 3f, the binding energies of MoS₂/Ag/g-C₃N₄ are located at 369.56 and 375.62 eV with a set of strong spectral peaks corresponding to the 3d_5/2 and 3d_3/2 characteristic peaks of metallic Ag monomers, respectively, indicating that the samples contain metallic Ag, which confirms that the Ag nanoparticles loaded into the composites successfully. In summary, after the combination of MoS₂ and Ag/g-C₃N₄, the shift of element binding energy indicates a change in its chemical environment. MoS₂ contacts with Ag/g-C₃N₄ to form a heterojunction, which is beneficial for subsequent catalytic degradation reactions.

  Figure 3

  

  Figure 3.  (a) Survey, (b) C1s, (c) N1s, (d) S2p, (e) Mo3d, and (f) Ag3d XPS spectra of the samples

  下载: 全尺寸图片 幻灯片

  2.3   Energy band structures

  The composites were analyzed using DRS tests to determine their absorption and energy band structure across different wavelengths. Fig. 4a displays the UV-Vis absorption spectra of MoS₂, g-C₃N₄, Ag/g-C₃N₄, and MoS₂/Ag/g-C₃N₄. The light absorption region of Ag/g-C₃N₄ had a limited response of only about 550 nm, while the light absorption region of MoS₂ exhibited higher absorption in the visible range. However, the UV-visible absorption wavelength range of MoS₂/Ag/g-C₃N₄ was broader than that of the individual catalysts Ag/g-C₃N₄ and g-C₃N₄. Importantly, the formation of a Z-scheme heterojunction restricts the recombination of photogenerated electrons and holes, and the silver atoms can serve as a bridge for the photogenerated electron and hole pairs. Consequently, the MoS₂/Ag/g-C₃N₄ composites demonstrated high catalytic activity under the same irradiation conditions^[19]. Additionally, the VB (valance band)-XPS spectra of g-C₃N₄ and MoS₂ are shown in Fig. 4b and 4c. VB-XPS detected the VB potentials of g-C₃N₄ and MoS₂ in a small range^[20-21]. The VB energies of g-C₃N₄ and MoS₂ were 1.62 and 1.86 eV, respectively. Therefore, using the formula E_VB-NHE=φ+E_VB-XPS-4.44 [where E_VB-NHE is the VB energy based on the normal hydrogen electrode (NHE), φ is the electron work function of the XPS analyzer with a value of 4.55 eV, and E_VB-XPS is the VB energy tested by VB-XPS]^[22], the calculated values of the VB energies of g-C₃N₄ and MoS₂ were 1.73 and 1.97 eV, respectively. Understanding the band gaps and energy band structures of the materials is the key to exploring the photocatalytic reaction mechanism. According to the analysis of the Mott-Schottky curves (Fig. 4d and 4e). The curve of MoS₂ was positively sloped, indicating that MoS₂ is an n-type semiconductor with a flat band potential of 0.08 V (vs Ag/AgCl). The curve of g-C₃N₄ was positively sloped, indicating that g-C₃N₄ is an n-type semiconductor with a flat band potential of -0.85 V (vs Ag/AgCl). Therefore, since the flat band potential of n-type semiconductor is closer to its conduction band (CB) potential, according to the formula E=E′-0.197 (where E′ is the potential relative to Ag/AgCl electrode, E is the potential relative to the NHE), the CB potentials of g-C₃N₄ and MoS₂ could be obtained as -1.05 and -0.12 V (vs NHE), respectively^[23]. The band energy levels of g-C₃N₄ and MoS₂ are shown in Fig. 4f.

  Figure 4

  

  Figure 4.  (a) UV-Vis absorption spectra of the samples; VB-XPS spectra of (b) g-C₃N₄ and (c) MoS₂;Mott-Schottky curves of (d) g-C₃N₄ and (e) MoS₂; (f) Band energy levels of g-C₃N₄ and MoS₂

  Inset: corresponding enlarged spectra.

  下载: 全尺寸图片 幻灯片

  2.4   Electrochemical properties and photocatalytic activity

  The electron-hole recombination rates of the samples were analyzed by PL spectroscopy, as shown in Fig. 5a. Ag/g-C₃N₄ had high fluorescence intensity and electron-hole recombination rate. However, the photogenerated carrier recombination rate and fluorescence intensity decreased after the formation of composite samples of Ag/g-C₃N₄ with MoS₂^[24-25]. This indicates that the electron-hole recombination efficiency was significantly suppressed, suggesting that the photocatalytic properties were enhanced and the performance was improved.

  Figure 5

  

  Figure 5.  (a) PL spectra, (b) TRPL spectra, and (c) EIS of the samples

  下载: 全尺寸图片 幻灯片

  To further investigate the separation and fluorescence lifetime of photoexcited charges, time-resolved photoluminescence spectra (TRPL) were measured. The decay curves were smoothly fitted with a double-exponential equation as shown in Fig. 5b. The average lifetime (τ_a) was calculated according to the following Eq.1:

  $ \tau_{\mathrm{a}}=\left(A_1 \tau_1^2+A_2 \tau_2^2\right)/\left(A_1 \tau_1+A_2 \tau_2\right) $

  (1)

  where A₁ and A₂ are the amplitudes of distinct decay processes, while τ₁ and τ₂ represent the corresponding lifetimes. The fitted decay curves disclose the lifetime (τ_a). It is noted that the unrecombined radiative carriers get involved in surface photocatalytic reactions. Therefore, the longer the fluorescence lifetime of the material, the more conducive to carrier separation, the greater the chance of photogenerated charge participation in the photocatalytic reaction, and the stronger the catalytic activity^[26]. As can be seen from Fig. 5b, the fluorescence lifetimes of Ag/g-C₃N₄ and MoS₂/Ag/g-C₃N₄ were 1.78 and 2.07 ns, respectively. The fluorescence lifetime of MoS₂/Ag/g-C₃N₄ was larger than that of Ag/g-C₃N₄, which suggests that the photogenerated electron-hole separation is higher due to the formation of heterojunction.

  Fig. 5c illustrates the Nyquist curves for Ag/g-C₃N₄, MoS₂, and MoS₂/Ag/g-C₃N₄. MoS₂/Ag/g-C₃N₄ exhibited a smaller arc radius compared to Ag/g-C₃N₄. This suggests a reduction in resistance alongside an enhancement in both electron-hole separation efficiency and photocatalytic performance, which is attributed to the incorporation of MoS₂ onto Ag/g-C₃N₄^[27-28].

  Fig. 6a shows the results of the photocatalytic degradation experiments on the samples. The degradation rate (η) can be calculated according to η=(1-A_t/A₀)×100% (where A_t is the absorbance of RhB at time t, and A₀ is the absorbance of the initial RhB solution). In the absence of a photocatalyst, the degradation rate of RhB remained unchanged after 120 min of light exposure, indicating that there was weak self-degradation of the RhB solution. However, when the catalyst was added, the degradation rate of the RhB solution gradually increased with prolonged light exposure time, demonstrating the catalytic activity of the sample under illumination. Among them, the photocatalytic activity of MoS₂/Ag/g-C₃N₄ was the highest, reaching a 98% degradation rate after 120 min. In addition, for the RhB aqueous solution, its photocatalytic degradation process can be expressed as a first-order reaction with the following reaction Eq.2:

  Figure 6

  

  Figure 6.  (a) Visible photocatalytic degradation performances of RhB from the samples and (b) corresponding first-order kinetic curves; (c) TOC changes during the photodegradation process for RhB

  下载: 全尺寸图片 幻灯片

  $ -\ln \left(A_t/A_0{ }^{\prime}\right)=k t $

  (2)

  where A₀′ is the absorbance after adsorption equilibrium and k is the degradation rate constant. In Fig. 6b, the k value of MoS₂/Ag/g-C₃N₄ was 0.024 min^-1, which was two and three times higher than that of MoS₂ (0.012 min^-1) and Ag/g-C₃N₄ (0.008 1 min^-1), respectively. Thus, the composite of MoS₂ and Ag/g-C₃N₄ can significantly improve the photocatalytic degradation rate.

  The extent of mineralization during the photocatalytic degradation of RhB by the photocatalyst can be assessed through the rate of change in total organic carbon (TOC). As shown in Fig. 6c, the mineralization rates of RhB by Ag/g-C₃N₄, MoS₂, and MoS₂/Ag/g-C₃N₄ within 2 h were 52%, 59%, and 70%, respectively. MoS₂/Ag/g-C₃N₄ had the highest degree of mineralization of RhB, indicating that the formation of heterojunction enhances the catalytic activity.

  2.5   Cyclic degradation stability and degradation mechanism

  The cyclic degradation stability of the samples is shown in Fig. 7a and 7b, which illustrates that the degradation rate of MoS₂/Ag/g-C₃N₄ catalyzed degradation of RhB showed a decreasing trend with the increase in the number of cyclic degradation, but the decrease was not significant. After five cycles of degradation, the degradation rate remained at about 85%. In the five- cycle degradation experiments, the composite degradation performance was significantly reduced, possibly due to the loss of catalyst during recycling or as a result of silver leaching. Cyclic degradation experiments showed that the degradation rate could still be maintained at 85.3% in the presence of loss during catalyst recycling, indicating that the composite photocatalyst had good cyclic degradation stability. In addition, the XRD patterns of MoS₂/Ag/g-C₃N₄ before and after photocatalytic degradation are shown in Fig. 7c. The intensity of the diffraction peaks after photocatalytic degradation changed but not significantly, indicating that MoS₂/Ag/g-C₃N₄ had a stable crystal structure during the photocatalytic degradation process.

  Figure 7

  

  Figure 7.  (a) Cyclic degradation curves and (b) degradation rates of MoS₂/Ag/g-C₃N₄ for RhB; (c) XRD patterns of MoS₂/Ag/g-C₃N₄ before and after cyclic degradation experiments

  下载: 全尺寸图片 幻灯片

  The mechanism of photocatalysis in the MoS₂/Ag/g-C₃N₄ degradation reaction system was investigated by introducing isopropanol (IPA, 20 mmol·L^-1), disodium ethylenediaminetetraacetate (EDTA-2Na, 20 mmol·L^-1), and benzoquinone (BQ, 20 mmol·L^-1). Fig. 8a shows that the degradation rate of RhB did not decrease significantly after adding IPA [hydroxyl radical (·OH) scavenger] during the reaction process, indicating that ·OH is not an active substance in the degradation reaction process. However, the degradation rate of RhB was significantly reduced after the addition of BQ [superoxide radicals (·O₂^-) scavenger] during the degradation process. It was shown that ·O₂^- was the main active substance in the degradation reaction. Furthermore, the addition of EDTA-2Na, which acts as a scavenger for h⁺, led to a reduction in the degradation rate of RhB, indicating that h⁺ plays a role in the RhB degradation process^[29-30]. To further investigate the influence of ·O₂^-, an EPR experiment was performed under xenon lamp irradiation. As shown in Fig. 8b, no signal was detected in the absence of light, but after 5 min of exposure to visible light, a clear peak corresponding to the ·O₂^- signal was observed. This confirms that the sample had a strong response to visible light and sufficient oxidizing ability to convert O₂ to ·O₂^{- [31]}. The formation of the MoS₂/Ag/g-C₃N₄ system by the Z-scheme mechanism was further confirmed by UPS. The UPS spectra of g-C₃N₄ and MoS₂ are shown in Fig. 8c and 8d. The work functions (W_F) of g-C₃N₄ and MoS₂ were calculated from Fig. 8c and 8d to be 4.95 and 5.44 eV, respectively, indicating that MoS₂/Ag/g-C₃N₄ should be the reduced and oxidized photocatalysts, respectively. The transfer of photogenerated electrons at the interface between semiconductors depends on their figure of merit; therefore, once g-C₃N₄ and MoS₂ are in close contact, the photogenerated electrons in MoS₂ will transfer to g-C₃N₄ through the heterojunction until their Fermi energy levels reach an equilibrium state^[32-33]. As a result of the work function differences, the surface of g-C₃N₄ will accumulate a positive charge, whereas the MoS₂ surface will retain a negative charge. This leads to the formation of an internal electric field at the interface between g-C₃N₄ and MoS₂, which aids in the efficient separation of photogenerated charges. On the contrary, due to the energy band bending and internal electric field, the photogenerated electrons cannot be transferred in a way that follows the conventional type Ⅱ pathway, which is thermodynamically unfavorable for sustained charge transfer^[34]. Thus, under visible light irradiation, the photogenerated electrons on the CB of MoS₂ are transferred to the VB of g-C₃N₄ via silver nanoparticles due to the formation of an electric field and combined with the remaining holes^[35]. Based on the above analysis, the photocatalytic mechanism is illustrated in Fig. 9. In this process, silver acts as a bridge for electron transport, facilitating the rapid recombination of holes in the VB of g-C₃N₄ with electrons in the CB of MoS₂. This recombination leads to the generation of a significant number of electrons. As a result, a large number of electrons accumulate in the CB of g-C₃N₄, while holes gather in the VB of MoS₂. This separation of charge carriers enhances photocatalytic efficiency by promoting the generation of reactive species that degrade pollutants^[36]. The electrons in the g-C₃N₄ CB have sufficient energy to convert O₂ to ·O₂^- [-0.33 V (vs NHE)]. Then, ·O₂^- can directly oxidize organic pollutants. Meanwhile, h⁺ in the VB of MoS₂ directly oxidizes organic pollutants^[37].

  Figure 8

  

  Figure 8.  (a) Photocatalytic radical trapping degradation curves of MoS₂/Ag/g-C₃N₄; (b) DMPO-·O₂^- EPR detection singles under dark and visible light irradiation; UPS spectra of (c) g-C₃N₄ and (d) MoS₂

  下载: 全尺寸图片 幻灯片

  Figure 9

  

  Figure 9.  Mechanism of photocatalytic degradation of RhB by MoS₂/Ag/g-C₃N₄

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  The paper presents the preparation of the photocatalyst MoS₂/Ag/g-C₃N₄ Z-scheme heterojunction using a simple hydrothermal synthesis method. The photocatalytic effect of MoS₂/Ag/g-C₃N₄ catalysts on the degradation of RhB was evaluated, and the results showed a degradation rate of 98% within 120 min. The MoS₂/Ag/g-C₃N₄ photocatalyst exhibited excellent catalytic performance. The material′s efficient photocatalytic performance is attributed to its high separation efficiency of photogenerated carriers and effective Z-scheme electron transfer mechanism, as well as its excellent photo-oxidation-reduction ability. This study presents an effective strategy for developing applied photocatalysts with high efficiency, narrow band gap, and excellent cyclic degradation stability at low cost.

  
  
• 1. [1]

     ZHAO B B, ZHONG W, CHEN F, WANG P, BIE C B, YU H G. High-crystalline g-C₃N₄ photocatalysts: Synthesis, structure modulation, and H₂-evolution application[J]. Chin. J. Catal., 2023, 52:  127-143. doi: 10.1016/S1872-2067(23)64491-2

  2. [2]

     NGUYEN T K A, PHAM T T, GENDENSUREN B, OH E S, SHIN E W. Defect engineering of water-dispersible g-C₃N₄ photocatalysts by chemical oxidative etching of bulk g-C₃N₄ prepared in different calcination atmospheres[J]. J. Mater. Sci. Technol., 2022, 103:  232-243. doi: 10.1016/j.jmst.2021.07.013

  3. [3]

     MISHRA A, MEHTA A, BASU S, SHETTI N P, REDDY K R, AMINABHAVI T M. Graphitic carbon nitride (g-C₃N₄)-based metal-free photocatalysts for water splitting: A review[J]. Carbon, 2019, 149:  693-721. doi: 10.1016/j.carbon.2019.04.104

  4. [4]

     NIUY Q, HU F G, XU H L, ZHANG S Z, SONG B, WANG H L, LI M L, SHAO G, WANG H L, LU H X. Exploration for high performance g-C₃N₄ photocatalyst from different precursors[J]. Mater. Today Commun., 2023, 34:  105040. doi: 10.1016/j.mtcomm.2022.105040

  5. [5]

     LIU S Z, WANG S B, JIANG Y, ZHAO Z Q, JIANG G Y, SUN Z Y. Synthesis of Fe₂O₃ loaded porous g-C₃N₄ photocatalyst for photocatalytic reduction of dinitrogen to ammonia[J]. Chem. Eng. J., 2019, 373:  572-579. doi: 10.1016/j.cej.2019.05.021

  6. [6]

     KONG L R, WANG J G, MU X J, RUI LI, LI X X, FAN X X, SONG P, MA F C, SUN M T. Porous size dependent g-C₃N₄ for efficient photocatalysts: Regulation synthesizes and physical mechanism[J]. Mater. Today Energy, 2019, 13:  11-21. doi: 10.1016/j.mtener.2019.04.011

  7. [7]

     CUI L, LIU S L, WANG F K, LI J Y, SONGY H, SHENGY , ZOU H F. Growth of uniform g-C₃N₄ shells on 1D TiO₂ nanofibers via vapor deposition approach with enhanced visible light photocatalytic activity[J]. J. Alloy. Compd., 2020, 826:  154001. doi: 10.1016/j.jallcom.2020.154001

  8. [8]

     LIU Q W, MENG Y, LIU Q M, XU M, HU Y H, CHEN S K. Synthesis of Ag₃PO₄/Ag/g-C₃N₄ composite for enhanced photocatalytic degradation of methyl orange[J]. Molecules, 2023, 28(16):  6082. doi: 10.3390/molecules28166082

  9. [9]

     WANG Y Y, YANG X J, LOU J H. Enhance ZnO photocatalytic performance via radiation modified g-C₃N₄[J]. Molecules, 2022, 27(23):  8476. doi: 10.3390/molecules27238476

  10. [10]

      FELLIPE M C, CAUE R, EDUARDO B A. Improving g-C₃N₄:WO₃ Z-scheme photocatalytic performance under visible light by multivariate optimization of g-C₃N₄ synthesis[J]. Appl. Surf. Sci., 2021, 537:  147904. doi: 10.1016/j.apsusc.2020.147904

  11. [11]

      CHEN S K, JIANG D C, CAO Y M, ZENG G, CHI H B, MIAO Y X, ZHANG Y F, LI L, HE Y X, KE F, YE S. Doping and heterojunction construction dual-regulation: Magnetically recoverable CoFe_1.9Y_0.1O₄/g-C₃N₄ nanosheets with enhanced visible-light-driven photocatalytic activity[J]. Mater. Lett., 2021, 287:  129275. doi: 10.1016/j.matlet.2020.129275

  12. [12]

      JU L, LIU C, SHI L R, SUN L. The high-speed channel made of metal for interfacial charge transfer in Z-scheme g-C₃N₄/MoS₂ water-splitting photocatalyst[J]. Mater. Res. Express, 2019, 6:  115545. doi: 10.1088/2053-1591/ab509c

  13. [13]

      ZHOU D F, QIU C Q. Study on the effect of Co doping concentration on optical properties of g-C₃N₄[J]. Chem. Phys. Lett., 2019, 728:  70-73. doi: 10.1016/j.cplett.2019.04.060

  14. [14]

      MAGED S, EL-BORADY O M, EL-HOSAINY M, MAGED E K. Efficient photocatalytic reduction of p-nitrophenol under visible light irradiation based on Ag NPs loaded brown 2D g-C₃N₄/g-C₃N₄ QDs nanocomposite[J]. Environ. Sci. Pollut. Res., 2023, 30:  117909-117922. doi: 10.1007/s11356-023-30010-z

  15. [15]

      ZHAI N X, LUO J H, SHU P C, MEI J, LI X P, YAN W X. 1D/2D CoTe₂@MoS₂ composites constructed by CoTe₂ nanorods and MoS₂ nanosheets for efficient electromagnetic wave absorption[J]. Nano Res., 2023, 16:  10698-10706. doi: 10.1007/s12274-023-5777-9

  16. [16]

      LI Z C, YAN X X, TANG Z K, HUO Z Y, LI G L, JIAO L Y, LIU L M, ZHANG M, LUO J, ZHU J. Direct observation of multiple rotational stacking faults coexisting in freestanding bilayer MoS₂[J]. Sci. Rep., 2017, 7:  8323. doi: 10.1038/s41598-017-07615-9

  17. [17]

      TRUONG N X, DIEN N T, TUE N N, KHANH D Q, MIKLÓS N, SHOAIB M, OTTÓ H. Effect of ruthenium modification of g-C₃N₄ in the visible-light-driven photocatalytic reduction of Cr(Ⅵ)[J]. Catalysts, 2023, 13(6):  964.

  18. [18]

      YU M, NIKOLAY K, LENNART V, PATRICIA J K, EMIEL J M H. Investigation of the active phase in K-promoted MoS₂ catalysts for methanethiol synthesis[J]. ACS Catal., 2020, 10(3):  1838-1846.

  19. [19]

      SENTHILNATHAN S, GANESH K K, SUGUNRAJ S, AMMAL D M, RAJENDRAPRASAD M, SUGANTHI M, SHREE K K, SASIKUMAR P, MOHAMED A, WIREEN L T D. MoS₂ modified g-C₃N₄ composite: A potential candidate for photocatalytic applications[J]. J. Saudi Chem. Soc., 2023, 27(5):  101717.

  20. [20]

      MOHAMMED A B, KHALED A, GUBRAN A, SUJAY SHEKAR G C, BASHEER M A, MAHMOOD M S A, AMAR A K, LOKANATH N K, NEPPOLIAN B, BHOJYA NAIK H S. Tailoring morphology and structure of 1D/2D isotype g-C₃N₄ for sonophotocatalytic hydrogen evaluation[J]. Surf. Interfaces, 2023, 42:  103511.

  21. [21]

      XIE H Y, WANG K, LI S L, JIN Z L. Construction of Co₉S₈/MoS₂/Ni₂P double S-scheme heterojunction for enhanced photocatalytic hydrogen evolution[J]. Surf. Interfaces, 2023, 42:  103353.

  22. [22]

      LI X B, XIONG J, GAO X M, MA J, CHEN Z, KANG B B, LIU J Y, LI H, FENG Z J, HUANG J T. Novel BP/BiOBr S-scheme nano-heterojunction for enhanced visible-light photocatalytic tetracycline removal and oxygen evolution activity[J]. J. Hazard. Mater., 2020, 387:  121690.

  23. [23]

      YAO Y, SHEN Q H, CHEN Q F, HE Y Y, JIANG L B, LIU J, LI Y, CAO X D, YANG H. Ag/AgX (X=Cl, Br, or I) nanocomposite loaded on Ag₃PO₄ tetrapods as a photocatalyst for the degradation of contaminants[J]. ACS Appl. Nano Mater., 2024, 7:  3711-3723.

  24. [24]

      YU J S, HWANG J J, LEE J Y, HA D H, KIM J H. Tuning photoluminescence spectra of MoS₂ with liquid crystals[J]. Nanoscale, 2021, 13:  16641-16648.

  25. [25]

      ADITYA S, MAYORA V, KEUN H C, SUNG O W. Mechanistic investigations on emission characteristics from g-C₃N₄, g-C₃N₄@Pt and g-C₃N₄@Ag nanostructures using X-ray absorption spectroscopy[J]. Curr. Appl. Phys., 2018, 18(11):  1458-1464.

  26. [26]

      QI H J, WANG C Y, SHEN L P, WANG H M, LIAN Y, ZHANG H X, MA H X, ZHANG Y, ZHANG J Z. α-NiS/g-C₃N₄ nanocomposites for photocatalytic hydrogen evolution and degradation of tetracycline hydrochloride[J]. Catalysts, 2023, 13:  983.

  27. [27]

      DU H, GAO X H, MA Q X, YANG X J, ZHAO T S. Cu/PCN metal-semiconductor heterojunction by thermal reduction for photoreaction of CO₂-aerated H₂O to CH₃OH and C₂H₅OH[J]. ACS Omega, 2022, 7(19):  16817-16826.

  28. [28]

      SHAO G L, LU Y H, HONG J H, XUE X X, HUANG J Q, XU Z Y, LU X C, JIN Y Y, LIU X, LI H M, HU S, KAZU S, HAN Z, JIANG Y, LI S S, FENG Y X, PAN A L, LIN Y C, CAO Y, LIU S. Seamlessly splicing metallic Sn_xMo_1-xS₂ at MoS₂ edge for enhanced photoelectrocatalytic performance in microreactor[J]. Adv. Sci., 2020, 7(24):  2002172.

  29. [29]

      LI X B, KANG B B, DONG F, ZHANG Z Q, LUO X D, HAN L, HUANG J T, FENG Z J, CHEN Z, XU J L, PENG B L, WANG Z L. Enhanced photocatalytic degradation and H₂/H₂O₂ production performance of S-pCN/WO_2.72 S-scheme heterojunction with appropriate surface oxygen vacancies[J]. Nano Energy, 2021, 81:  105671.

  30. [30]

      DU X Y, SONG S, WANG Y T, JIN W F, DING T, TIAN Y, LI X G. Facile one-pot synthesis of defect-engineered step-scheme WO₃/g-C₃N₄ heterojunctions for efficient photocatalytic hydrogen production[J]. Catal. Sci. Technol., 2021, 11:  2734-2744.

  31. [31]

      ZHANG H J, DENG L, CHEN H, ZHANG Y T, LIU Y H, CHEN Y F, ZENG H X, SHI Z. How MoS₂ assisted sulfur vacancies featured Cu₂S in hollow Cu₂S@MoS₂ nanoboxes to activate H₂O₂ for efficient sulfadiazine degradation[J]. Chem. Eng. J., 2022, 446:  137364.

  32. [32]

      SWEETY S, SEKHAR C R. Trigonal (1T) and hexagonal (2H) mixed phases MoS₂ thin films[J]. Appl. Surf. Sci., 2019, 474:  227-231.

  33. [33]

      SHI Y N, CHEN J J, MAO Z Y, BRADLEY D F, WANG D J. Construction of Z-scheme heterostructure with enhanced photocatalytic H₂ evolution for g-C₃N₄ nanosheets via loading porous silicon[J]. J. Catal., 2017, 356:  22-31.

  34. [34]

      XIONG W, DONG Y H, PAN A H. Fabricating a type Ⅱ heterojunction by growing lead-free perovskite Cs₂AgBiBr₆ in situ on graphite-like g-C₃N₄ nanosheets for enhanced photocatalytic CO₂ reduction[J]. Nanoscale, 2023, 15:  15619-15625.

  35. [35]

      LUO X L, YUAN J H, LIU J H, HU H K, YANG Z Y, HOU X P, SUN Q, XU D. Construction of type Ⅱ ZnBi₂O₄/g-C₃N₄ heterojunction photocatalysts for efficient degradation of acid red B[J]. New J. Chem., 2023, 47:  21944-21959.

  36. [36]

      KEBENA G M, WU C, SZUTSEN L. Novel Ag₃PO₄@ZIF-8 p-n heterojunction for effective photodegradation of organic pollutants[J]. J. Water Process Eng., 2023, 52:  103586.

  37. [37]

      HASEEB U, ZAHID H, ABRAR A, IAN S B, REBWAR N D, ZIAUR R. MoS₂ and CdS photocatalysts for water decontamination: A review[J]. Inorg. Chem. Commun., 2023, 153:  110775.

• 

  Figure 1  XRD patterns of the samples

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) SEM, (b) TEM, and (c) HRTEM images of MoS₂/Ag/g-C₃N₄

  Inset: Fast Fourier transform phase profiles of MoS₂ (002) and Ag (111) crytal planes in the d-space of the MoS₂/Ag/g-C₃N₄ composites.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) Survey, (b) C1s, (c) N1s, (d) S2p, (e) Mo3d, and (f) Ag3d XPS spectra of the samples

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) UV-Vis absorption spectra of the samples; VB-XPS spectra of (b) g-C₃N₄ and (c) MoS₂;Mott-Schottky curves of (d) g-C₃N₄ and (e) MoS₂; (f) Band energy levels of g-C₃N₄ and MoS₂

  Inset: corresponding enlarged spectra.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) PL spectra, (b) TRPL spectra, and (c) EIS of the samples

  下载: 全尺寸图片 幻灯片
  

  Figure 6  (a) Visible photocatalytic degradation performances of RhB from the samples and (b) corresponding first-order kinetic curves; (c) TOC changes during the photodegradation process for RhB

  下载: 全尺寸图片 幻灯片
  

  Figure 7  (a) Cyclic degradation curves and (b) degradation rates of MoS₂/Ag/g-C₃N₄ for RhB; (c) XRD patterns of MoS₂/Ag/g-C₃N₄ before and after cyclic degradation experiments

  下载: 全尺寸图片 幻灯片
  

  Figure 8  (a) Photocatalytic radical trapping degradation curves of MoS₂/Ag/g-C₃N₄; (b) DMPO-·O₂^- EPR detection singles under dark and visible light irradiation; UPS spectra of (c) g-C₃N₄ and (d) MoS₂

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Mechanism of photocatalytic degradation of RhB by MoS₂/Ag/g-C₃N₄

  下载: 全尺寸图片 幻灯片
•