Used oxygen-doped C₃N₄ to enhance the photocatalysis performance of O-C₃N₄/Ag@AgCl composite photocatalyst

Abstract

In this study, an oxygen doped-C₃N₄/Ag@AgCl (O-C₃N₄/Ag@AgCl) composite was synthesized by deposition–precipitation method and subsequent photoassisted reduction treatment. The photocatalytic degradation performances of both Rhodamine B (RhB) and phenol were investigated, and the results indicated that the photocatalytic performance of the O-C₃N₄/Ag@AgCl composite exhibited significant enhancement as compared with that of the graphite-like C₃N₄/Ag@AgCl composite. The enhancement in the photocatalytic performance of O-C₃N₄/Ag@AgCl was attributed to the following reasons: on one hand, the oxidizing groups induced by the doping of oxygen on the C₃N₄ surface, were easily combined with the Ag⁺ ions, and subsequently, the interfacial chemical bonds were formed between them, these bonds transforming into channels that would enhance the transfer capacity of the photogenerated electrons between O-C₃N₄ and Ag@AgCl, which improved the electron–hole separation efficiency; on the other hand, the specific surface area of O-C₃N₄ was enlarged during the oxygen doping process, which increased the effective contact area between O-C₃N₄ and Ag@AgCl, thereby further improving the separation efficiency of the photogenerated electrons and holes.

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1. Introduction

Nowadays, energy shortage and environmental pollution have become two main issues of concern. For solving these issues, researchers have been continuously investigating clean, high-efficient new energies, such as solar and wind energy, for reducing the use of traditional energies or even replacing them. Solar energy has attracted increasing attention because it is clean, environment-friendly, and inexhaustible.^1–3 Photocatalytic technology is considered as an effective approach to cope with the aforementioned issues,⁴ and it can be utilized by the continuous conversion of solar photons to chemical energy. During several decades of struggle, photocatalytic technology has developed considerably;^5,6 however, for increasing the potential possibility in industrial application, some issues of the photocatalyts such as low photo quantum yield and narrow photons absorption range, need to be solved.^7,8

Recently, Ag@AgCl has emerged as a new photocatalyst. Currently, the type method to synthesize Ag@AgCl was as follows: AgCl particles are first prepared by ion exchange, followed by the conversion of Ag⁺ to Ag particles by white light illumination, which in turn leads to the covering of Ag particles on the AgCl surface.^9,10 For this photocatalyst, Ag particles exhibit a strong surface plasmon resonance (SPR) effect, which enables them to absorb visible light with high efficiency; hence, it exhibits good performance in the field of photocatalytic organic degradation.^11–13 Ma et al.¹⁴ have prepared quasi-shell–core composite Ag/AgCl and studied its photocatalytic mechanism for organic dyes; the results indicated that photocatalysts can generate electrons and holes by photoexcitation, and the photoinduced holes, serving as strong oxidants, can react with water to generate ·OH free radicals, followed by dye degradation after a series of oxidation. Simultaneously, the photogenerated electrons can also react with O₂ to form O₂· active species, and these active oxygen radicals are also involved in the reaction.¹⁵ For further improving the photocatalytic property of Ag@AgX, researchers have attempted to prepare composites of Ag@AgX with other materials for forming a heterojunction interface system, thereby improving the separation efficiency of photoinduced electrons and holes in the photocatalyst.^16–18 When Ag@AgX comes in contact with other materials, because of their different Fermi level, the interface exchange of electrons and holes occurs, and an interfacial electric field is formed; the existence of this electric field can significantly increase the separation efficiency of photogenerated electrons and holes, which in turn can improve the photocatalytic performance.¹⁹ Kang et al.²⁰ have covered g-C₃N₄ on the surface of spherical Ag/AgCl and have reported a significant improvement in the photocatalytic property toward methyl orange. Li et al.²¹ has synthesized a ZnO/Ag@AgCl composite and have demonstrated that an interface heterojunction was formed between ZnO and Ag@AgCl, and the composite exhibited good performance for the degradation of organic dyes under visible-light.

Recently, graphite-like carbon nitride (g-C₃N₄) has become the focus of research in the field of photocatalysis because of its visiblelight response, high cost performance, and high stability;^22–24 the most important characteristic of g-C₃N₄ is its special structure and appropriate bandgap (2.7 eV).^25,26 As compared to layer-structured graphite, g-C₃N₄ exhibits relatively weak forces between layers, leading to a significantly easier ultrasonic dispersion in an organic medium or even in pure water. The large surface area of g-C₃N₄ can increase the effective contact area when combined with other materials, thereby improving the photocatalytic property. However, further studies have observed that the less photocatalytic reaction active sites, lower separation efficiency,²⁷ and migration rates of the photogenerated electrons, as well as the more negative valence band potential, restrict the further promotion of the photocatalytic performance of g-C₃N₄.²⁸ To solve these issues, researchers have conducted a large number of theoretical and experimental studies. Zhao et al.²⁹ prepared 2D g-C₃N₄ with excellent performance by alerting the precursors mass. Sun et al.³⁰ prepared Ag/g-C₃N₄ nanoparticles with enhanced photocatalytic performance for NOx removal. Dong et al.³¹ fabricated a novel semimetal-organic Bi spheres-g-C₃N₄ nanohybrid photocatalyst that consisted of g-C₃N₄ sheets and well-coupled semimetal Bi nanospheres, which exhibited highly enhanced visible light photocatalytic activity and stability for NO purification. Besides, the simple combination of g-C₃N₄ with other inorganic conductor materials is effective for the improvement of its photocatalytic property. If a bond can be formed between the C₃N₄ layer and inorganic semiconductor nanoparticles, the transfer ability of the photoinduced electrons between them can be further enhanced, leading to a further increase in the photocatalytic performance of the composite materials. Recently, Li et al.³² have subjected g-C₃N₄ to hydrothermal treatment using hydrogen peroxide and have observed that oxygen atoms are doped into g-C₃N₄ molecules, leading to the formation of residual C–O and O–C–N bonds on the g-C₃N₄ surface, and oxidation results in the increase of its surface area, expansion of its response wavelength to visible light, and improvement in the separation efficiency of photoinduced electrons and holes. Hence, we first treated g-C₃N₄ with hydrogen peroxide to form O-C₃N₄ and then synthesized Ag@AgCl on the surface of O-C₃N₄. On the one hand, a heterojunction electric field can be formed between the interface of the O-C₃N₄ and Ag@AgCl heterojunction. On the other hand, the C–O and O–C–N–O residual bonds on the surface of O-C₃N₄ can bond with Ag⁺, resulting in the formation of an effective electron transfer channel between C₃N₄ and the inorganic semiconductor materials, thereby enhancing the separation efficiency of the photoinduced electrons and holes, as well as the photocatalytic property.

2. Results and discussion

Fig. 1 shows the XRD patterns of the prepared materials. Curves 1a and 1b show the XRD patterns of g-C₃N₄ and O-C₃N₄ materials, respectively. A high-intensity diffraction peak was observed at 2θ = 27.4°(002), which is the characteristic index for the interlayer stacking of carbon nitride. By the comparison of the two peaks, the peak intensity of curve 1b was clearly weaker than that of curve 1a, possibly attributed to the fact that the layered structure of carbon nitride material is partially destroyed after oxidation. Curve 1c and curve 1d show the XRD patterns of g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl materials, respectively. Peaks characteristic of AgCl were easily observed; the peaks were observed at 2θ = 27.82°, 32.24°, 46.25°, 54.81°, and 57.56° (those marked with “□”), which can be attributed to the (111), (200), (220), (311), and (222) crystal planes of AgCl (JCPDS cards no. 31-1238), respectively.

Fig. 1 XRD patterns of (a) g-C₃N₄; (b) O-C₃N₄; (c) g-C₃N₄/Ag@AgCl; (d) O-C₃N₄/Ag@AgCl.

Diffraction peaks characteristic of Ag at 2θ = 38.1° and 44.3° (those marked with “○”), which are attributed to the (111) and (200) crystal planes of Ag (JCPDS cards no. 04-0783), were not observed. The absence of these peaks could be attributed to the low yield or low crystallinity of Ag formed on AgCl. Notably, carbon nitride exhibited no characteristic peak in both curves 1c and 1d; this result indicates that Ag@AgCl particles on the surface of the composite disturb the diffraction peaks of carbon nitride; the results obtained from these diffraction curves were in agreement with those reported in the studies by Bu¹⁹ and Kang²⁰ et al.

To ascertain the elemental composition and distribution of the O-C₃N₄/Ag@AgCl composite, we analyzed the element composition by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) mapping, and the results are shown in Fig. 2. From Fig. 2A and B, the composition of the material was consisted of C, N, O, Cl, and Ag, with no impurity elements. The specific state in which silver element exists was further confirmed by X-ray photoelectron spectroscopy (XPS).

Fig. 2 (A) EDS spectrum and (B) elemental composition of O-C₃N₄/Ag@AgCl.

Fig. 3 shows the results obtained from the XPS analysis of the surface valence state of the O-C₃N₄/Ag@AgCl. Fig. 3A shows the survey scan spectrum of O-C₃N₄/Ag@AgCl; elements C, N, O, Cl, and Ag were observed, with no impurity elements. Fig. 3B shows the XPS core-level spectra of Ag 3d: binding energy peaks were observed at 367.6 eV and 373.6 eV, attributed to the electron orbits of Ag 3d_5/2 and Ag 3d_3/2, respectively. This result is in agreement with the theoretical peak of Ag 3d orbits, confirming the existence of Ag⁺ from AgCl. Meanwhile, binding energy peaks were observed at 368.2 eV and 374.2 eV, attributed to Ag⁰, confirming the existence of elemental silver.^33,34 Fig. 3C shows the XPS spectrum of Cl 2p; binding energy peaks were observed at 198.1 eV and 199.6 eV, attributed to the characteristic peaks of Cl 2p_5/2 and Cl 2p_3/2, respectively. This result demonstrates that Cl exists as Cl⁻. Fig. 3D shows the XPS spectra of N 1s; binding energy peak were observed at 398.8 eV and 400.2 eV, attributed to sp²-hybridized C=N–C and N–O, respectively. Fig. 3E shows the XPS core-level spectra of O 1s; two characteristic core-level peaks were observed at 532.2 eV and 531.2 eV, attributed to the adsorbed water molecules and sp²-hybridized C=O, respectively. According to the results, after treatment with hydrogen peroxide, g-C₃N₄ was oxidized to O-C₃N₄, and results in the formation of chemical bonds between O and N, O and C to form N–O and C=O, respectively; it also demonstrated that the O-C₃N₄/Ag@AgCl composite was successfully synthesized.²⁸

Fig. 3 XPS spectra of O-C₃N₄/Ag@AgCl (A) survey scan spectrum; (B) Ag 3d; (C) Cl 2p; (D) N 1s; (E) O 1s.

Fig. 4 shows the SEM images of the prepared materials. Fig. 4A shows the micro morphology of pure g-C₃N₄ material, the g-C₃N₄ sample exhibited a typical stacked lamellar structure. Fig. 4B shows the surface morphology of O-C₃N₄ after treatment with H₂O₂. By comparison of Fig. 4B and A, graphite-like g-C₃N₄ was corroded and appeared as irregular porous structures by H₂O₂ treatment. Fig. 4C shows the SEM morphology of the g-C₃N₄/Ag@AgCl composite; several Ag@AgCl nanoparticles were observed on the surface of g-C₃N₄, with a size of approximately 20–80 nm, and slight agglomeration was observed. Fig. 4D shows the morphology of O-C₃N₄/Ag@AgCl composite: Ag@AgCl nanoparticles were densely distributed on the surface of O-C₃N₄, with an average size of approximately 50 nm. To further characterize the loading state of Ag@AgCl on the surface of O-C₃N₄, we conducted field-emission high-resolution transmission electron microscopy (FE-HRTEM) to observe.

Fig. 4 Surface morphology of (A) g-C₃N₄; (B) O-C₃N₄; (C) g-C₃N₄/Ag@AgCl; (D) O-C₃N₄/Ag@AgCl.

To characterize the distribution of Ag@AgCl on the surface of O-C₃N₄, we further observed O-C₃N₄/Ag@AgCl using an FE-HRTEM instrument, and Fig. 5 shows the results. Fig. 5A present the low resolution image of the O-C₃N₄/Ag@AgCl composite. From this image we could find that several Ag@AgCl particles were deposited on the surface of sheet-like O-C₃N₄, with a size of approximately 50 nm. On the other hand, as shown in the high-resolution image in Fig. 5B, spherical AgCl particles were observed, with a size of approximately 25 nm. The black shadow in Fig. 5B is possibly attributed to the Ag nanoparticles grown on the AgCl surface, with a size of approximately 4 nm.

Fig. 5 HRTEM morphology of O-C₃N₄/Ag@AgCl (A) low resolution; (B) high resolution.

Fig. 6 shows the results of the specific surface area of four prepared materials and their N₂ adsorption–desorption isotherms. According to the series curve shape, a type IV isotherm was observed, indicating the presence of porous structure material.³⁵ For the series of materials, at relative low pressure (0 < p/p⁰ < 0.45), the adsorption amount increased slowly, and N₂ molecules were adsorbed in the inner surface of the mesopores as a monolayer or multilayers. Moreover, near a p/p⁰ of 0.4–0.9, the adsorption amount suddenly increased, and the adsorption at p/p⁰ of 0.9–1 was possibly related to the formation of macropores by the secondary aggregates of the particles. The change in the surface area will result in the change in the pore size of the porous structure, which can affect the pore volume. The SBET of g-C₃N₄ is 11.6 m² g⁻¹. However, after oxidation, the specific surface area was significantly increased, and the SBET of O-C₃N₄ increased to 50.8 m² g⁻¹, suggesting that a large number of porous structures form on the surface of O-C₃N₄, thus enhancing the adsorption capacity. After forming a composite with Ag@AgCl, the surface area of both g-C₃N₄ and O-C₃N₄ significantly decreased: the SBET of g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl was 8.3 m² g⁻¹ and 26.4 m² g⁻¹, respectively. Dense growth of Ag@AgCl particles was observed on the surface of carbon nitride, and some particles could fill inside the pores; hence, the specific surface area of the g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl decrease.

Fig. 6 Nitrogen adsorption–desorption isotherms of (a) g-C₃N₄; (b) O-C₃N₄; (c) g-C₃N₄/Ag@AgCl; (d) O-C₃N₄/Ag@AgCl.

Fig. 7 shows the UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS) curve of each prepared material for investigating the optical absorption properties of the series of photocatalytic materials. Curve 7a and 7b represent the DRS spectrum of g-C₃N₄ and O-C₃N₄, respectively. The optical light absorption threshold of g-C₃N₄ was clearly 460 nm, attributed to a band gap of 2.7 eV. Both g-C₃N₄ and O-C₃N₄ exhibited a strong light absorption ability below a wavelength of 460 nm; however, above 460 nm, g-C₃N₄ exhibited a sharp decline in its light absorption capacity, while O-C₃N₄ exhibited a slower decline, and its light absorption range extended to approximately 600 nm, which resulted in a change of colour from light yellow of g-C₃N₄ to orange of O-C₃N₄.

Fig. 7 UV-Visible diffuse reflectance spectrum of (a) g-C₃N₄; (b) O-C₃N₄; (c) g-C₃N₄/Ag@AgCl; (d) O-C₃N₄/Ag@AgCl.

The above changes can be attributed to the width shortening of the energy band gap, which is caused by the new doping level formed below the CB by the introduction of the O element into C₃N₄.³⁶ Curve 7c and 7d represent the DRS spectra of the g-C₃N₄/Ag@AgCl composite and O-C₃N₄/Ag@AgCl composite, respectively. Interestingly, pure g-C₃N₄ and O-C₃N₄ almost did not respond optically in the visible light region of 400–750 nm, while O-C₃N₄/Ag@AgCl and g-C₃N₄/Ag@AgCl photocatalysts exhibited broad and strong absorption, which is attributed to the surface plasmon resonance (SPR) of Ag NPs originating from the photodegradation of AgCl.^19–21,33 The above results indicate that after Ag@AgCl combines with O-C₃N₄, the SPR effect of Ag@AgCl will broaden the range of material absorbing visible light, which would enhance the photocatalytic properties of O-C₃N₄/Ag@AgCl.

To characterize the photocatalytic degradation ability of O-C₃N₄/Ag@AgCl composite materials under visible light, we tested and compared the degradation toward Rhodamine B (RhB) and phenol using these materials under visible light. Fig. 8A shows the degradation curve of the RhB solution. From curve 8(A)a, pure g-C₃N₄ absorbed nearly 12% of the RhB dye in 30 min, and under visible light illumination for 50 min, the degradation rate was approximately 70%. Curve 8(A)c shows the degradation curve of the g-C₃N₄/Ag@AgCl composite. As compared with curve 8(A)a, the adsorption capacity of the composite was almost the same as that of g-C₃N₄, and after visible light illumination, its degradation speed was fast at the beginning. However, it was decreased with time going. Curve 8(A)b and 8(A)d represent the degradation of O-C₃N₄ and the O-C₃N₄/Ag@ AgCl composite, respectively. By comparison of curves 8(A)b and 8(A)d with those of 8(A)a and 8(A)c, after oxygen doped, the adsorption capacity of O-C₃N₄ clearly increased, and pure O-C₃N₄ completely finished degradation in 35 min, while the O-C₃N₄/Ag@AgCl composite finished degradation in only 15 min. Possible reasons as follows: the lamellar structure of g-C₃N₄ is destroyed after oxidation, and several holes are formed on the surface of carbon nitride, thus the specific surface area is increased, hence, the adsorption capacity increases; on the other hand, Ag⁺ on the surface of Ag@AgCl can combine with O on the surface of O-C₃N₄, thereby forming an interface bridge. This bridge evolve into an effective channel for the transfer of photogenerated electrons; thus, the separation efficiency of electrons and holes is increased, as well as the photocatalytic degradation property.

Fig. 8 Photocatalysis property of the prepared materials for (A) 10 mg L⁻¹ RhB solution; (B) 10 mg L⁻¹ phenol solution; (C) photocatalytic degradation stability test of O-C₃N₄/Ag@AgCl composite for RhB.

As RhB exhibits a sensitization effect on the photocatalytic material, which results in interference to the performance evaluation of photocatalysis properties of these materials. We further tested the photocatalytic degradation of g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl for a phenol solution. Fig. 8B shows corresponding results. After 30 min of dark-state adsorption, no significant reduction in phenol concentration was observed for both photocatalysts. When O-C₃N₄/Ag@AgCl served as the photocatalyst, the phenol was degraded completely in 20 min. While with the use of g-C₃N₄/Ag@AgCl, the degradation rate was 75% in 50 min. This result further confirmed the speculation mentioned above: the large surface area of O-C₃N₄ created complete contact between O-C₃N₄ and Ag@AgCl and increased the effective area at the interface between the material. Furthermore, Ag⁺ can combine with O which on the C=O and N–O bonds, to form an interface bridge, increasing the transfer capacity of the photogenerated electrons between C₃N₄ and Ag@AgCl, effectively inhibiting the recombination of photogenerated electrons and holes. Hence, the photocatalytic property is improved.

In order to investigate the stability of the O-C₃N₄/Ag@AgCl composite in the process of RhB degradation, five successive cyclic RhB degradation tests were performed and the relevant results are shown in Fig. 8C. From the results shown in Fig. 8C, the photocatalytic RhB degradation efficiency of the composite material does not show noticeable decline after five successive cycle tests and it can still degradate 93% RhB after 45 min of illumination by visible light at the 5^th degradation cycle, demonstrating that O-C₃N₄/Ag@AgCl composite possesses very high photocatalytic degradation stability.

To investigate the functions of photoinduced electrons or holes of the O-C₃N₄/Ag@AgCl composite in the photocatalysis degradation process, we conducted experiments by addition of photocatalytic inhibitors into the RhB solution, and the results are show in Fig. 9. Curve 9a shows the degradation of pure-phase O-C₃N₄/Ag@AgCl composite materials for RhB, and curve 9b shows the RhB photocatalysis degradation process after the addition of BuOH, which is an inhibitor for the photogenerated electrons, in the reaction system. The almost overlapped curves of 9a and 9b indicated that the photogenerated electrons by O-C₃N₄/Ag@AgCl haven't any contribution in RhB degradation process. Curve 9c shows the degradation curve after the addition of photogenerated hole inhibitor EDTA-Na₂ in the degradation system. By comparison of curve 9c with curve 9a, irrespective of the dark-state adsorption or the illumination process, the addition of a hole inhibitor clearly decreased the photocatalytic properties of O-C₃N₄/Ag@AgCl composite materials significantly, indicating the important role played by the photogenerated holes in the entire process of photocatalytic degradation. This result demonstrates that Ag⁺ can rapidly transfer the photogenerated electrons by combining with the C=O and N–O bonds, and the remaining holes can participate in a series of redox reactions, finally completing the degradation of RhB, hence, the photocatalytic activity of the composite is enhanced.

Fig. 9 Photocatalytic mechanism exploration of O-C₃N₄/Ag@AgCl composite.

The separation and recombination rate of the photogenerated electrons and holes is directly related to the photocatalytic properties of the photocatalyst: the higher separation efficiency, the better photocatalytic property. The fluorescence emission intensity is a measure to assess the recombination rate of the photogenerated electrons and holes. Fig. 10 shows the microscopic fluorescence spectra of the four prepared materials; curves 10a–10d represent g-C₃N₄, O-C₃N₄, g-C₃N₄/Ag@AgCl, and O-C₃N₄/Ag@AgCl, respectively. From curves 10a to 10d, the fluorescence intensity successively weakened, indicating the successive decrease in the intensity of the photogenerated electron–hole recombination, thereby increase in the separation efficiency. From this result, we can speculate that when O-C₃N₄ forms a composite with Ag@AgCl, Ag⁺ can combine with –C=O and N–O, rapidly transfer the photogenerated electrons on Ag@AgCl to O-C₃N₄, so as to effectively suppress the recombination and enhance the separation efficiency of the photogenerated electrons and holes.

Fig. 10 Microscopic fluorescence spectra of (a) g-C₃N₄; (b) O-C₃N₄; (c) g-C₃N₄/Ag@AgCl; (d) O-C₃N₄/Ag@AgCl.

To further discuss the reason for the improvement of the photocatalytic property of O-C₃N₄/Ag@AgCl, the electro-chemical impedance and photoinduced i–t methods were employed. The electron migration ability within the thin-film electrode and the electrochemical reaction capability on the surface of electrode can be characterized by electrochemical impedance spectrum (EIS): the smaller the radius of the spectrum, the higher the separation efficiency of photogenerated electrons and holes. Fig. 11A shows the results obtained from the tests. The radius of O-C₃N₄/Ag@AgCl was less than that of g-C₃N₄/Ag@AgCl, indicating a higher migration rate for the photogenerated electrons in the former thin film. Under light illumination, the photogenerated electrons in O-C₃N₄ will be rapidly transferred between the two materials through –O–Ag– interface bond bridge, thus enhancing the separation efficiency of the photogenerated electrons and holes. Fig. 11B shows the photoinduced i–t curve of g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl thin-film electrodes. From this results, under visible-light excitation, the photocurrent density of the g-C₃N₄/Ag@AgCl thin-film electrode was relatively low, approximately 1.2 μA cm⁻², while the photocurrent density of the O-C₃N₄/Ag@AgCl thin-film electrode was 5 μA cm⁻², which indicates a significant improvement over the former. This result is attributed to the following two aspects: first, the interface bridge formed between O-C₃N₄ and Ag@AgCl, which accelerated the migration of the photogenerated electrons within the film; second, the larger surface area of O-C₃N₄/Ag@AgCl offered more reactive sites in the electrochemical reaction, thus enhancing the photocurrent density.

Fig. 11 (A) Electrochemical impedance spectrum and (B) photoinduced i–t curve of g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl.

Photocatalytic mechanism of O-C₃N₄/Ag@AgCl photo-catalyst for RhB and phenol degradation was shown in Scheme 1. In this composite system, Ag⁺ on the surface of Ag@AgCl can combine with O on the surface of O-C₃N₄, thereby forming an interface bridge which evolve into an effective channel for the photogenerated electrons transfer. As shown in Scheme 1, when the light irradiates on the surface of O-C₃N₄/Ag@AgCl, as Ag⁰ has strong SPR effect, amount of photogenerated electrons were generated rapidly, and rapidly transferred to O-C₃N₄ through –O–Ag– interface bond bridge, completing reduction reaction with O₂. The photogenerated holes on the Ag@AgCl were involved in the oxidizing reaction with around OH⁻ and generate OH·, and underwent further oxidizing decomposition reaction with around RhB dye and phenol.

Scheme 1 Photocatalytic mechanism illustration of O-C₃N₄/Ag@AgCl composite.

Scheme 2 Schematic illustration of the relevant preparation process of the O-C₃N₄/Ag@AgCl photocatalyst.

3. Experimental section

3.1. Preparation of O-C₃N₄

All chemicals were analytical grade and used without further purification. The first step is preparation of g-C₃N₄ powder by directly heating dicyanodiamide. Simply, put dicyanodiamide in a semiclosed ceramic crucible with a cover, heated at 520 °C in a muffle furnace for 4 h at a heating rate of 10 °C min⁻¹, the light yellow powder collected being g-C₃N₄. Then prepare oxidized carbon nitride material by hydrothermal reaction method. In a typical procedure, 0.6 g of prepared g-C₃N₄ powder was added into 60 mL of 30 wt% hydrogen peroxide solution. After mixing thoroughly by stirring, the mixture was transferred into a 100 mL Teflon hydrothermal reaction kettle and reacted at 150 °C for 5 h. After centrifugation, the centrifugate was washed twice with deionized water and then dried at 60 °C for 4 h. Finally, O-g-C₃N₄ orange powder was obtained. And the preparation process shows in Scheme 2.

3.2. Preparation of O-C₃N₄/Ag@AgCl composite

O-C₃N₄/Ag@AgCl was prepared by deposition-precipitation and light reduction method. Firstly, 0.3 g prepared O-C₃N₄ powder was dispersed in 20 mL deionized water, and the resulting mixture was ultrasonically vibrated for 30 min. Subsequently, silver nitrate ammonia (SNA) solution, which was prepared by dissolving 0.49 g of AgNO₃ into 2.0 mL NH₄OH solution (25 wt% NH₃), was then slowly added drop by drop into the dispersed liquid mixture of O-C₃N₄ and stirred for 10 min. After that, HCl solution, prepared by diluting 0.24 mL concentrated hydrochloric acid into 30 mL deionized water, was slowly added drop by drop into the liquid mixture of O-C₃N₄ and SNA solution under stirring. After 24 h of stirring at room temperature, 10 mL of methanol was added into the liquid mixture and subsequently this liquid mixture was transferred into a beaker with the temperature controlled at 25 °C by a water cooling system and irradiated for 30 min using a 300 W Xenon lamp under vigorous stirring. Some of the AgCl formed on the surface of O-C₃N₄ will reduced to elemental Ag under light illumination. O-C₃N₄/Ag@AgCl composite was obtained by suction filtration and was then rinsed with deionized water and ethanol, and the dark grey powder O-C₃N₄/Ag@AgCl was obtained after 6 h of vacuum drying at 60 °C. g-C₃N₄/Ag@AgCl was prepared by the same method.

3.3. Characterization of the prepared composites

The morphologies and the microstructures of the synthetic composites were analyzed using a field emission scanning electron microscope (FESEM) and a high-resolution transmission electron microscope (HRTEM). The elemental composition of the synthetic composites were analyzed by energy dispersive spectrometer (EDS) and SEM mapping method. The crystalline structures and bonding information of the synthetic composites were analyzed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The optical absorption properties were investigated using a UV/Vis diffuse reflectance spectrophotometer (UV-Vis DRS) and fluorescence microscopic (FM).

3.4. Photocatalysis property measurement

The photocatalysis degradation property for organic dye and phenol of the prepared composites were measured respectively. We chose commercially available RhB for the organic dye degradation measurement, in a typical procedure, 0.1 g photocatalysis was added into 100 mL prepared RhB solution (10 mg L⁻¹), then the mixture was stirred for 30 min in the dark to make sure the photocatalysis fully adsorb the dye. Afterwards, open the light. The light source was a 300 W Xenon lamp, a 420 nm cutoff filter was used to remove light with wavelengths less than 420 nm and ultimately generate visible light (power energy density is 150 mW cm⁻²). The distance between the light source and the dye liquid level is 10 cm. The temperature of the dye liquid was maintained steady using circulating water. All of the photocatalytic dye degradation measurements were performed under visible light. The photocatalytic degradation stability of the O-C₃N₄/Ag@AgCl composite was studied in this work. The photocatalyst was recycled by centrifugation of the solution after dye degradation and the centrifugate was used for the next cycle of RhB degradation after sufficiently drying. Photocatalyst recycling tests were performed for five times in this work.

The photocatalysis degradation for phenol was examinated with the same method, 0.1 g photocatalysis for 100 mL 10 mg L⁻¹ phenol solution, other conditions were also unanimous with RhB degradation measurement, the only difference was that: the dye absorbance was determined by visible spectrophotometer, while the phenol absorbance by UV spectrophotometer.

3.5. Photodegradation mechanism explore of O-C₃N₄/Ag @AgCl composite

To explore whether the photoinduced electrons or the holes play an important role in photodegradation process of O-C₃N₄/Ag@AgCl composite, we conducted experiment by add photocatalytic inhibitors into RhB solution. Simply, photogenerated electrons inhibitors butyl alcohol (BuOH) or photogenerated holes inhibitors disodium ethylenediamine tetraacetate (EDTA–Na₂) was added into the liquid mixture of 50 mL 10 mg L⁻¹ RhB solution with 0.05 g photocatalyst respectively. Control the more concentration of the inhibitors as 2 mmol L⁻¹, measure its photocatalysis degradation property by the same method mentioned above.

Afterwards, to characterize transfer ability and separation efficiency of photogenerated electrons and holes, we prepared thin-film electrodes of the synthesized g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl materials for electrochemical tests. The specific method is as follows: 4 mg photocatalyst powder was put into an agate mortar, then add in minute quantity of Nafion perfluorinated resin, isopropanol and distilled water respectively, grind the mixture to form homogenate suspension, then, 0.025 mL of the as-prepared suspension was evenly distributed onto the exposed area of the conductive side of the FTO glass (the effective area is 1 cm × 1 cm), the insulating tape on the edge of the FTO glass was removed after the suspension had dried in the air. A copper wire was connected to the conductive side of the FTO glass using conductive silver paste, uncoated parts of the conductive side of the FTO glass were isolated with paraffin after the conductive silver paste had dried. Electrochemical tests were carried out on a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A traditional three-electrode system was adopted for the electrochemical tests with Pt having a large area as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. The electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential (OCP) over the frequency range between 10⁵ and 10⁻¹ Hz and in the dark, 0.1 M Na₂SO₄ solution was selected as the electrolyte solution for the experiments. The variations of the photoinduced current density with time (i–t curve) were measured at a 0 V bias potential (vs. SCE) under visible light off and on.

4. Conclusions

In this study, we first prepared g-C₃N₄ powder by high-temperature polycondensation method, subsequently, prepared O-C₃N₄ by hydrothermal method. Next, we synthesized g-C₃N₄/Ag@AgCl and O-C₃N₄/Ag@AgCl by deposition–precipitation and photoassisted reduction. A series of results showed that the oxygen-doped carbon nitride (O-C₃N₄) has a surface area greater than that of g-C₃N₄, and a wider range of light absorption. The photocatalyst of O-C₃N₄/Ag@AgCl composite shows the best photocatalysis degradation performance of RhB and phenol, and the composite shows high stability in the cycle RhB degradation tests. Further tests show that the enhancement properties were primarily attributed to the following reasons: the large surface area of O-C₃N₄ created a complete contact between O-C₃N₄ and Ag@AgCl, thereby increasing the effective area of the interface between the O-C₃N₄ and Ag@AgCl; Furthermore, Ag⁺ can combine with the C=O and N–O bonds on the O-C₃N₄ to form an interface bridge, that the transfer speed of the photogenerated electrons between the two phases increased rapidly, thus inhibiting the recombination of the photogenerated electrons and holes effectively and enhancing their separation efficiency.

Acknowledgements

This work was financially supported by Foundation of Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences (MCKF201410).

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