English

• 1. INTRODUCTION

  With the deterioration of the environment and the shortage of energy all over the world, the semiconductor-based photo-catalysis technology using the pollution-free sunlight has attracted enormous research interest, but effective removal of the organic pollutants from wastewater remains a great global challenge^[1-3]. Photo-catalysis technology is regarded to be efficient and promising because photocatalysts can completely mineralize harmful organic pollutants in the environment into harmless carbon dioxide and water by absorbing the safe and cheap sunlight^[4]. The photo-catalysis technology can also be used to decompose water into hydrogen^[5-8], reducing carbon dioxide to fuel^{[9, 10]} and solar cell^{[11, 12]} which can help mitigate global energy shortage. Though the photo-catalysis technology has been demonstrated as a promising environment treatment technology, the single-component photocatalysts can only utilize small amount of sunlight and the recombination rate of the photon-generated carrier is rapid. The traditional single-component photocatalyst, such as TiO₂ and ZnO, on account of their wide band gap, can only absorb the ultraviolet light that just occupies 5% of the sunlight. Some photocatalysts owning narrow band-gap can be used in the visible and ultraviolet regions, nevertheless, it has poor stability.

  Graphite carbon nitride (g-C₃N₄), a novel metal-free polymeric organic semiconductor, has aroused ever-growing attention of researchers since Wang et al. first reported using g-C₃N₄ as a photocatalyst to decompose water into hydrogen and oxygen under visible light in 2009^[13-16]. g-C₃N₄ featured an extended delocalized π electron system which is a prototypical two-dimensional conjugated polymer^[17]. g-C₃N₄ has a relatively narrow band gap (~2.7 eV) and strong covalent between C–N bonds, which lead to the wonderful photocatalytic activity and unexceptionable photo-corrosion resistance performance^{[18, 19]}. Moreover, the fascinating properties such as non-toxicity, good thermal and chemical stability, high specific surface area and low-cost synthesis make g-C₃N₄ an ideal candidate for visible light responding photocatalyst^[20-23]. In spite of owning many advantages, its relatively low quantum efficiency (quantum efficiency=0.1% at 420~460 nm) under visible light and high recombination rate of photo-generated electron-hole pairs hinder the practical application of g-C₃N₄^{[5, 24]}.

  To improve the photocatalytic performance of g-C₃N₄, many strategies such as hybridization with semiconductor cocatalysts, non-metal/metal doping, noble metal (Pt, Pd, Ag, Au) deposition and the creation of mesoporous g-C₃N₄ have been carried out. Among them, noble metal deposition can not only effectively restrain the recombination of photogenerated electron-hole pairs, but also form a metal-semiconductor heterojunction (Schottky barrier). The potential well on the metal-semiconductor interface can capture electrons, which will efficiently separate the photo-generated carriers and lead to a distinct enhancement of photocatalystic activity of the hybrid catalyst. Ag is a relatively cheap noble metal, attracting extensive attention for its potential applications in catalysis, biosensors, biomedicine and other fields. Ag is one of the most effective cocatalysts because it can collect photo-generated electrons and enhance the visible light absorption on account of the surface plasmon resonance effect^[25]. GO is the oxide of graphene and the property of electron-accepting and electron-transporting is wonderful. Therefore, g-C₃N₄/Ag/GO (CNAG) nanocomposite catalyst is proposed and investigated.

  Herein, we report the preparation of CNAG photocatalyst by a simple two-step method, and the photocatalytic activity of the as-prepared samples was evaluated by estimating the degradation rate of Rhodamine B (RhB) under simulated sunlight irradiation. The toxicity of as-prepared samples was assessed by seed germination experiment. Moreover, the stability of the as-prepared CNAG photocatalyst was investigated by repeated experiments. Finally, the mechanism of the enhanced photocatalytic activity of CNAG nanocomposite catalysts was proposed based on the experimental results.

  2. EXPERIMENTAL

  2.1 Materials and reagents

  Hydrogen peroxide (H₂O₂) was obtained from Laiyang Kantian chemical co. LTD. Sodium nitrate (NaNO₃) was purchased from Tianjin basf chemical co. LTD. Melamine (99%) was available from Aladdin Co. Other materials were obtained from Sinopharm Chemical Reagent Corp, P. R. China. All chemicals in this research were not further purified and deionized water was used throughout the experiment to prepare the solutions.

  2.2 Preparation of the photocatalyst

  Bulk g-C₃N₄ was synthesized by heating melamine in a muffle furnace. In a typical procedure, the melamine was put into a semi-closed ceramic crucible and calcined at 550 ℃ for 4 hours at a ramping rate of 5 ℃/min. The obtained yellow product was cooled to room temperature and grounded into powder by an agate mortar.

  The GO powder was synthesized via a modified Hummers' method by oxidation of graphite powder. Firstly, 1 g graphite powder and 0.5 g sodium nitrate were dispersed into 24 mL sulfuric acid (95%) with vigorous magnetic stirring for 30 min under ice-bath condition. Secondly, 3 g KMnO₄ was slowly added into the mixture solution under vigorous stirring for 6 h to obtain the slurry solution. Thirdly, 30 mL deionized water was added slowly to the slurry solution. Subsequently, the slurry solution was stirred constantly for 24 hours until the solution turned yellow. Then, 3.5 mL H₂O₂ (30%) was added dropwise into the slurry solution. Finally, the obtained GO powders were collected by filtration and washed several times with moderate amounts of HCl (5%), then dried at 80 ℃.

  The g-C₃N₄/Ag (CNA) composite photocatalysts were synthesized by photo-deposition method. Briefly, 0.2 g g-C₃N₄ nanosheets and 40 mg AgNO₃ were added into 150 mL deionized water with ultrasonic treatment for 2 h under dark conditions. Subsequently, the suspension was illuminated under 35W Xenon lamp for different time. The resultant samples were centrifuged, washed several times with deionized water and ethanol, and then dried in an oven at 65 ℃ for 12 h. The CNA-t photocatalysts (t representing the illumination time was 5, 10, 15, 20 min) were prepared.

  A certain mass ratio of as-prepared GO powder and CNA-15 composite were immersed into 150 mL distilled water. After ultrasonic treatment for 2 h, the suspension solution was stirred constantly for 24 h at room temperature. Finally, the CNAG samples were obtained by centrifuging, washing and drying. The as-prepared samples were denoted as CNAG-X, where X (X was 1%, 4%, 6% and 8%) represents the theoretical mass percent of GO to CNA-15 composite.

  2.3 Characterization

  The purity and crystal structure of the as-prepared samples were recorded by X-ray diffractometer (Rigaku D/Max-rb, Japan) with Cu-Kα radiation. The surface functional groups and bonding modes of pure g-C₃N₄, CAN and CNAG were detected and analyzed by Fourier transform infrared (FTIR, IRAffinity-1, Shimadzu) in the range of 400~4000 cm^-1. The surface composition and chemical bonds of CNAG-6% were confirmed via a PHI 5700 ESCA X-ray photoelectron spectrometer (XPS). The UV-Vis light absorption spectra of the photocatalysts were obtained by a UV-2550 spectrophotometer (Shimadzu) equipped with an integrating sphere. Barium sulfate (BaSO₄) was taken as the reference to measure all the samples and the range of the scanning wavelength was 200~800 nm. The photoluminescence (PL, FLS-1000, Edinburgh Instruments) was carried out with an excitation wavelength of 300 nm in order to analyze the energy band structure and light response intensity of the as-prepared samples. The photocurrent response of as-prepared samples under simulated sunlight was measured by electrochemical workstation (PARSTAT 3000A-DX, Princeton Applied Research). The morphologies of the samples were observed by transmission electron microscopy (TEM, JEOL 2100, JEOL Ltd).

  2.4 Evaluation of photocatalytic activity

  The experimental apparatus of degradation organic pollutants was made up of Xenon lamp, reaction solution, quartz reactor and photocatalysts. The Xenon lamp and sunlight had similar spectral region, so the Xenon lamp could be used to simulate sunlight in the experiment. In a typical photocatalytic experiment, 25 mg of as-prepared photocatalyst was dispersed in 50 mL of the RhB solution (5 mg/L). Before exposure to simulated solar irradiation, the suspension liquid was stirred in darkness for 30 min to reach the adsorption and desorption equilibrium.

  During the degradation process, approximately 4 mL of the reaction solution was put into a 5 mL centrifuge tube at a given interval, and then the photocatalyst was removed through being centrifuged at 11,000 rpm for 10 min. The degradation rate can be calculated by the formula:

  $ {X}{=}\frac{{{C}}_{\mathit{0}}{-}{{C}}_{{t}}}{{{C}}_{\mathit{0}}}{}\times {100\%} $

  (1)

  where X means the amount of organic decomposed (%), C₀ refers to the initial concentration of organic (mg/L) prior to irradiation and C_t represents the concentration of organic solution after t min simulated solar irradiation. The optimal mass ratio of Ag and GO was confirmed by evaluating the degradation rate of RhB in the irradiation of simulated sunlight. In order to confirm the reusability of the as-prepared CNAG-6% catalysts, the cyclic tests were implemented under the same experimental conditions. After each cycle, the CNAG-6% photocatalysts were collected by centrifugation and washed with deionized water. After that, the solid was dried at 60 ℃ and reused for the next run.

  2.5 Evaluation of reactive oxygen species

  Since a large number of reactive species may participate in the photocatalytic process, an essential active species trapping experiments were conducted. Different types of radical scavenger were introduced into degradation experiment to confirm the role of reactive groups (such as ·OH, ·O₂^-, H₂O₂ and h⁺). According to the reports, isopropyl alcohol (IPA, 1mM) is a trapping agent for hydroxyl radicals (·OH), ammonium oxalate (AO, 1 mM) is a scavenger for hole (h⁺), benzoquinone (BQ, 1 mM) can quench the superoxide radical (·O₂^-) and Fe(Ⅱ)-ethylene diamine tetraacetic acid (Fe(Ⅱ)-EDTA, 1mM) can obliterate the hydrogen peroxide (H₂O₂)^{[26, 27]}. The scavengers can partially inhibit the photocatalytic oxidation reaction and the inhibition level corresponds to the importance degree of the reactive species.

  2.6 Toxicity assessment

  Given that one of the main targets for the developed photocatalysts is applied to treat the wastewater, the leaching toxicity of the prepared samples should be confirmed. To investigate the influence of as-prepared samples for the plant growth, the toxicity of as-prepared samples was assessed by pakchoi seed germination experiment. Briefly, 25 mg sample was added into 50 mL deionized water (DW), and then the mixture was stirred magnetically for 8 hours and left to rest for 16 hours. Subsequently, the liquid supernatant was removed and filtrated through 0.45 μm membrane. Finally, the pakchoi seeds and 5 mL DW or treated supernatant (TS) were put into Petri dishes and incubated at 25 ℃ under dark condition for 3 days. Three parallel tests were performed for each sample. The percentage of inhibition was obtained by the following formula:

  $ \% \;\text{inhibition=}\frac{\text{(}{\text{average root length)}}_{\text{DW}}\text{-}{\text{(average root length)}}_{\text{TS}}}{\text{(}{\text{average root length)}}_{\text{DW}}}\text{}\times {100\%} $

  (2)

  3. RESULTS AND DISCUSSION

  3.1 Structure and composition of the CNAG photocatalyst

  The structure and phase of as-prepared photocatalyst were characterized by XRD. As shown in Fig. 1a, g-C₃N₄ has two characteristic diffraction peaks at 27.6° and 12.6°. The peak at 12.6° belongs to (002) plane, which is the in-plane structure packing motif of tris-triazine units. The strong diffraction peak at 27.6° corresponds to (100) plane, which belongs to the characteristic interlayer stacking aromatic segments. The CNA-15 and CNAG-6% have similar profile with pure g-C₃N₄, indicating that the addition of Ag and GO does not change the crystal structure of g-C₃N₄. No obvious impurity peak was observed in CNAG-6% composite, which may illustrate that the quantity of Ag and GO is so low and highly dispersed that could't be observed.

  Figure 1

  

  Figure 1.  (a) XRD patterns of pure g-C₃N₄, CNA-15 and CNAG-6%; (b) FTIR spectra of g-C₃N₄, CNA-15, GO and CNAG-X

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  The functional groups information of the pure g-C₃N₄, CNA and CNAG composite samples is further verified by FTIR spectra. As shown in Fig. 1b, the samples basically maintain the characteristic peaks of g-C₃N₄. The broad peak at 3000~3500 cm^-1 can be attributed to the stretching mode of O–H groups and the N–H vibration mode^[28]. The peaks at 1550~1650 cm^-1 region can be assigned to the typical stretching modes of the C=N groups and the peaks at 1200~1500 cm^-1 region to C–N groups in aromatic heterocycles^[29]. The intensity of the several peaks in 1200~1650 cm^-1 region is nearly identical, indicating the heterocycle structure has no obvious change. Moreover, the shape peaks at 803 and 880 cm^-1 may be related to the breathing mode of triazine units of g-C₃N₄^{[30, 31]}.

  The XPS analysis was carried out to reveal the surface composition and valence state of CNAG-6% composite photocatalyst. Fig. 2a shows the XPS survey spectrum of CNAG-6% ternary composites and can be concluded that the as-prepared sample mainly consists of C, N, Ag and O elements. Fig. 2b shows the high-resolution XPS spectrum of C1s in CNAG-6% composite. C1s spectrum of CNAG-6% ternary composites can be deconvoluted into four individual peaks with the binding energies of 284.6, 286.1, 288.2 and 290.2 eV. The two strong peaks at 284. 6 and 288.2 eV can be contributed to the binding energies of C–C and N=C–N bonds, respectively, which are the characteristic peaks of C in g-C₃N₄^{[32, 33]}. The other two peaks at the positions of 286.1 and 290.2 eV can be attributed to the C–N and O–C=O bonds, respectively. The weak intensity of the peak at 290.2 eV suggests the low content of O–C=O species^[34]. The O1s high-resolution XPS spectrum is shown in Fig. 2c. It is clear to see that there exist two main peaks at 532.0 and 533.4 eV, which can be identified as the functional group of GO and the oxygen species of the adsorption hydroxyl group, respectively. Fig. 2d presents the Ag3d XPS spectrum. The two individual peaks at 367.7 and 373.7 eV with 6.0 eV splitting between the two peaks can be attributed to the metal Ag0 species, indicating the existence of Ag in the composite^[35]. In addition, the atomic molar ratio of Ag was 1.88%. The high-resolution spectrum of N1s is shown in Fig. 2e, which can be deconvoluted into three peaks that correspond to nitrogen atoms in different functional groups: sp² hybridization of C=N bonds in triazine rings at 398.7 eV^[36], tertiary nitrogen (N–(C)₃) at 400.2 eV, and amino functional groups with a hydrogen atom (N–H) at 401.4 eV^[37].

  Figure 2

  

  Figure 2.  XPS spectra of CNAG-6% survey scan (a), C1s (b), O1s (c), Ag3d (d) and N1s (e)

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  3.2 Optical absorption performance of CNAG photocatalysts

  In order to examine the mechanism for the improved photocatalystic degradation efficiency of CNAG samples, two possible reasons of light absorption and charge separation efficiency were investigated. Fig. 3a shows the UV-vis diffuse reflectance spectra (UV-vis DRS) of pure g-C₃N₄, CNA-15 and CNAG-6% samples. Light absorbance over the range of 420~800 nm was observed for pure g-C₃N₄, CNA-15 and CNAG-6% samples, which can be responsible for the visible-light induced photocatalytic activity. Relative to pure g-C₃N₄, the CNA-15 sample showed the stronger absorption, which may be attributed to the surface plasmon resonance effect of silver nanoparticles. The absorption intensity of CNAG-6% sample has great light harvesting efficiency relative to pure g-C₃N₄ and CNA-15, which is attributed to the strong absorption of GO in the visible-light region. The maximum absorption wavelength of the sample can be obtained by making a tangent at the key descending part of the diffuse reflectance spectrum and extending the tangent line to the X axis. The absorption edges of pure g-C₃N₄, CNA-15 and CNAG-6% were observed at 461, 470 and 481 nm, respectively. The band gaps of the samples can be estimated by the following formula:

  $ {{E}}_{\text{g}}{=1240/}{{ \lambda }}_{\text{g}} $

  (3)

  Figure 3

  

  Figure 3.  (a) UV-vis diffuse reflectance spectra, (b) Photoluminescence spectra and (c) Transient photocurrent responses of the as-prepared g-C₃N₄, CNA-15 and CNAG-6% samples

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  where E_g is the gap energy of the semiconductor and λ_g is the maximum absorption edge of the semiconductor^[38]. The band gaps of g-C₃N₄, CNA-15 and CNAG-6% samples were determined to be 2.69, 2.64 and 2.58 eV, respectively. The result of UV-Vis DRS showed that the introduction of Ag and GO increased the absorption capacity of visible light and narrowed the bandgap, improving the visible light response.

  Photoluminescence (PL) may be caused by the recombination of free carrier^{[39, 40]}, so the separation efficiency of photogenerated electrons and holes in semiconductor can be investigated by the PL analysis. Generally, the faster recombination rate of photon-generated charge carrier induces stronger intensity of PL emission and unfavorable photocatalytic activity^[41]. As shown in Fig. 3b, g-C₃N₄ has a relatively dense emission peak at 453 nm, which can be attributed to the rapid recombination of electrons and holes. For CNA-15 sample, the position of the emission peak in the PL spectrum is similar to that of g-C₃N₄, but the intensity dropped obviously. This characterization result indicates that the addition of Ag inhibits the recombination efficiency of photo-generated charge, demonstrating that the efficiency of electron-cavity separated for CNA-15 catalyst increases compared with g-C₃N₄. The intensity of PL luminescence is further inhibited by the introduction of GO, meaning that the CNAG-6% composite has better charge mobility and separation ability than the CNA-15 sample.

  The photocurrent response is widely considered as the most efficient evidence for explaining the electrons and holes separation in the composite photocatalysts^[42]. To further detect the photo-induced charge and the separation rate of carriers, the photocurrent responses of g-C₃N₄, CNA-15 and CNAG-6% samples were investigated and shown in Fig. 3c. Compared with pure g-C₃N₄, the photogenerated current density increases after adding Ag, indicating the better separation efficiency of photo-induced carriers in CNA-15 catalyst. For the CNAG-6% composite, the photocurrent response is further enhanced in comparison with that of CNA-15 catalyst. The results illustrate that the separation and transfer efficiency of photo-induced electron-hole pairs in CNAG-6% composite are better than that in g-C₃N₄ and CNA-15 catalysts, which can improve the photocatalytic performance. The results of photocurrent response are in good accordance with the above PL results.

  The results of UV-Vis DRS, PL and photocurrent response show a good interaction between g-C₃N₄, Ag and GO. Compared with g-C₃N₄, CNA-15 sample shows the higher light harvesting efficiency and electron hole separation efficiency, which can improve the photocatalytic activity. Compared with CNA-15 sample, the light harvesting efficiency and the electron hole separation efficiency of CNAG-6% were further enhanced, which can reveal the better photocatalytic degradation efficiency.

  3.3 Morphology of CNAG photocatalyst

  The detailed microscopic structures of pure g-C₃N₄, CNA-15 and CNAG-6% were investigated by TEM. It can be observed from Fig. 4a that pure g-C₃N₄ has the typical two-dimensional layered structure, which is consistent with the previous reports^[43]. As shown in Fig. 4b, a lot of Ag nanoparticles attache to pure g-C₃N₄ sheets for CNA-15 sample. Noticeably, the TEM image of CNAG-6% composite is shown in Fig. 4c, from which we can see that the GO nanosheets are attached to the surface of CNA-15 sample and the as-prepared CNAG-6% composite still possesses a stratified structure.

  Figure 4

  

  Figure 4.  TEM images for (a) g-C₃N₄, (b) CAN-15 and (c) CNAG-6%

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  3.4 Photocatalytic activity and toxicity evaluation

  3.4.1   Photocatalytic activity

  The photocatalytic performance of CNA-T samples was obtained by estimating the degradation efficiency of RhB under the simulated sunlight irradiation, and the results are shown in Fig. 5. The photocatalyst was dispersed in 50 mL (5 mg/L) RhB solution, and then all the samples experienced 30 min in the dark before visible light irradiation to reach the equilibrium of adsorption and desorption. As shown in Fig. 5, the RhB degraded hardly without adding photocatalyst under simulated solar irradiation, implying that RhB is very stable and its self-degradation could be negligible. The degradation rate was enhanced with the increase of Ag content, and CNA-15 catalyst exhibits the highest degradation rate towards RhB. This may be assigned to the formation of schottky barrier between Ag and g-C₃N₄ which could promote the separation of photogenic carriers^[35]. Nevertheless, the photocatalytic performance decreases with further increasing the concentration of Ag. The probable reason is that Ag acts as the receiver of electrons to prevent photo-generated electrons from recombining with holes at a low content, but it forms a new charge recombination center with the increase concentration of Ag. Therefore, the optimal illumination time was 15 min.

  Figure 5

  

  Figure 5.  Degradation rate of RhB under simulated sunlight irradiation without catalyst and in the presence of g-C₃N₄ and CNA-T samples

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  To confirm the effect of combining GO with CNA-15 composite, a range of photocatalytic degradation experiments were carried out under simulated sunlight irradiation, and the results are shown in Fig. 6. As depicted in Fig. 6a, the degradation efficiency of RhB aqueous solutions was significantly improved after combining GO with CAN-15 composite. The reaction kinetics of photocatalytic degradation of RhB solution was quantitatively analyzed via the pseudo-first-order kinetics equation under simulated sunlight irradiation, with the result shown in Fig. 6b. It was found that the photocatalytic degradations of RhB over all the as-prepared samples follow the formula:

  $ -{ln}{\mathit{(}}{{C}}_{{t}}{/}{{C}}_{\mathit{0}}{\mathit{)}}=kt $

  (4)

  Figure 6

  

  Figure 6.  (a) Degradation rate of RhB under simulated sunlight irradiation without catalyst and in the presence of g-C₃N₄, CNA-15 and CNAG-X, (b) Plots of –ln(C/C₀) versus irradiation time for RhB representing the fit using a pseudo-first-order reaction rate

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  where k is the apparent reaction rate constant, C₀ the initial concentration of RhB and C_t the concentration of RhB at time t.

  The apparent reaction rate constant of RhB degradation under simulated sunlight is 0.018 min^-1 for pure g-C₃N₄. The CNAG-6% ternary catalyst reveals unexceptionable photocatalytic performance under simulated sunlight. The photocatalytic activity of the catalyst on RhB was gradually enhanced with the increase of GO before the mass ratio reaches 6%. The possible reason is that the superior e-transfer performance of GO nanolayer could reduce the composite probability of photo-induced carriers, which is beneficial to the photocatalytic performance.

  However, the catalytic effect decreased in the case of the mass ratio of GO more than 6%. Possible reasons are as follows (1) Excessive GO may work as the recombination center of photo-induced carrier rather than as the transmission path of photo-generated electrons. (2) Excessive GO may overlay the active sites on the surface of CNA-15 catalyst, thus reducing the separation efficiency of electron hole pairs.

  3.4.2   Toxicity assessment

  The percentage of inhibition was confirmed by the formula given in Eq. (2). The average root lengths (ARL) and percentage of inhibition are shown in Table 1. Fig. 7 is the photo of germinated seeds cultured with different solutions. As shown in Table 1, the ARL of pakchoi seeds was 20.6, 20.1, 20.3, 19.6 and 21.0 mm for DW, g-C₃N₄, CNA-15, CNAG-1% and CNAG-6%. The ARL of pakchoi seeds cultured with treated supernatant did not decrease significantly compared to those cultured with DW. The percentage of inhibition fluctuates within a relatively normal range.

  Table 1

  

  Table 1.  Average Root Lengths of Pakchoi Seeds and Percentages of Inhibition

  DownLoad: CSV

  Samples	DW	g-C3N4	CNA-15	CNAG-1%	CNAG-6%
  ARL (mm)	20.6	20.1	20.3	19.6	21.0
  % Inhibition	----	2.43	1.46	4.85	–1.94

  Figure 7

  

  Figure 7.  Photo of germinated seeds cultured with different infusion solutions

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  3.5 Mechanism of photocatalytic reactions

  3.5.1   Reactive oxygen species

  During photocatalytic degradation process, it is generally believed that oxidative species should be responsible for the photodegradation activity. For the sake of better understanding the catalysis mechanism of as-synthesized composites, a series of free radical capture experiments were carried out, in which AO, IPA, Fe(Ⅱ)-EDTA and BQ were used as scavengers for the major active species h⁺, ·OH, H₂O₂ and ·O₂^-, respectively.

  Figs. 8a and 8b present the influence of different trapping agents on the degradation efficiency of RhB in pure g-C₃N₄ photocatalytic system. The introductions of BQ and Fe-EDTA cause a significant depression for the degradation of RhB, illustrating that ·O₂^- and H₂O₂ have a crucial role in the degradation process and ·O₂^- is the major reactive oxidative species. There is a slight change in the photodegradation efficiency of RhB after the addition of IPA, indicating that ·OH also participates in the degradation reaction. The addition of AO has a barely discernible change of the photocatalytic reaction, indicating that ·OH hardly participates in the degradation progress. The importance degree of oxidized species follows the order: ·O₂^- > H₂O₂ > ·OH > h⁺.

  Figure 8

  

  Figure 8.  Influence of various scavengers on the photocatalytic degradation processes of (a) pure g-C₃N₄ and (c) CNAG-6% toward the degradation of RhB; The value of the removal rate of the photo-degradation of RhB in the presence of (b) pure g-C₃N₄ and (d) CNAG-6%

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  As shown in Figs. 8c and 8d, the degradation efficiency of RhB is extremely inhibited with the addition of AO, indicating that the h⁺ is the dominant active species which plays a decisive role for the photocatalytic degradation. The photocatalytic efficiencies are also obviously decreased when adding Fe-EDTA, but less than that of AO addition, so the H₂O₂ is the second important active species in the photocatalytic degradation process by CNAG-6%. With the addition of IPA and BQ, the degradation efficiency of RhB was lightly depressed, implying that ·OH and ·O₂^- involved in the degradation process are the minor active species. On the basis of the above results, the order of contribution to the photocatalytic removal rate of RhB is h⁺ > H₂O₂ > ·O₂^- > ·OH. The results are totally different from pure g-C₃N₄, suggesting that the introduction of Ag and GO changes the reaction mechanism.

  3.5.2   Proposed mechanism

  Based on the abovementioned results and analysis, the photodegradation mechanism of as-prepared CNAG-6% ternary composites under simulated solar irradiation was proposed, and the scheme is shown in Fig. 9. The type of metal Ag and semiconductor g-C₃N₄ heterojunction belong to Schottky barrier, and the potential well on the Ag/g-C₃N₄ interface can capture electrons and promote the separation of electron holes. In addition, due to the surface plasma resonance (SPR) effect of Ag nanoparticles, Ag not merely acts as an electron pool but also can be excited to generate electron hole pairs, to generate active species and to oxidize Ag to Ag⁺ ions.

  Figure 9

  

  Figure 9.  photocatalytic mechanism scheme of as-prepared CNAG-6% samples

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  It is essential to research the CB and VB potentials of the photocatalyst for the sake of better understanding the separation of photogenerated electro-hole pairs over the composites. The VB and CB of g-C₃N₄ can be obtained according to the empirical formula:

  $ {{E}}_{\text{VB}}\text{=}\chi–{{E}}^{\text{C}}\text{+ 0.5}{{E}}_{\text{g}} $

  (5)

  $ {\text{}{E}}_{\text{CB}}\text{=}{{E}}_{\text{VB}}–{{E}}_{\text{g}} $

  (6)

  where E_VB is the valence band edge potential, E_CB the conduction band edge potential and E_g the band gap energy of the semiconductor. χ, the absolute electronegativity of a semiconductor, is 4.72 eV according to the previous report. E^C is the energy of free electrons (about 4.5 eV vs. NHE). The band gap of pure g-C₃N₄ is 2.69 eV obtained from UV-vis DRS. On the basis of the above equations, the values of E_vB and E_CB can be calculated to be 1.56 and −1.13 eV, respectively. Because the CB (−1.13eV, vs NHE) of g-C₃N₄ is more negative than that of Ag⁺/Ag (0.7991v, vs NHE), the photo-induced electron on the CB of g-C₃N₄ can in situ reduce Ag⁺ ions to Ag again.

  It is worth noting that coupling GO with CNA-15 markedly enhanced the photocatalytic activity of CNAG catalyst and this can be explained by the following. Due to the unique electrophilic group of GO, the light-generated electrons can quickly inject to GO, while photo-generated holes remain stick in VB of g-C₃N₄, promoting further separation of photo-induced electron hole pairs, improving the separation efficiency of charge carriers and enhancing photocatalytic activity. Based on the above analysis, the charge carrier transfers and photodegradation reactions in CNAG-6% composite system can be depicted as the following equations:

  $ g\text{-}{{\text{C}}_{\text{3}}}{{\text{N}}_{4}}+h\nu \to g\text{-}{{\text{C}}_{\text{3}}}{{\text{N}}_{4}}\left( {{e}^{-}}+{{h}^{+}} \right)$

  (7)

  $ \text{Ag}+h\nu \to \text{A}{{\text{g}}^{*}}({{e}^{-}}+{{h}^{+}}) $

  (8)

  $ \text{A}{{\text{g}}^{*}}\left( {{e}^{-}}+{{h}^{+}} \right)+{{\text{O}}_{2}}\to \text{A}{{\text{g}}^{+}}+\cdot {{\text{O}}_{2}}^{-}$

  (9)

  $ \text{A}{{\text{g}}^{+}}+g\text{-}{{\text{C}}_{\text{3}}}{{\text{N}}_{\text{4}}}({{e}^{-}})\to \text{Ag}+{g-}{{\text{C}}_{\text{3}}}{{\text{N}}_{\text{4}}}$

  (10)

  $ \cdot {{\text{O}}_{2}}^{-}+{{\text{H}}_{\text{2}}}\text{O}\to \text{H}{{\text{O}}_{2}}\cdot +\text{O}{{\text{H}}^{-}}$

  (11)

  $ 2\text{H}{{\text{O}}_{\text{2}}}\cdot \to {{\text{O}}_{2}}+{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$

  (12)

  $ {{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}+{{e}^{-}}\to \cdot \text{OH+O}{{\text{H}}^{\text{-}}} $

  (13)

  $ {g}\text{-}{\text{C}}_{\text{3}}{\text{N}}_{\text{4}}\text{(}{{h}}^{+}\text{)/oxidative species+RhB}\to {\rm{C}}{\text{O}}_{\text{2}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (14)

  3.5.3   Stability evaluation

  The stability of photocatalyst is a crucial factor in practical and theoretical applications. To confirm the stability of CNAG-6% sample, the cyclic tests for photocatalytic degradation of RhB were performed and the photodegradation performance in the cyclic tests is shown in Fig. 10a. From Fig.10a, the photocatalytic activity decreased slightly after three cycles but the CNAG-6% composite catalyst still possessed preferable photocatalytic activity compared with pure g-C₃N₄ and CNA-15 samples, which indicates that the catalyst has high stability. As shown in Fig. 10b, the crystal phase structure of CNAG-6% displays no distinct changes before and after the degradation progress, indicating that the sample is very stable.

  Figure 10

  

  Figure 10.  (a) Recyclability of the CNAG-6% composite in three successive experiments for the photocatalytic degradation of RhB under simulated solar irradiation irradiation; (b) XRD of CNAG-6% sample used before and after the degradation progress

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  4. CONCLUSION

  In summary, CNAG composite photocatalysts with different percents of GO were successfully prepared by a simple two-step process, and the structure of g-C₃N₄ was not changed with the addition of Ag and GO. Compared with pure g-C₃N₄ and CNA, the as-prepared CNAG composite showed superb photocatalytic performance under simulated sunlight irradiation. In particular, when the theoretical addition amount of GO is 6%, the optimal degradation rate constant of CNAG photocatalyst for the degradation of RhB was 4.3 and 2.5 times higher than that of pure g-C₃N₄, respectively. The enhanced photocatalytic activity could be attributed to the SPR effect of Ag nanoparticles, the excellent interface connection between Ag and g-C₃N₄, and the strong electron mobility between the GO layer caused by the unique electrophilic groups. The pakchoi seed germination experiment was carried out and the results demonstrated that the percentage of inhibition fluctuates within a relatively normal range. In the RhB degradation process, h⁺ was the crucial active species, followed by H₂O₂. Moreover, ·OH and ·O₂^- were involved in the photocatalytic process as well, but played a minor role in the RhB degradation reaction. The CNAG composite had good stability and photocatalytic activity in the degradation process of RhB. This study offered insights for the photocatalytic mechanism of CNAG composite photocatalyst as well as the good idea for the rational design of high efficiency catalysts.

  
  
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• 

  Figure 1  (a) XRD patterns of pure g-C₃N₄, CNA-15 and CNAG-6%; (b) FTIR spectra of g-C₃N₄, CNA-15, GO and CNAG-X

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  Figure 2  XPS spectra of CNAG-6% survey scan (a), C1s (b), O1s (c), Ag3d (d) and N1s (e)

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  Figure 3  (a) UV-vis diffuse reflectance spectra, (b) Photoluminescence spectra and (c) Transient photocurrent responses of the as-prepared g-C₃N₄, CNA-15 and CNAG-6% samples

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  Figure 4  TEM images for (a) g-C₃N₄, (b) CAN-15 and (c) CNAG-6%

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  Figure 5  Degradation rate of RhB under simulated sunlight irradiation without catalyst and in the presence of g-C₃N₄ and CNA-T samples

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  Figure 6  (a) Degradation rate of RhB under simulated sunlight irradiation without catalyst and in the presence of g-C₃N₄, CNA-15 and CNAG-X, (b) Plots of –ln(C/C₀) versus irradiation time for RhB representing the fit using a pseudo-first-order reaction rate

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  Figure 7  Photo of germinated seeds cultured with different infusion solutions

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  Figure 8  Influence of various scavengers on the photocatalytic degradation processes of (a) pure g-C₃N₄ and (c) CNAG-6% toward the degradation of RhB; The value of the removal rate of the photo-degradation of RhB in the presence of (b) pure g-C₃N₄ and (d) CNAG-6%

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  Figure 9  photocatalytic mechanism scheme of as-prepared CNAG-6% samples

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  Figure 10  (a) Recyclability of the CNAG-6% composite in three successive experiments for the photocatalytic degradation of RhB under simulated solar irradiation irradiation; (b) XRD of CNAG-6% sample used before and after the degradation progress

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  Table 1.  Average Root Lengths of Pakchoi Seeds and Percentages of Inhibition

  Samples	DW	g-C3N4	CNA-15	CNAG-1%	CNAG-6%
  ARL (mm)	20.6	20.1	20.3	19.6	21.0
  % Inhibition	----	2.43	1.46	4.85	–1.94

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•