Ag Loading Enhanced Photocatalytic Activity of g-C3N4 Porous Nanosheets for Decomposition of Organic Pollutants

The g-C3N4 porous nanosheets with different loading amount of Ag nanoparticles (NPs) are successfully prepared by a simple liquid-phase reduction method. These Ag/g-C3N4 composites have an improved photocatalytic performance for decomposing organic pollutants compared with that of pure g-C3N4 nanosheets. Many measurements have been used for characterizing the samples, such as XRD, FTIR, UV-Vis DRS, PL, XPS, EDS, SEM, and TEM. In Ag/g-C3N4, the Ag NPs are uniformly coated on the g-C3N4 surface, the diameter is mainly in the range of 8~18 nanometers. Loading of Ag NPs expand the response to the visible light for g-C3N4 and increasing the producing rate of photogenerated e−-h+ pairs. The loading of silver NPs obviously enhances the photocatalytic activity of C3N4 nanosheets toward the Rhodamine B (RhB) decomposition under the simulated sunlight irradiation. With different loading amounts of Ag NPs, Ag/g-C3N4 (3 wt% of Ag) showed the highest photocatalytic activity for RhB decomposition among these as-prepared samples, which is 10 times of the rate of pure C3N4. Based on the experimental results, a possible photocatalytic mechanism for Ag/g-C3N4 is proposed.


INTRODUCTION
The organic pollutants in waste water has become a serious problem that threatens human health (Zhang and Wen, 2008). Solar energy, as a clean energy, has been widely concerned. It is a very effective way to solve the problem of water pollution by using solar energy (George et al., 2015;Wang et al., 2017Wang et al., , 2018aQi et al., 2018a;Shi et al., 2018;Wei et al., 2018;Yan et al., 2018a,b;Zhong et al., 2018). Since Fujishima et al. reported the phenomenon of water splitting for hydrogen production on TiO 2 photoanode (Fujishima and Honda, 1972), the study of semiconductor photocatalysts has become very popular Wei et al., 2015;Wen et al., 2015;Park et al., 2016;Qi et al., 2017bQi et al., , 2018bXia et al., 2017).
The photocatalysis technology has been widely used to treat the organic polluted waste water because of its simpler equipment, convenient operation, energy saving, environmental protection, and strong oxidizing ability. Semiconductor photocatalysts mainly include graphite nitride (g-C 3 N 4 ) (Cao et al., 2015), metal oxide (Qi et al., 2017a), and metal sulfide (Hong et al., 2015). Among these materials, g-C 3 N 4 , as an important photocatalyst, has received widely attention, because of its unique characteristics, including metal free, non-toxic, easy preparation, suitable band gap, and cheap (Cao and Yu, 2014). However, the photocatalytic activity of g-C 3 N 4 is still very low, it is difficult to use in real life, due to the low utilization efficiency of sunlight and the fast recombination of photogenerated e − -h + pairs (Martha et al., 2013;Zhu et al., 2018). Up to now, many efforts have been devoted to improve the photocatalytic performance of g-C 3 N 4 (Xiang et al., 2011;Akple et al., 2015;Xu et al., 2018;Fu et al., 2019). For example, Liu et al. used metal doping to enhance the visible light adsorption of g-C 3 N 4 and found that it enhanced the photocatalytic activity in the photocatalysis of water splitting for hydrogen production (Niu et al., 2012). Cheng et al. demonstrated that through building heterojunction of ZnO/g-C 3 N 4 , the photocatalytic activity for the decomposition of organic dyes is enhanced (Cheng et al., 2013). Fina et al. reported that loading Pt nanoparticles (NPs) can enhance activity of g-C 3 N 4 photocatalytic water splitting into H 2 production (Fina et al., 2015). In these methods, depositing noble metals (Au, Ag, Pt, or Pd) on the g-C 3 N 4 surface is useful to enhance the photocatalytic activity of g-C 3 N 4 (Wen et al., 2017;Tong et al., 2018;Wang et al., 2018b). However, the mechanism of interaction between the loading noble metals and C 3 N 4 , and how they work to enhance the photocatalytic activity of g-C 3 N 4 are limited to know.
This work reports a simple liquid-phase reduction method to prepare the Ag/g-C 3 N 4 composites. The structure, morphology, optical property, and photocatalytic activity of the as-prepared Ag/g-C 3 N 4 samples are investigated. The effects of loading content of Ag on the light absorbency and photocatalytic activity of C 3 N 4 are studied. Under the simulated sunlight, the photocatalytic performance of g-C 3 N 4 for Rhodamine B (RhB) photodegradation is obviously improved after loading Ag NPs. Finally, a possible photocatalytic mechanism of the Ag/g-C 3 N 4 composite is given.

EXPERIMENTAL Synthesis
The g-C 3 N 4 nanosheet was prepared via thermal polycondensation of urea. Fifteen grams urea was placed into a covered ceramic crucible and heated to 500 • C for 5 h in air, at the heating rate of 10 • C min −1 . After the reaction, it cooled down to room temperature naturally, the product was collected and grind to powder. The Ag/g-C 3 N 4 composite was synthesized by a liquid-phase reduction method. First 0.5 g of g-C 3 N 4 was put in 50 mL of water and ultrasonic treated for 5 min. Second, a certain amount of AgNO 3 (5 mM) aqueous solution was put into the above solution and maintain stirring. Third, a certain amount of NaBH 4 [the molar ratio of n(AgNO 3 ):n(NaBH 4 ) = 1:5] dissolved in 30 mL of water, and then put into the above solution, stirring for 1 h. Following, the product was centrifuged and washed with absolute ethanol and distilled water, respectively. Finally, these samples are dried in vacuum oven at 70 • C for 5 h. By varying the amount of using AgNO 3 , a series of samples with different ratios of Ag to g-C 3 N 4 [m(Ag):n(g-C 3 N 4 ) = 1, 2, 3, 4, and 5%] were prepared and labeled as 1%-Ag/g-C 3 N 4 , 2%-Ag/g-C 3 N 4 , 3%-Ag/g-C 3 N 4 , 4%-Ag/g-C 3 N 4 , and 5%-Ag/g-C 3 N 4 , respectively.

Characterization
The crystal phases of products were studied by X-ray diffraction (XRD) (X-ray diffractometer, Cu Kα, λ = 1.54056 Å) (Bruker D5005, Germany). Fourier transform infrared (FT-IR) spectra were conducted using a Nicolet Magna 560 (US) spectrophotometer. X-ray photoelectron spectroscopy (XPS) was measured on a PHIQ 1,600 XPS (US) instrument. The weight percentages of Ag in the Ag/g-C 3 N 4 photocatalysts was studied by inductively coupled plasma atomic emission spectrometry (ICP-AES, Shimadzu ICP-7510, Japan). High resolution transmission electron microscopy (HRTEM) was taken by a JEOL JEM-2100F (Japan) electron microscope. UVvis absorbance spectra were collected on a Shimadzu UV-3100 (Japan) spectrophotometer, using BaSO 4 as reference. The photoluminescence (PL) spectra of g-C 3 N 4 and Ag/g-C 3 N 4 samples were studied on a Varian Cary Eclipse (US) spectrometer equipped with an excitation wavelength of 325 nm.

Photocatalytic Performance
The photocatalytic activity of pure g-C 3 N 4 and Ag/g-C 3 N 4 samples was examined by photodegradation of RhB under the simulated sunlight irradiation, which was obtained from an 500 W Xe lamp. Ten milligram of samples were dispersed in 25 mL of RhB aqueous solution (10 mg/L RhB aqueous solution). Prior to the irradiation, the reaction solution was magnetically stirred in the dark for 30 min to get adsorptiondesorption equilibrium for the dyes on photocatalyst surface. During the photocatalytic degradation, 2 mL of the sample was withdrawn from the reaction solution at the time intervals of every 15 min and then centrifuged to remove the particles. Then the concentration of RhB was examined by UV-vis spectrophotometer, at the absorbance wavelength of 553 nm. The photodegradation rate of RhB was calculated by the formula: D = C/C 0 × 100%, where C 0 is the initial concentration of RhB, and C is the concentration of RhB at a time t.

Photoelectrochemical Measurement
The photoelectrochemical performance was studied on a CHI 660D electrochemical work station with a standard threeelectrode system. Put g-C 3 N 4 or Ag/g-C 3 N 4 on the ITO glass surface as the working electrode. A piece of Pt wire and a calomel electrode were used as the counter electrode and reference electrode, respectively. The electrolyte is 0.1 mol/L Na 2 SO 4 aqueous solution. Five milligram photocatalysts were mixed with 1 mL ethanol and then the mixture was coated on 2 × 4 cm ITO glass for use as an electrode. Electrochemical impedance spectroscopy (EIS) Nyquist plots were conducted at an open current potential with an amplitude of 5 mV and the frequency range was from 10 5 to 1 Hz.

XRD Patterns
The crystal phase of as-prepared samples is studied by XRD measurements, and the XRD patterns are shown in Figure 1. The pure g-C 3 N 4 nanosheets and Ag/g-C 3 N 4 nanocomposites have two dominant peaks at 13.1 • and 27.5 • , indexed to g-C 3 N 4 (JCPDS87-1526) (Yang et al., 2013b). The peak at 27.5 • is ascribed to the typical (002) plane with planar distance of 0.33 nm corresponding to interlayer-stacking of aromatic segments. The peak at 13.1 • with distance of 0.675 nm is indexed to the (100) plane corresponding to in-plane structural packing (Dong et al., 2011;Liu et al., 2011). Compared with the pure g-C 3 N 4 nanosheets, the intensity of the diffraction peak at 27.5 • becomes weaker with increasing content of loading Ag NPs. The diffraction peak related to Ag NPs is not found, because of the low Ag loading amount and the high dilution effect of Ag NPs on the g-C 3 N 4 surface (Zhou et al., 2014;Fu et al., 2015). As follows, the XPS and EDS data demonstrate the existence of Ag loading on the g-C 3 N 4 surface.

FTIR Analysis
The FTIR spectra are similar between the pure g-C 3 N 4 nanosheets and Ag/g-C 3 N 4 composites with different Ag loading amounts (Figure 2). The peak at 1,639 cm −1 can be ascribed to the stretching vibration of C-N groups, and the peaks at 1,242, 1,327, 1,568 and 1,408 cm −1 can be attributed to the aromatic C-N stretching vibration (Aghdam et al., 2017). The peak at 809 cm −1 corresponds to the breathing mode of triazine units (Sun et al., 2012). The peak at 3171 cm −1 is attributed to the stretching vibration of N-H group (Yang et al., 2013a). All these characteristic FTIR peaks suggest that the overall structure of g-C 3 N 4 maintains the original form after Ag NPs loading.
FIGURE 1 | XRD patterns of pure g-C 3 N 4 and Ag/g-C 3 N 4 samples.

TEM Images
The morphology and microstructure of the pure g-C 3 N 4 and 3%-Ag/g-C 3 N 4 samples were investigated by TEM measurements. The TEM image of pure g-C 3 N 4 shows that it is a twodimensional nanosheet with some holes in the size range of 10-30 nm ( Figure 3A). TEM image of the 3%-Ag/g-C 3 N 4 sample ( Figure 3B) shows that Ag NPs, observed as black dots, uniformly disperse on g-C 3 N 4 surfaces. The size of Ag NPs is from 6 to 20 nm, indicating that these Ag NPs are Ag clusters on the surface of g-C 3 N 4 . The size distribution of Ag NPs on 3%-Ag/g-C 3 N 4 is presented in Figure 3C, which is mainly in the range of 8-18 nm. The energy dispersive X-ray spectrum (EDS) also confirms that Ag NPs exist on the surface of g-C 3 N 4 ( Figure 3D). Also, it shows that the 3%-Ag/g-C 3 N 4 sample is consisted of C, N, and Ag elements, which confirms that Ag NPs successfully adsorbed on the g-C 3 N 4 surface.

XPS Analysis
The surface elemental composition and chemical states of Ag/g-C 3 N 4 are studied by XPS, here 3%-Ag/g-C 3 N 4 is selected for study (Figure 4). The elements C, N, O, and Ag are clearly observed in the survey spectrum ( Figure 4A). The peak located at 531 eV is assigned to O, which may be the water molecules at the sample surface (Hu et al., 2015). Two C 1 s peaks locate at 284.8 eV and 288.3 eV ( Figure 4B). The peak located at 284.8 eV is assigned to sp 2 -hybridized C atoms, and the 288.3 eV peak can be assigned as N-C=N 2 groups . As shown in Figure 4C, the peaks of N 1 s locate at 398.8, 400.5, and 401.5 eV, which can be ascribed to sp 2 bonded nitrogen C-N-C groups, sp 3 tertiary nitrogen N-(C) 3 and amino functional groups (C-N-H), respectively (Qi et al., 2019). The spectrum of Ag 3d ( Figure 4D) shows that the peaks located at 367.4 and 374.0 eV can be assigned as Ag 3d 5/2 and Ag 3d 3/2 , respectively (Yang et al., 2016). This confirms that Ag NPs are successfully coasted on g-C 3 N 4 surfaces. The peak at 368.1 eV is assigned as Ag(I), which indicates the formation of Ag 2 O on the surface of metallic Ag (Tian et al., 2015). The actual content of Ag in the Ag/g-C 3 N 4 FIGURE 2 | FTIR spectra of pure g-C 3 N 4 and Ag/g-C 3 N 4 samples.
Frontiers in Chemistry | www.frontiersin.org  composites was studied by ICP-AES analysis. The result shows that the weight percentages of Ag in 1%-Ag/g-C 3 N 4 , 2%-Ag/g-C 3 N 4 , 3%-Ag/g-C 3 N 4 , 4%-Ag/g-C 3 N 4 , and 5%-Ag/g-C 3 N 4 were measured to be 0.72, 1.43, 2.09, 2.74, and 3.55, respectively. The measured value by ICP-AES is a little smaller than the theoretical value for the weight percentages of Ag in Ag/g-C 3 N 4 composites, but both of the two changing trends are the same.

UV-vis Diffuse Reflectance Spectra
The UV-DRS measurement is used to study the optical adsorption property of the pure g-C 3 N 4 nanosheets and Ag/g-C 3 N 4 composites ( Figure 5). Figure 5A shows that the light absorption edge of the pure g-C 3 N 4 is at 440 nm, which agrees with the intrinsic band gap of bulk g-C 3 N 4 (Chen et al., 2016). Compared with pure g-C 3 N 4 nanosheets, Ag/g-C 3 N 4 composites have an additional weak and broad absorption peak around 450-600 nm, which is characteristic of the silver surface plasmon resonance band . The Ag/g-C 3 N 4 composite shows similar light absorption range with that of pure g-C 3 N 4 , but the visible light adsorption is increased, as shown in Figure 5B. Thus, the samples with the increasing of Ag loading amount change the color from yellow to dark gray.

PL Spectra
Photoluminescence measurement is an useful method to analyze the separation efficiency and the life time of photogenerated carriers, as shown in Figure 6. PL spectra of pure g-C 3 N 4 and Ag/g-C 3 N 4 are taken by the exciting light of 325 nm. A strong broad peak at ∼460 nm is observed. Compared with pure g-C 3 N 4 , the PL intensity of Ag/g-C 3 N 4 composites decreases significantly. The weaker peak intensity of PL results in a slower recombination rate of photogenerated carriers (Ong et al., 2014). In Ag/g-C 3 N 4 composites, Ag NPs combine with the g-C 3 N 4 surface strongly, and effectively reduce the recombination rate of e − -h + pairs. This improved separation efficiency of photogenerated carriers leads to increasing e − and h + to join the photocatalytic process of Ag/g-C 3 N 4 . However, over loading of Ag on g-C 3 N 4 , such as 5%-Ag/g-C 3 N 4 , the PL intensity is getting increase again, indicating the increase of the combination of photogenerated carriers. It clearly sees that the Ag/g-C 3 N 4 composites with proper loading amounts of Ag have a potential for using as photocatalysts with high activity.

Photocatalytic Activity
The photocatalytic activity of pure g-C 3 N 4 and Ag/g-C 3 N 4 for photodecomposition of RhB is tested under the simulated sunlight irradiation. As shown in Figure 7A, compared with that of pure g-C 3 N 4 , the Ag nanoparticle modified g-C 3 N 4 shows an improved photocatalytic performance for decomposition of RhB aqueous. After irradiation for 100 min, the degradation of RhB is about 20% for pure g-C 3 N 4 nanosheets and almost 100% for 3%-Ag/g-C 3 N 4 . Figure 7B shows the apparent reaction rate constant (k) of RhB photodegradation, which shows that the kinetic constant of 3%-Ag/g-C 3 N 4 is almost 10 times higher than that of pure g-C 3 N 4 . When the mass ratio of Ag is in the range of 1-5 wt%, the enhanced photocatalytic activity is observed, due to effective enhanced the separation efficiency of photogenerated e − -h + pairs at the Ag/g-C 3 N 4 interface and the surface plasmon resonance (SPR) effect of Ag NPs (Duan et al., 2014), which are FIGURE 6 | PL spectra of the pure g-C 3 N 4 , 1%-Ag/g-C 3 N 4 , 3%-Ag/g-C 3 N 4 , and 5%-Ag/g-C 3 N 4 sample. Frontiers in Chemistry | www.frontiersin.org also supported by PL and UV-vis DRS results. Obviously, 3%-Ag/g-C 3 N 4 has the highest photocatalytic activity, which may be due to that the excess Ag NPs will be working as recombination centers, or the active site on g-C 3 N 4 surface is blocked (Ge et al., 2011). The photocatalytic activity of as-prepared Ag/g-C 3 N 4 composites is compared to the typical photocatalysts including TiO 2 and ZnO. The photocatalytic activity of 3%-Ag/g-C 3 N 4 (k = 0.0326 min −1 ) in this work is higher that TiO 2 (k = 0.0084 min −1 ) and ZnO (k = 0.0062 min −1 ) reported previously (Carvalho et al., 2015;Hao et al., 2019). In order to check the reusability of as-prepared Ag/g-C 3 N 4 photocatalysts, the recycling test was carried out. As shown in Figure 7C, the photodegradation percentage of RhB is >90% after six cycles, which indicates the as-prepared photocatalysts owns a good stability. A gradually decreased photocatalytic activity is due to the loss of photocatalyst in the recovery process.

Photoelectrochemical Performance
The charge separation efficiency is studied by using the photoelectrochemical measurements. Figure 8A shows that the photocurrent response of pure g-C 3 N 4 , 1%-Ag/g-C 3 N 4 , 3%-Ag/g-C 3 N 4 , and 5%-Ag/g-C 3 N 4 samples under the simulated sunlight irradiation, which shows stable reproducible photocurrent responses over five on-off cycles. The photocurrent starts when the light was turned on, the photocurrent is close to zero when the light was turned off. The photocurrent density FIGURE 7 | (A) photodegradation of RhB using pure g-C 3 N 4 , Ag/g-C 3 N 4 with different Ag contents under simulated sunlight irradiation. (B) kinetic data for the degradation of RhB. (C) the photodegradation rate of RhB for six cycles using 3%-Ag/g-C 3 N 4 .
(0.50 µA/cm 2 ) of 3%-Ag/g-C 3 N 4 is bigger than the pure g-C 3 N 4 (0.07 µA/cm 2 ) and the 5%-Ag/g-C 3 N 4 (0.38 µA/cm 2 ). The stronger photocurrent is due to the higher separation efficiency of the photogenerated e − -h + pairs of Ag/g-C 3 N 4 , which is consistent with its higher activity on photocatalytic decomposition of organic dyes.
In order to study the charge separation efficiency, the electrochemical impedance spectroscopy (EIS) Nynquist plots are used, and the EIS Nyquist plot of pure g-C 3 N 4 , 1%-Ag/g-C 3 N 4 , 3%-Ag/g-C 3 N 4 , and 5%-Ag/g-C 3 N 4 sample is as shown in Figure 8B. Usually, the smaller the radius of EIS Nyquist plots is, the higher the separation efficiency of charge carriers is . The radius on the EIS Nynquist plot of C 3 N 4 is getting smaller after Ag modification, the order is 3%-Ag/g-C 3 N 4 < 5%-Ag/g-C 3 N 4 < 1%-Ag/g-C 3 N 4 < g-C 3 N 4 , indicating that Ag modification indeed reduces the recombination of charge carries and increase the separation efficiency of photogenerated e − -h + pairs, 3%-Ag/g-C 3 N 4 is the best, which agrees well with PL spectra. Figure 9 shows the photocatalytic mechanism of Ag/g-C 3 N 4 composites during RhB decomposition under sunlight irradiation. Ag nanoparticle modification enhances the photocatalytic performance of g-C 3 N 4 due to the synergistic effect of two aspects, one is the SPR effect of metal Ag, another is the decrease of the recombination rate of photogenerated e − -h + pairs (Ingram et al., 2011). When Ag/g-C 3 N 4 is irradiated by the simulated sunlight irradiation, the e − -h + pairs are separated, e − is excited to CB of g-C 3 N 4 , h + remains at VB of g-C 3 N 4 . Then e − transfers to Ag NPs due to the high Schottky barrier of Ag, finally, transfers to the photocatalyst surface to join the reduction reaction. The generated e − from two routes one from the plasmon excited Ag NPs and the other from the photoexcited g-C 3 N 4 nanosheets. These e − react with O 2 to generate O 2−• , and O 2−• radicals can discompose of RhB molecules to CO 2 and H 2 O. Thus it is concluded that the adsorbed silver NPs have two functions, one is as the electron pool and the other is capture of the photoinduced electrons.

CONCLUSION
In this work, Ag NPs modified g-C 3 N 4 nanosheets are successfully prepared by a simple liquid-phase reduction method. In the Ag/g-C 3 N 4 composites, the Ag NPs uniformly coasted on the g-C 3 N 4 surface with the diameter range of 6∼20 nm. After Ag loading, the Ag/g-C 3 N 4 composites expand the visible light response and show an enhanced photocatalytic activity on RhB decomposition. The enhanced photocatalytic activity of Ag/g-C 3 N 4 is due to the two reasons, one is the SPR effect of metal Ag, and another is the decrease of the recombination of the photogenerated e − -h + pairs. Especially, 3%-Ag/g-C 3 N 4 demonstrates the highest photocatalytic activity among the asprepared samples for RhB decomposition, which is 10 times faster than the pure g-C 3 N 4 nanosheet for decomposition of RhB. This work indicates that the Ag/g-C 3 N 4 photocatalyst is one of promising candidates to treat organic pollutants in the waste water.