Scholarly article on topic 'AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light-Driven Photocatalytic Activity and Mechanism'

AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light-Driven Photocatalytic Activity and Mechanism Academic research paper on "Nano-technology"

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Academic research paper on topic "AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light-Driven Photocatalytic Activity and Mechanism"

Nano-Micro Lett. (2016) 8(2):182-192

DOI 10.1007/s40820-015-0076-y CrossMark

ARTICLE

AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light-Driven Photocatalytic Activity and Mechanism

Yongchao Bao1'2 . Kezheng Chen1'3

Received: 21 September 2015/Accepted: 29 October 2015/Published online: 11 December 2015 © The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract The ternary plasmonic AgCl/Ag/g-C3N4 photocatalysts were successfully fabricated by a modified deposition-precipitation method, through which Ag/AgCl nanoparticles (5-15 nm in size) were evenly dispersed on the surface of g-C3N4. The AgCl/Ag/g-C3N4 composites exhibited higher photocatalytic activity than Ag/AgCl and g-C3N4. The enhanced photocatalytic performance could be attributed to an efficient separation of electron-hole pairs through a Z-scheme mechanism, in which Ag nanoparticles acted as charge separation centers.

Keywords AgCl/Ag/g-C3N4 • Hybrid • Photocatalytic activity

1 Introduction

As a new metal-free semiconductor, polymeric graphitic carbon nitride (g-C3N4) has been developed to cope with environmental pollutants due to its outstanding mechanical, optical, electronic, and catalytic properties as well as its high thermal and chemical stability [1-4]. However, its practical application is quite limited owing to its appreciable drawbacks, including low specific surface area, high photogenerated electron-hole recombination rate, and the limited range of visible light photo-responses [5, 6]. To tackle these issues, many methods have been proposed, such as doping extraneous elements [7-10], designing porous structures [11-15], depositing noble metals [16-

& Kezheng Chen kchen@qust.edu.cn

1 Lab of Functional and Biomedical Nanomaterials, Qingdao University of Science and Technology, Qingdao 266042, People's Republic of China

2 College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao 266042, People's Republic of China

3 College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, People's Republic of China

19], and coupling with other semiconductors [20-23]. Although some progresses have been achieved, the light harvesting ability and quantum efficiency of these modified g-C3N4 systems are still poor.

Noble metal nanoparticles have attracted considerable attention due to their application as active components for the preparation of various efficient visible light photocat-alysts. Logar et al. reported Ag/TiO2 plasmonic photocat-alyst that showed high efficiency for degradation of methyl orange (MO) [24]. Parida et al. developed Au/g-C3N4 plasmonic photocatalyst with enhanced photocatalytic activity under irradiation of visible light [17]. Besides, Ag/ AgCl [25-27], Ag/AgBr [28], and Ag/AgI [29, 30] have also been used as co-catalysts to enhance the photocatalytic activity of semiconductors under visible light irradiation. It is believed that noble metal nanoparticles can act as active sites and play vital roles in effective visible light absorption and subsequent photocatalytic reactions. The possible reason is that noble metal nanoparticles can strongly absorb visible light because of their localized surface plasmon resonance (LSPR), which can be tuned by varying their size, shape, and surroundings [31].

Recently, two types of Ag/AgCl/g-C3N4 composites were fabricated by Yao et al. [32] and Zhang et al. [33]. The as-prepared products showed efficient photocatalytic degradation activity. However, their precipitation methods

not only lack precise control over the morphology and size of final products, but also induce reunion reaction of AgCl [34].

In this work, AgCl/Ag/g-C3N4 composites were fabricated by a modified deposition-precipitation method to overcome the above shortcomings. The photocatalytic activities of the AgCl/Ag/g-C3N4 composites were evaluated by the photocatalytic degradation of Rh B and MO aqueous solution under irradiation of visible light. A plasmonic Z-scheme photocatalytic mechanism was then proposed to explain the enhancement of the photocatalytic activity of the AgCl/Ag/g-C3N4 photocatalysts.

2 Experimental

All reagents supplied by Sinopharm Chemical Reagent co., Ltd. are of analytical grade and used as received without further purification.

2.1 Preparation of Photocatalysts

2.1.1 Preparation of g-C3N4 Powders

The metal-free g-C3N4 powders were fabricated by heating melamine in a muffle furnace. Typically, 5 g of melamine was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 550 °C at a heating rate of 10 °C min-1 and held for 4 h. After the reaction, the alumina crucible was cooled to room temperature. The products were collected and ground into powders.

2.1.2 Preparation of AgCl/Ag/g-C3N4 Hybrid Composites

In a typical preparation process, 0.4 g of g-C3N4 powders and 0.64 g of hexadecyl trimethyl ammonium chloride (CTAC) were added into 200 mL of deionized water, and the suspension was stirred for 30 min and sonicated for 30 min. Then, 4.4 mL of 0.1 M AgNO3 was quickly added to the above mixture. During this process, the excessive surfactant CTAC not only adsorbed onto the surface of g-C3N4 to limit the number of nucleation sites for AgCl to grow, resulting in homogenously dispersed AgCl, but also induced Cl- to precipitate Ag? in the suspension. The resulting suspension was stirred for 1 h and then placed under irradiation of 300 W Xe lamp for 30 min. The suspension was filtered, washed using deionized water, and dried at 80 °C for 8 h. And then, the gray powder was calcined at 300 °C for 3 h. Different molar ratios of AgCl/ Ag/g-C3N4 photocatalysts (3, 5, 10, 15, 20, and 40 at%) were fabricated with the similar procedure.

2.2 Characterization of Photocatalysts

X-ray diffraction (XRD) data were collected on a D-MAX 2500/PC diffractometer (Japan). The surface morphologies of the as-prepared samples were characterized with field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100F). Photoluminescence (PL) spectra were measured at room temperature on F-4600 fluorescence spectrometer (Hitachi, Japan) with an excitation wavelength of 365 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250 X-ray photoelectron spectrometer. The Fourier transform infrared spectra (FTIR) of the samples were recorded using IRAffinity-1 spectrometer (Shimadzu, Japan). Ultraviolet visible (UV-Vis) diffuse reflectance spectra (DRS) of the samples were obtained on an UV-vis spectrophotometer (Shimadzu UV-2600, Japan) in the range of 200 to 800 nm and BaSO4 as a standard reference. The Brunauer-Emmett-Teller (BET) surface area (SBET) of the samples were measured by nitrogen adsorption-desorption isotherm measurements on a micromeritics ASAP2020 system. The electron spin resonance (ESR) signals of OH and O2- radicals spin-trapped with the spin-trap reagent DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) in water and methanol were examined on an ESR spectrometer (ER200-SCR, Germany), respectively.

2.3 Adsorption Experiment

Adsorption experiments were carried out in the dark. In a typical adsorption procedure, 100 mg of 5 at% AgCl/Ag/g-C3N4 powers mixed with 100 mL 10 mg L- Rh B aqueous solution in a glass conical beaker was shaken at ambient temperature. At given time intervals, about 3 mL solution suspension was sampled and immediately cen-trifuged. The concentration of Rh B solution was analyzed with a UV-Vis spectrophotometer at the maximal absorption wavelength of Rh B, whose characteristic absorption peak was chosen to be 554 nm.

2.4 Test of Photocatalytic Activity

The photocatalytic activities of samples were evaluated by the degradation of Rh B and MO under 300 W Xe lamp with a 420 nm cutoff filter. In brief, 100 mg of photocat-alyst was dispersed in 100 mL of a 10 mg L-1 aqueous solution of Rh B or MO in a reactor with a double layer cooled by running water to keep the temperature unchanged. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium between the photocata-lyst and dye molecules. Then, the suspension was

illuminated by the Xe lamp combined with magnetic stirring. At given time intervals, about 3 mL solution suspension was sampled and centrifuged. The concentrations of Rh B and MO were measured by UV-Vis spectrophotometer. Additionally, the recycle experiments were performed for five consecutive cycles to test the durability. After each cycle, the catalyst was centrifuged and washed thoroughly with distilled water several times to remove residual dye impurities and then dried at 80 °C for the next test.

3 Results and Discussion

3.1 Characterization of Photocatalysts

The crystal structures of the as-prepared samples were analyzed by the XRD pattern. Figure 1 shows the XRD patterns of the pure g-C3N4 and AgCl/Ag/g-C3N4 hybrid composites with different Ag/AgCl contents. It is observed that two broad peaks around 27.4° and 13.0° in the XRD patterns of the pure g-C3N4 are well ascribed to the (002) and (100) diffraction planes, respectively. The former, which corresponds to the interlayer distance of 0.326 nm, is attributed to the long-range interplanar stacking of aromatic units; the latter, with a much weaker intensity, which corresponds to a distance d = 0.681 nm, is associated with interlayer stacking [35]. It is obvious that the diffraction peaks at 27.7°, 32.2°, 46.2°,54.8°,57.5°,67.4°, 74.5°, and 76.7° gradually appear and the intensity increases with the increase of Ag/AgCl content, and the peaks are assigned to the (111), (200), (220), (311), (222), (400), (331), and (420) planes of AgCl crystal, respectively. A weak

10 20 30 40 50 60 2 Theta (degree)

diffraction peak at 38.2° is also seen, which corresponds to the (111) plane of Ag crystal. It is difficult to distinguish the characteristic peak of g-C3N4 (27.4°) and the AgCl (111) plane (27.8°) because they are very close to each other. However, with the increase in the amount of Ag/ AgCl, the relative intensity of (111)/(200) diffraction of AgCl decreases. This result could be attributed to pure AgCl, the relative intensity of (111) diffraction is about one half of that of (200) diffraction. Therefore, in the XRD analysis, no other crystal phases are observed, which indicated that the AgCl/Ag/g-C3N4 composites were successfully fabricated.

The morphology and microstructure of the as-prepared samples were investigated by SEM and TEM analysis. Figure 2 shows the SEM images of the pure g-C3N4 and the 5 at% AgCl/Ag/g-C3N4 composite. The pure g-C3N4

Fig. 1 XRD patterns of a pure g-C3N4, b 5 at% AgCl/Ag/g-C3N4 composite, c 15 at% AgCl/Ag/g-C3N4 composite, d 20 at% AgCl/Ag/ g-C3N4 composite, and e 40 at% AgCl/Ag/g-C3N4 composite

Fig. 2 Typical SEM images of a pure g-C3N4 and b 5 at% AgCl/Ag/ g-C3N4 composite

sample displays an aggregated morphology with a large size and lamellar structure. The surface of the aggregation is very smooth, showing the layer structure of g-C3N4. Figure 2b shows Ag/AgCl nanoparticles which might be deposited on the surface of g-C3N4 and a rough surface was obtained. It is observed from Fig. 3 that the g-C3N4 displays a lamellar shape and has an amorphous structure, whereas the Ag/AgCl exhibits uniform spherical particles. It is also seen that there are dark spherical particles and gray areas. The dark particles can be assigned to Ag/AgCl, whereas the gray areas can be assigned to g-C3N4. The Ag/ AgCl particles are evenly dispersed on the surface of g-C3N4, and are approximately 5-15 nm in size. Figure 3c shows that the lattice fringe of 0.236 and 0.277 nm, corresponding to the (111) plane of Ag and (200) plane of AgCl, is clearly observed in the AgCl/Ag/g-C3N4 composite, which verifies the formation of an AgCl/Ag/g-C3N4 heterojunction.

The chemical composition of the as-prepared 5 at% AgCl/Ag/g-C3N4 was analyzed by XPS, as shown in Fig. 4. The survey spectrum of 5 at% AgCl/Ag/g-C3N4 shows peaks of elements of Ag, Cl, C, N, and O. No peaks for other elements is found, indicating that the hybrid composite is primarily composed of Ag, Cl, C, and N elements. To investigate and demonstrate the different chemical states of Ag, C, N, and Cl, the high-resolution XPS peaks of the different elements are provided in Fig. 5. It can be seen from the C1s spectrum that the two C1s peaks are located at 284.7 and 288.0 eV. The former is ascribed to the adventitious contamination and defect-containing sp -hybridized carbon atoms present in graphitic domains, whereas the latter one is assigned to C-N-C coordination [35, 36]. In the N1s spectrum, several binding energies can be separated. The main N1s peak at 398.4 eV corresponds to sp2-hybridized aromatic N bonded to C atoms (C=N-C). The peak at 399.3 eV is assigned to the

200 400 600

Binding energy (eV)

Fig. 4 XPS survey spectrum of 5 at% AgCl/Ag/g-C3N4 composite

tertiary N bonded to C atoms in the form of N-(C)3. The peak at 400.7 eV is from the N-H structure. The weak peak at 403.9 eV is attributed to charging effects [4]. The highresolution XPS spectra of Ag3d are shown in Fig. 5c. The two peaks at approximately 368.3 and 374.2 eV can be ascribed to the binding energies of Ag3d5/2 and Ag3d3/2, respectively. These two peaks can be further deconvoluted into two peaks, at about 367.9/368.6 eV and 373.9/ 374.6 eV, respectively [37]. The peaks at 367.9 and 373.9 eV are attributed to Ag? of AgCl, and those at 368.6 and 374.6 eV are ascribed to the metal Ag0. On the basis of XPS peak areas, the mole ratio between Ag0 and Ag? was calculated to be 2:3. The Cl2p XPS peaks can also be resolved into two typical peaks, 197.9 and 199.5 eV, which are ascribed to AgCl [38]. All of these results further confirm the coexistence of Ag/AgCl and g-C3N4 in the AgCl/Ag/g-C3N4 composite.

Fig. 3 TEM images of a pure g-C3N4 and b 5 at% AgCl/Ag/g-C3N4 composite. c HRTEM images of 5 at% AgCl/Ag/g-C3N4 composite

280 282 284 286 288 290 292 294 Binding energy (eV)

(c)Ag 3d5/2

364 366 368 370 372 374 376 378 380 Binding energy (eV)

394 396 398 400 402 404 406 Binding energy (eV)

Cl 2P3/2

,|Cl 2pi/2

192 194 196 198 200 202 204 206 Binding energy (eV)

Fig. 5 High-resolution XPS spectra of 5 at% AgCl/Ag/g-C3N4 composite: a C1s, b N1s, c Ag3d, and d Cl2p

—AgCl/Ag/g-C3N4(40%) —AgCl/Ag/g-C3N4(20%) —AgCl/Ag/g-C3N4(15%) —AgCl/Ag/g-C3N4(10%) AgCl/Ag/g-C3N4(5%) AgCl/Ag/g-C3N4(3%) — Pure g-C3N4

200 300 400 500 600 Wavelength (nm)

Fig. 6 UV-Visible diffuse reflectance spectra of pure g-C3N4 and AgCl/Ag/g-C3N4 composites

The UV-Vis diffuse reflectance spectra of the as-prepared samples are shown in Fig. 6. Pure g-C3N4 has an absorption edge at about 460 nm, which originates from its

band gap of 2.7 eV. Compared with the pure g-C3N4, the series of AgCl/Ag/g-C3N4 hybrid composites show a slight red shift of the absorption edge, which is attributed to the surface plasmon resonance (SPR) effect of Ag nanocrystal formed in situ on the surfaces of the AgCl nanoparticles. In addition, the absorption intensities of AgCl/Ag/g-C3N4 composites show a significant enhancement in the visible light regions, which can also be attributed to the SPR effect of Ag nanoparticles.

PL spectra analysis has been widely used to investigate the separation efficiency of photogenerated electrons and holes in semiconductor particles [39]. Figure 7 shows the PL spectra of pure g-C3N4 and AgCl/Ag/g-C3N4 composites. It can be found that the strong emission peak of the pure g-C3N4 centered at 460 nm suggests a high recombination probability of the photogenerated electron-hole pairs, while for AgCl/Ag/g-C3N4 composites, a significant quenching of PL is observed in comparison with g-C3N4, which indicates that these composites have lower recombination rates of photogenerated electrons and holes. This demonstrates that after the formation of a heterojunction between g-C3N4 and Ag/AgCl, the recombination of

— Pure g-C3N4

X AgCl/Ag/g-C3N4(5%)

\ AgCl/Ag/g-C3N4(20%)

\ AgCl/Ag/g-C3N4(40%)

---_ , , -----

shows that most of the pores fall into the size range from 3 to 100 nm. The specific surface area of the 5 at% AgCl/Ag/ g-C3N4 composite was calculated to be 11.46 m g- , which is lower than that of pure g-C3N4 (14 m2 g-1). The decrease of the surface area after decoration can be attributed to the disappearance of the small pore, which can be further proved by the pore size distribution shown in the inset of Fig. 8. The results illustrate that the enhanced photocatalytic performance is not a result of the change of the BET surface areas of the samples.

400 450 500 550 Wavelength (nm)

Fig. 7 PL spectra of pure g-C3N4 and AgCl/Ag/g-C3N4 composites

photogenerated charge carriers is greatly suppressed. Therefore, the photogenerated electron-hole pairs of the AgCl/Ag/g-C3N4 composites can efficiently transfer at the interface of heterostructure, resulting in the higher photo-catalytic activity than pure g-C3N4 and Ag/AgCl.

Full nitrogen adsorption isotherms of pure g-C3N4 and 5 at% AgCl/Ag/g-C3N4 composite were measured to gain the information about the specific surface area, as shown in Fig. 8. It is true that the pure g-C3N4 and the 5 at% AgCl/ Ag/g-C3N4 composites have type IV isotherms and type H3 hysteresis loops, which indicates the mesoporous structure of the samples. The pore size distribution of the samples

3.2 Adsorption Kinetics

The adsorption kinetics of Rh B on 5 at% AgCl/Ag/g-C3N4 is shown in Fig. 9. The adsorption capacity of Rh B increases firstly and then remains unchanged with time. The adsorption equilibrium is achieved after about 30 min. The adsorption kinetics can be well fitted by a pseudo-second-order model, expressed by Eq. (1) t 1 t

_ = T2 + - (1)

—t k-2 —e

where k is the rate constant [g (mg min)-1], qe is equilibrium adsorption capacity (mg g-1), and qt is the amount of Rh B (mg g-1) adsorbed at time t. By plotting t/qt versus t, one can determine the values of qe and k from the slope and intercept of the fitted line, respectively. The correlation coefficient (R2) value of the fitted plot is near unity

Fig. 8 N2 adsorption-desorption isotherms of pure g-C3N4 and 5 at% AgCl/Ag/g-C3N4 composite. The inset shows the corresponding BJH pore size distribution curves of the samples

Fig. 9 a The adsorption-kinetics curve and b Pseudo-second-order kinetics of Rh B on 5 at% AgCl/Ag/g-C3N4 at 25 °C

(0.9999), indicating the applicability of the pseudo-second-order model for the above adsorption kinetics.

3.3 Photocatalytic Activities of the AgCl/Ag/g-C3N4 Hybrid Composites

The photocatalytic activities of the as-prepared AgCl/Ag/g-C3N4 composites were evaluated by monitoring the degradation of Rh B in an aqueous solution under visible light irradiation. For comparison, the performances of pure g-C3N4 and Ag/AgCl photocatalyst were also investigated under same condition. Figure 10 shows the photocatalytic activities of pure g-C3N4, Ag/AgCl, and AgCl/Ag/g-C3N4

20 30 40 Irradiation time (min) ■ Ag/AgCl -»-AgCl/Ag/g-C3N4(10%)

-•- Pure g-C3N4 AgCl/Ag/g-C3N4(15%)

AgCl/Ag/g-C3N4(3%) -♦- AgCl/Ag/g-C3N4(20%) AgCl/Ag/g-C3N4(5%) -•- AgCl/Ag/g-C3N4(40%)

Fig. 10 Photocatalytic activities of pure g-C3N4, Ag/AgCl, and AgCl/Ag/g-C3N4 composites on the degradation of Rh B under visible light irradiation

0 10 20 30 40 50 60 70 80 90 Irradiation time (min)

Fig. 11 Photocatalytic activities of pure g-C3N4, Ag/AgCl, and 5 at% AgCl/Ag/g-C3N4 composite on the degradation of MO under visible light irradiation

tl0le(tub) 60

Fig. 12 Cycling degradation efficiency of Rh B solution in the presence of 5 at% AgCl/Ag/g-C3N4 composites under visible light irradiation

Before reaction

After reaction

0 200 400 600 800 1000 364 366 368 370 372 374 376 378 380 Binding energy (eV) Binding energy (eV)

Fig. 13 a XPS survey spectra of 5 at% AgCl/Ag/g-C3N4 before and after 5 recycling runs. b Ag3d XPS spectra of 5 at% AgCl/Ag/g-C3N4 before and after 5 recycling runs

Irradiation time (min)

Fig. 14 Photocatalytic performances for the degradation of Rh B with 5 at% AgCl/Ag/g-C3N4 composite with different sacrificial agents under visible light irradiation

composites. It can be clearly observed that the photocat-alytic activities of AgCl/Ag/g-C3N4 composites increase firstly and then decrease with the increase of Ag/AgCl content, and the 5 at% AgCl/Ag/g-C3N4 composite exhibits the highest photocatalytic activity. The decreased photo-catalytic activity may be attributed to the fact that the higher content of Ag/AgCl may easily result in the agglomeration of Ag/AgCl particles causing a low dis-persibility on the surface of g-C3N4. This influences the transfer of photogenerated charge carriers. The degradation efficiency of 5 at% AgCl/Ag/g-C3N4 photocatalyst exceeds 1.14 times more than that of Ag/AgCl and 2.7 times larger than that of g-C3N4.

The photocatalytic activities of pure g-C3N4, Ag/AgCl, and 5 at% AgCl/Ag/g-C3N4 hybrid composite for the degradation of MO were also investigated, as shown in Fig. 11. These results indicate that the photocatalytic activity of 5 at% AgCl/Ag/g-C3N4 hybrid composite

Fig. 15 Schematic illustration of the charge separation and transfer in the AgCl/Ag/g-C3N4 composites under visible light irradiation

toward MO is also much higher than that of either pure g-C3N4 or Ag/AgCl. It suggests that a suitable molar ratio between g-C3N4 and Ag/AgCl is significant for effectively enhancing the photocatalytic activity.

Because renewable catalytic was another important factor for a photocatalyst, the stability of the 5 at% AgCl/ Ag/g-C3N4 composite was investigated by a recycling test, as shown in Fig. 12. After five cycles, there was no significant loss of activity, indicating that the photocatalyst was stable during the photocatalytic test.

The excellent stability was further confirmed by the XPS of 5 at% AgCl/Ag/g-C3N4 composite before and after photodegradation. Figure 13 shows that there is no noticeable difference between the sample before and after 5 recycling runs. These results indicated that the chemical

(a)DMPO-O2 An Light on 5 min Dark

3460 3480 3500 3520 3540 3560 3460 3480 3500 3520 3540 3560 Magnetic field (G) Magnetic field (G)

Fig. 16 DMPO spin-trapping ESR spectra of 5 at% AgCl/Ag/g-C3N4 composite in a methanol dispersion (for DMPO — -O2—), b aqueous dispersion (for DMPO — OH)

states of the AgCl/Ag/g-C3N4 composite surface remained almost unchanged during the photocatalytic degradation of Rh B or MO.

3.4 Photocatalytic Mechanism

To further investigate the photocatalytic mechanism of the AgCl/Ag/g-C3N4 hybrid composites, a series of radicals trapping experiments were performed by using ammonium oxalate (AO), N2 and t-butyl alcohol (TBA) scavengers for holes, O2— and OH radicals, respectively. As is clear from Fig. 14, the addition of TBA did not affect the degradation rate of Rh B over 5 at% AgCl/Ag/g-C3N4 composite, suggesting that -OH was not the main reactive species in the photocatalytic process. On the contrary, the photocat-alytic degradation of Rh B was obviously suppressed after the addition of AO and N2 purging. According to these results, it can be clearly seen that h? and O2— are main reactive species for 5 at% AgCl/Ag/g-C3N4 hybrid composite in the photocatalytic degradation process of Rh B under visible light irradiation.

The g-C3N4 with a band gap of 2.7 eV is a novel metalfree visible light photocatalyst [40]. Under visible light irradiation, the g-C3N4 absorbs visible light photons to produce photogenerated electrons and holes. The photogenerated electrons react with O2 that existed in the photodegradation system, reducing it to superoxide radical anion O2—. The dye molecules are degradated by photogenerated holes and O2—. Ag/AgCl has been demonstrated to be an efficient visible light photocatalyst [31]. As AgCl cannot absorb visible light due to its wide band gap of 3.25 eV, the visible light absorption in Ag/AgCl is attributed to the plasmonic absorption of Ag nanoparticles which can absorb visible light and convert the plasmonic energy into LSPR oscillation. Then, the plasmon-induced electrons from the photoexcited Ag nanoparticles transfer to

the CB of AgCl and the electrons on the surface of AgCl are trapped by the adsorbed O2 to form O2— active species, and the plasmon-induced holes stay on the surface of Ag nanoparticles and oxidize the dye molecules [41, 42].

When the Ag/AgCl nanoparticles are coupled with g-C3N4 to form AgCl/Ag/g-C3N4 composites, a hetero-junction structure is formed in the interface between g-C3N4 sheets and Ag/AgCl nanoparticles. The improved photocatalytic performances are mainly attributed to the separation efficiency of photogenerated electrons and holes in the composites. On the basis of the above results, a plasmonic Z-scheme mechanism of AgCl/Ag/g-C3N4 composites is proposed and illustrated in Fig. 15. Under visible light irradiation, both Ag and g-C3N4 absorb visible light photons to produce photogenerated electrons and holes. The plasmon-induced electrons of Ag nanoparticles are transported to the CB of AgCl to reduce oxygen, while the holes remain on the Ag nanoparticles. Meanwhile, the photogenerated electrons of g-C3N4 transfer to the Ag nanoparticles to recombine with the plasmon-induced holes produced by plasmonic absorption of Ag nanoparticles, while the VB holes remain on g-C3N4 to oxidize organic substances. Therefore, for the AgCl/Ag/g-C3N4 composites, the reduction active site is on the CB of AgCl while the oxidation active site is on the VB of g-C3N4, it is because Ag nanoparticles act as the charge separation center to form the visible light-driven AgCl/Ag/g-C3N4 Z-scheme system.

The photocatalytic mechanism is further investigated by the ESR technique. DMPO was generally applied to trapping radicals of O2— and OH. As shown in Fig. 16, O2— and OH radicals could be detected by the ESR under visible light irradiation. The signal of O2— is much stronger than that of OH. Considering the band structure of g-C3N4, the VB holes (1.40 eV) from g-C3N4 cannot directly oxidize OH—/H2O into OH radicals (1.99 and

2.38 eV for OH-/OH and H2O/OH potential). The OH radicals should be generated via the O2-?H2O2 ? OH route [43]. This fact demonstrates that O2- radicals and h ? are the main active species which play important roles during the photodegradation process. Based on the result, it demonstrates again that the transport process of the pho-toexcited carriers of the AgCl/Ag/g-C3N4 composites is in accordance with Fig. 15. Thus, the AgCl/Ag/g-C3N4 composites exhibit excellent photocatalytic performance through Z-Scheme photocatalytic mechanism under visible light.

4 Conclusion

The hybrid AgCl/Ag/g-C3N4 photocatalysts were successfully fabricated by a modified deposition-precipitation method, which was effective for the control of photocata-lyst morphology and size. The Ag/AgCl particles with the size of approximately 5-15 nm were evenly dispersed on the surface of g-C3N4. The AgCl/Ag/g-C3N4 composites exhibited the higher photocatalytic performance than Ag/ AgCl and g-C3N4 over the degradation of Rh B or MO dyes, which was attributed to Ag nanoparticles act as the charge separation center to form the visible light-driven AgCl/Ag/g-C3N4 Z-scheme system. This study provides new insight into the design of highly efficient and stable g-C3N4-based plasmonic Z-Scheme photocatalysts and facilitates their practical application.

Acknowledgments The authors are grateful to the financial aid from the National Natural Science Foundation of China (NSFC No. 51472133).

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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