Ag2WO4 composite for enhanced photocatalytic activity of contaminants degradation

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Contents lists available at ScienceDirect

Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

AgBr and GO co-decorated g-C3N4/Ag2WO4 composite for enhanced photocatalytic activity of contaminants degradation

T

⁎

Yi Wanga, Xinyan Xiaoa, , Jiayi Chena, Xingye Zenga,b a School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, China b College of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China

A R T I C LE I N FO

A B S T R A C T

Keywords: g-C3N4/Ag2WO4/AgBr/GO composite CTAB-assisted co-deposition method n-n heterojunction Internal electric filed Photocatalytic mechanism

A novel AgBr and GO co-decorated g-C3N4/Ag2WO4 (g-C3N4/Ag2WO4/AgBr/GO) composite was successfully fabricated by hexadecyl trimethyl ammonium bromide (CTAB)-assisted co-deposition of AgBr and GO onto gC3N4/Ag2WO4 composite. The as-prepared composites were characterized by XRD, SEM, TEM, FT-IR, BET, XPS, UV–vis diffuse reflectance spectra (DRS), photoluminescence spectra (PL), and electrochemistry impedance spectroscopy (EIS). The photocatalytic efficiency for tetracycline (TC) degradation over the as-prepared g-C3N4/ Ag2WO4/AgBr/GO composite is 91.64% within 100 min simulated sunlight irradiation, which is nearly 2.59 and 5.25-folds greater than those of the g-C3N4/Ag2WO4 and g-C3N4 samples. The enhancement of photocatalytic performance is mainly ascribed to the introduction of AgBr and GO, which serve as photosensitizer and electron acceptor, respectively, confirmed by the results of DRS, PL, and EIS. The active species trapping experiments confirmed that %O2− and h+ are the dominant reactive species responsible for TC degradation. Furthermore, a reasonable n-n heterojunction-based photocatalytic mechanism for the g-C3N4/Ag2WO4/AgBr/GO composite was proposed.

1. Introduction Energy shortage, environmental pollution, and global warming problems are inevitable in recent years. Semiconductor-based photocatalysis technology can convert solar energy into chemical energy, recognized as an effective and sustainable strategy to resolve the energy and environment problems [1–3]. However, the practical application of semiconductor-based photocatalysis technology was restricted by the limitation in the absorption of visible light, insufficient carrier separation and poor photochemical stability of photocatalyst [4,5]. To address these challenges, many researchers have devoted to the fabrication of novel semiconductor-based photocatalysts [6–10]. Carbon-based nanomaterials have attracted much attention in the researches of visible-light-responded photocatalysts, because of their economical cost, nontoxicity, superior electrochemical property and stability [11,12]. Graphitic carbon nitride (g-C3N4) is a potential carbon-based material for visible-light-driven photocatalysis, owing to its narrow band gap of 2.7 eV [13,14]. Moreover, g-C3N4 has a melonbased two-dimensional (2D) π-conjugated planes structure with numerous reactive sites, while pristine g-C3N4 was stacked by layered gC3N4 nanosheets [15,16]. Thus, many efforts have been made to

⁎

exfoliate the pristine g-C3N4 into g-C3N4 nanosheets that contain more active sites and have shorter charge carrier transport distance [17,18]. However, the fast charge carrier recombination and poor light utilization had a negative impact on the photocatalytic performance of pure gC3N4 [19]. Recently, metal tungstates [20], sulfides [21], oxides [22], halides [23], and other semiconductors [24–28] have been selected to construct semiconductor-based heterojunction with g-C3N4, proved to be the promising strategy to overcome these difficulties. Among these semiconductors, silver (Ag)-containing compounds, such as AgX (X = Cl−, Br−, I−) [29], Ag2WO4 [30], Ag2O [31], Ag3PO4 [32], and Ag2CO3 [33], have been regarded as the great materials for constructing g-C3N4-based heterojunction, because of the filled 4d10 electronic configurations of Ag+ in the most of compounds. The corresponding results showed that the visible light absorption, the transport and separation efficiencies of charge carriers were adjusted, and thus the photocatalytic performance of g-C3N4-based Ag-containing composites was improved. Recently, coupling g-C3N4 with silver tungstate (Ag2WO4) with a wide band gap of 3.1 eV was used in many photocatalytic processes. Zhu et al. have prepared a g-C3N4/Ag2WO4 composite by an in-situ synthesis [34]. The enhanced photocatalytic

Corresponding author. E-mail address: [email protected] (X. Xiao).

https://doi.org/10.1016/j.jphotochem.2019.111957 Received 19 March 2019; Received in revised form 26 June 2019; Accepted 2 July 2019 Available online 03 July 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Scheme 1. Schematic illustration for the fabrication of g-C3N4/Ag2WO4/AgBr/GO.

2. Experimental section

performance of methyl orange (MO) degradation over the g-C3N4/ Ag2WO4 composite was attributed to a direct Z-scheme photocatalytic mechanism. Besides, silver bromide (AgBr), with a narrow band gap of 2.6 eV, was widely applied in visible-light-driven photocatalytic processes. Feizpoor and Habibi-Yangjeh have fabricated a ternary TiO2/ Ag2WO4/AgBr plasmonic nanocomposite by a one-pot strategy [35]. Comparing with the TiO2, TiO2/Ag2WO4, and TiO2/AgBr, the ternary nanocomposites exhibited higher photocatalytic activity under visible light for Rhodamine B (RhB), fuchsine and MO degradation, which was attributed to the tandem n-n heterojunctions. However, Ag-containing compounds were unstable, since metallic Ag0 would generate from them under light irradiation. And excessive Ag0 particles loading on the surface might present a shielding effect for the incidence of light [36,37]. To address this drawback of Ag-containing compounds, coupling with other suitable materials, such as g-C3N4, CNTs [38], graphene oxide (GO) [39], TiO2 [40], MoS2 [41], has been proved to be an effective method to suppress its photo-corrosion. Except for building semiconductor-based heterojunctions, introducing graphene acted as an electron-trapping material is also an effective strategy to promote the transfer and separation of charge carriers [42,43]. GO has been considered as promising catalyst support as well as a promoter (GO modified g-C3N4), because of its superior electron mobility, large specific surface area, flexibility, and excellent chemical stability [44–46]. The unique property of g-C3N4, Ag2WO4, AgBr, and GO is projected to be ideal for constructing a promising composite. Though several researches about g-C3N4/Ag2WO4, g-C3N4/AgBr, and Ag2WO4/AgBr composites have been reported, the photocatalytic performance and photochemical stability of g-C3N4 modified by Ag2WO4 were still not satisfied. Thus, an AgBr and GO co-decorated g-C3N4/Ag2WO4 (g-C3N4/ Ag2WO4/AgBr/GO) composite was fabricated by CTAB-assisted co-deposition route in the present work. Typically, a g-C3N4/Ag2WO4 composite was prepared by a simple in-situ growth method. And the gC3N4/Ag2WO4/AgBr/GO composite was fabricated through a CTABassisted co-deposition method. The photocatalytic activity of the asprepared samples was evaluated by degrading tetracycline (TC). The dominant reactive species for the TC degradation were determined by the active species trapping experiments. And a reasonable n-n heterojunction-based photocatalytic mechanism was proposed.

2.1. Materials Melamine, sodium tungstate (Na2WO4·2H2O), hexadecyl trimethyl ammonium bromide (CTAB), and silver nitrate (AgNO3) disodiumethylenediaminetetraacetate (EDTA-2Na), 1,4-benzoquinone (BQ), and isopropyl alcohol (IPA) were purchased from Aladdin Chemistry Co. Ltd. Tetracycline (TC), RhB, and absolute alcohol (C2H5OH, analytical reagent) were obtained from Sinopharm Chemical Reagent Co., Ltd. All the materials are employed without further purification. 2.2. Synthesis of photocatalysts 2.2.1. g-C3N4 nanosheets, GO nanosheets and Ag2WO4 microrods Bulk g-C3N4 was synthesized by pyrolysis of melamine in an air atmosphere. Typically, 10 g melamine was heated at 550℃ for 3 h at a heating rate of 5℃/min. After cooling naturally, the resultant powders were further heated to 500℃ and maintained for 3 h to prepare g-C3N4 nanosheets. GO nanosheets were synthesized through modified Hummer’s method following the previous work [47]. The Ag2WO4 microrods were obtained via a deposition method as follows: 0.3 mmol of Na2WO4 was dissolved in 50 mL of deionized water to form solution (A), and 50 mL of AgNO3 (0.3 mmol) solution (B) was added dropwise into the above solution A. After stirring for 2 h in the dark, the Ag2WO4 were collected and washed 3 times with deionized water and ethanol, then dried at 60℃ for 12 h. 2.2.2. g-C3N4/Ag2WO4/AgBr/GO The g-C3N4/Ag2WO4/AgBr/GO composite was fabricated by in-situ deposition and CTAB-assisted co-deposition method (as shown in Scheme 1). Firstly, the g-C3N4/Ag2WO4 composite was prepared via a simple insitu deposition method. 0.2 g of g-C3N4 nanosheets was dispersed into 50 mL deionized water by ultrasonic treatment. 25 mL of AgNO3 (0.25 mmol) solution was slowly added into the g-C3N4 nanosheets suspension and keep stirring for 1 h to reach Ag+ sorption equilibrium. Then the g-C3N4-Ag+ suspension was followed by drop-wise addition of Na2WO4 (0.25 mmol, 25 mL) solution. After stirring for 2 h, the g-C3N4/ Ag2WO4 suspension (A) was obtained. 2

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 1. XRD patterns: (a) g-C3N4, Ag2WO4, g-C3N4/Ag2WO4, and g-C3N4/Ag2WO4/GO; (b) AgBr; (b, c) g-C3N4/Ag2WO4/AgBr and g-C3N4/Ag2WO4/AgBr/GO.

Chenhua, Shanghai, China) in a typical three-electrode cell.

Secondly, an appropriate amount of GO and CTAB were dispersed in deionized water with the assistance of ultrasonication to form a uniform CTAB-modified GO suspension (B). Then the suspension B was added dropwise into suspension A and keeping stirring for another 2 h. The asprepared products (donated as g-C3N4/Ag2WO4/AgBr/GO) were collected by centrifugation, washing with deionized water and ethanol, following dry at 60℃ for 12 h. Similarly, the g-C3N4/Ag2WO4/AgBr was prepared via the same method but without GO.

2.4. Photocatalytic performance The photocatalytic performance of samples was evaluated by the photocatalytic degradation of organic contaminants (TC and RhB) under simulated sunlight irradiation. The light source was an XG500 xenon long-arc lamp surrounded by water to cool. Briefly, 0.1 g of photocatalyst was dispersed in 0.3 L of 10 mg/L organic contaminant aqueous solution by sonicating. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium between the photocatalysts and organic contaminants. Then the mixed solutions were exposed to light irradiation under stirring, and about 4 mL of suspensions were taken at 20 min intervals and centrifuged for subsequent organic contaminants concentration analysis. The catalyst-free solutions were measured using a Hitachi U-3900H spectrophotometer at 356 nm (or 553 nm) wavelength for TC (or RhB). The photocatalytic degradation efficiency (η) was calculated by Eq. (1):

2.3. Characterization The crystal structures were investigated by X-ray diffraction (XRD, D8 ADVANCE, Bruker) with Cu Kα radiation (λ = 0.15406 nm). The surface morphology and structure analyses were investigated using a field emission scanning electron microscopy (SEM, Hitachi, SU8220) and transmission electron microscope (TEM, JEOL JEM-2100). The Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet Avatar 370 spectrophotometer using the standard KBr as a reference. The Brunauer-Emmett-Teller specific surface area (SBET) was determined on Micromeritics Tristar 3020 analyzer at 77.3 K. The X-ray photoelectron spectroscopy (XPS) analysis was measured by a Thermo Fisher Scientific ESCALAB 250Xi XPS instrument. The UV–vis diffuse reflectance spectra (DRS) were analyzed by Hitachi UV-3010 spectrophotometer using BaSO4 as a reflectance standard. The photoluminescence (PL) spectra were analyzed on a fluorescence spectrometer (Hitachi F-4500). The electrochemistry impedance spectroscopy (EIS) was recorded on an electrochemical analyzer (CHI 660C,

η=

C0 − C × 100% C0

(1)

where C0 and C are the subsequent concentrations of organic contaminants at t0 and t, respectively. 2.5. Recycling experiments The recycling experiments of RhB degradation over the samples were carried out under the same condition. The used samples were 3

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 2. SEM images (a–e) and EDX spectrum (f, g) of g-C3N4 (a), Ag2WO4 (b), g-C3N4/Ag2WO4 (c), g-C3N4/Ag2WO4/AgBr (d), g-C3N4/Ag2WO4/AgBr/GO (e–g).

4

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Table 1 The weight and atomic percentages of g-C3N4/Ag2WO4/AgBr/GO. Element

Weight%

Atomic%

C N O Br Ag W Totals

28.43 45.31 9.40 3.56 9.41 3.90 100.00

37.32 51.00 9.26 0.70 1.38 0.33

centrifuged and washed with deionized water and ethanol after each cycle, then dried at 60℃ for 12 h and weighted. Then the used samples were dispersed into fresh RhB solution and repeated for five times. 2.6. Active species trapping experiments To confirm the main active radical species for TC degradation over g-C3N4/Ag2WO4/AgBr/GO, different scavengers of EDTA-2Na, BQ and IPA were conducted to detect the holes (h+), superoxide radical (%O2−) and hydroxyl radicals (·OH), respectively. During the photocatalytic performance tests, 1 mmol of the scavengers were added into the TC solution. All the measurements were performed at room temperature (25–27 °C).

Fig. 4. FT-IR spectra of g-C3N4, Ag2WO4, g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/ AgBr, g-C3N4/Ag2WO4/GO, and g-C3N4/Ag2WO4/AgBr/GO.

Ag2WO4 and g-C3N4 are similar to their pristine samples. In addition, all the peak intensity of g-C3N4 is weaker than that in pure g-C3N4 and the peak intensity of Ag2WO4 in g-C3N4/Ag2WO4, which can be ascribed of the inhibition of the incident X-ray into samples and the diffraction Xray from samples. After modified by CTAB, the XRD pattern shows new diffraction peaks at 31°, 44.4°, 55.1°, 64.6°, and 73.3°, which can be assigned to the (200), (220), (222), (400), and (420) crystal planes of AgBr [35]. While the characteristic peaks of Ag2WO4 become weak, since the Br− could react with Ag2WO4 and then AgBr would in-situ grow on the Ag2WO4. For the g-C3N4/Ag2WO4/GO and g-C3N4/ Ag2WO4/AgBr/GO composites, their XRD patterns show the similar characteristic peaks with their composites without GO. And the peaks intensity of AgBr is slightly stronger than those of Ag2WO4 in the enlarged XRD pattern of g-C3N4/Ag2WO4/AgBr/GO composite (Fig. 1c), which is contrary to the pattern of the g-C3N4/Ag2WO4/AgBr composite, implying that the more AgBr is exposed on the surface of g-C3N4/ Ag2WO4/AgBr/GO composite. As expected, the diffraction peaks of GO do not appear in the composites, due to the low content of GO. However, the present of GO in the as-prepared composites can be evidenced

3. Results and discussion 3.1. Characterization The XRD patterns of samples are illustrated in Fig. 1. The two diffraction peaks of pristine g-C3N4 are in good agreement with hexagonal phase (JCPDS No. 87-1526). The broad characteristic peak at 27.6° corresponding to the stacking of the conjugated aromatic systems can be indexed to the (002) planes, while the diffraction peak at 13.1° associated with the in-plane structural packing of tri-s-triazine units can be indexed to (100) planes [48]. For the pure Ag2WO4, all diffraction peaks are consistent with the metastable hexagonal-phase β-Ag2WO4 (JCPDS No. 33-1195) [49]. As for the pure AgBr, the pattern possessed the face-centered cubic structure of AgBr (JCPDS No. 06-0438). When Ag2WO4 was deposited on g-C3N4, it can be found that the peaks of

Fig. 3. TEM (a), HRTEM (b) and EDX mapping (c–i) images of g-C3N4/Ag2WO4/AgBr/GO. 5

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 5. XPS spectra of g-C3N4/Ag2WO4/AgBr/GO in survey (a), C 1s (b), N 1s (c), O 1s (d), W 4f (e), Ag 3d (f), and Br 3d (g).

6

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

introduced into the as-prepared composite. The microstructure of the g-C3N4/Ag2WO4/AgBr/GO composite was further analyzed by TEM and HRTEM. Fig. 3a shows that the uniform Ag2WO4/AgBr nanoplates dispersed on the surface of g-C3N4/GO nanosheets. As the HRTEM image (Fig. 3b) shown, g-C3N4/GO displays a typical lamellar structure and the decorated Ag2WO4/AgBr exhibits well defined crystal lattice fringes. The observed d-spacing of 0.297 nm and 0.285 nm can be assigned to the (0 2 2) crystal plane of β-Ag2WO4 and the (2 0 0) crystal plane of AgBr, respectively. As EDX mapping indicated (Fig. 3c–i), the distribution of C and N elements can be seen in the almost whole region, implying that the gray lamellar sheets are gC3N4. And the Ag, W, O and Br elements have an identical distribution in the composite, agreeing well with the dark region in the TEM image. All the results further reveal well assembly of Ag2WO4/AgBr on g-C3N4/ GO nanosheets by CTAB-assisted co-deposition method. The FT-IR spectra of as-prepared samples are shown in Fig. 4. In the pure g-C3N4 FT-IR spectrum, the broad adsorption band in the range of 3300‒3000 cm−1 is ascribed to the stretching vibration modes of NeH bonds, due to the incomplete condensation of amino groups (eNH2). The sharp absorption peak at 809 cm−1 is attributed to the typical breathing mode of tri-s-triazine units, and the other strong peaks from 1750 to 1200 cm−1 indicate the typical stretching modes of CeN heterocycles in g-C3N4 [51]. The Ag2WO4 shows the absorption band at 840 cm−1, implying that the stretching vibrations of OeWeO from distorted [WO6] clusters [52]. All the g-C3N4-based composites present analogous characteristic peaks to g-C3N4, due to the weak absorptions of Ag2WO4 and AgBr sheltered by that strong one from g-C3N4. Similarly, the absorption peaks of GO cannot be detected in the g-C3N4/ Ag2WO4/GO and g-C3N4/Ag2WO4/AgBr/GO composites, because of the low GO contents. The above phenomenon is consistent with the previous researches [53]. The chemical composition of the as-prepared composites and chemical status of elements was confirmed by XPS analysis. The XPS survey spectrum (Fig. 5a) exhibits that the g-C3N4/Ag2WO4/AgBr/GO composite consists of C, N, O, W, Ag and Br elements. The corresponding high-resolution XPS spectra are provided in Fig. 5b–g. The C 1s spectrum shows three deconvoluted peaks at 284.8, 286.3 and 288.3 eV could be ascribed to sp2 CeC, CeO and NeC]N bond in the composite [39,54], respectively. And the weak CeO peak is consistent to the low GO content in the composite. The N 1s spectrum could be well fitted into four peaks at 398.8, 400.3, 401.5 and 404.1 eV, which correspond to the sp2 hybridized nitrogen C]NeC, tertiary nitrogen Ne(C)3, uncondensed terminal amino groups and π-excitation [55,56,34], respectively. The high-resolution O 1s spectra could be fitted into three peaks at 530.6, 531.7 and 532.8 eV. The peak at 530.6 eV could be attributed to the oxygen in eOH group or water species adsorbed on the composite. And the peaks with the banding energy of 531.7 and 532.8 eV could be assigned to the oxygen in Ag2WO4 and OeC [57,39], respectively. The W 4f peaks (Fig. 5e) at 35.5 and 37.6 eV are assigned to the W 4d7/2 and W 4d5/2 binding energies of W (VI) in WO42−, respectively. Fig. 5f displays the Br 3d spectrum, and the peaks at 68.4 and 69.5 eV correspond to Br 3d5/2 and Br 3d3/2, which could be attributed to Br- in the composite. Ag 3d peaks at 368.1 and 374.1 eV in

Fig. 6. Nitrogen adsorption-desorption isotherms and pore-size distribution curves of g-C3N4, g-C3N4/Ag2WO4, and g-C3N4/Ag2WO4/AgBr/GO.

via the results of DRS spectra, EDX and XPS analyses. The morphologies and elemental composition of as-prepared samples were observed and analysed by SEM and EDX, respectively. As observed (Fig. 2a), the pure g-C3N4 exhibits a 2D layered structure composed of numerous wrinkle nanosheets with an average size of 3 μm, might provide accessible sites for the deposition of Ag2WO4. And the morphology of pure Ag2WO4 is in the form of 1D rod-like structures, organized randomly from several microrods with lengths of 3–9 μm and diameters of approximate 200–500 nm (Fig. 2b). For the g-C3N4/ Ag2WO4 composite (Fig. 2c), numerous 1D nanorods with lengths of about 60 nm and diameters of about 20 nm were uniformly anchored on the majority surface of g-C3N4 nanosheets. As shown in Fig. 2d, the gC3N4/Ag2WO4/AgBr composite exhibits a similar microstructure to the g-C3N4/Ag2WO4 composite. Specially, the nanorods deposited on the 2D nanosheets aggregate into a network-like structure. It suggests that the Ag2WO4 nanorods could be decorated by AgBr through ion exchange, and meanwhile 1D Ag2WO4/AgBr nanorods would self-assemble into a network-like hierarchical heterostructure. And Fig. 2e displays the SEM image of the g-C3N4/Ag2WO4/AgBr/GO composite. It could be seen that numerous irregulated nanoplates were anchored on the g-C3N4 nanosheets. Interestingly, through the CTAB-assisted codeposition method, 1D nanorods almost transform to 2D nanoplates, fabricating a face-to-face hierarchical heterostructure. Therefore, a large interfacial area among each of materials was achieved in the gC3N4/Ag2WO4/AgBr/GO composite due to face-to-face contact, resulting in effective separation and transfer of charge carriers [50]. From EDX spectrum (Fig. 2g), the g-C3N4/Ag2WO4/AgBr/GO composite is composed of C, N, O, Br, Ag, and W, and the weight and atomic percentages of the elements are shown in Table 1. The atomic ratio of Ag : W : Br is obtained as 1.38 : 0.33 : 0.7, which agrees well with that of Ag2WO4 (Ag : W = 2 : 1) and AgBr (Ag : Br = 1 : 1) phase. And the atomic ratio of W : O is obtained as 0.33 : 9.26, which is smaller than that of Ag2WO4 (W : O = 1 : 4), suggesting that GO was successfully

Table 2 Comparison of as-prepared g-C3N4, g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/AgBr/GO, and other g-C3N4-based Ag-containing composites for SBET. Samples

Synthetic procedures

SBET (m2/g)

Vpore (cm3/g)

dpore (nm)

Ref.

g-C3N4 g-C3N4/Ag2WO4 g-C3N4/Ag2WO4/AgBr/GO g-C3N4/Ag2WO4 g-C3N4/Ag2WO4 Ag@AgBr/g-C3N4 AgX(X = Br, I)/g-C3N4 Ag2CrO4/g-C3N4/GO

Thermal polymerization In-situ deposition CTAB-assisted co-deposition Deposition-precipitation In-situ deposition Deposition-precipitation Water bath method Self-assembly precipitation

151.6 89.3 66.2 21.0 8.0 30.8 4.1-3.5 38.0

0.76 0.62 0.53 – 0.05 – – 0.11

19.4 26.4 29.0 – 27.0 – – 12.1

This work This work This work [30] [34] [60] [61] [67]

7

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 7. UV–vis DRS spectra (a) and the Eg (b) of g-C3N4, Ag2WO4, and AgBr; UV–vis DRS spectra (c) and the Eg (d) of as-prepared samples.

Fig. 8. Photocatalytic degradation efficiency (a) and first-order kinetics fitted curves (b) of TC.

the as-prepared composites exhibited a wide pore size distribution in the range of 2–150 nm and a narrow peak centered at 2.35 nm. The specific surface areas (SBET) of g-C3N4, g-C3N4/Ag2WO4 and g-C3N4/ Ag2WO4/AgBr/GO were calculated to be 151.6 m2/g, 89.30 m2/g, and 66.19 m2/g. Compared with g-C3N4, the g-C3N4/Ag2WO4 and g-C3N4/ Ag2WO4/AgBr/GO possess lower SBET due to the deposition of Ag2WO4, AgBr and GO, which could block some mesopores of g-C3N4 nanosheets. And the similar phenomenon was reported by some previous researches of composites comprising g-C3N4 and Ag-containing compounds [59]. As listed in Table 2, the as-prepared g-C3N4/Ag2WO4/AgBr/GO composite in this work demonstrates higher SBET than most of other g-C3N4-

Fig. 5g are assigned to the Ag 3d5/2 and Ag 3d3/2 binding energies of Ag+, attributed to Ag+ of Ag2WO4 and AgBr. Thus, the g-C3N4/ Ag2WO4/AgBr/GO was successfully synthesized confirmed by above XPS results. The N2 adsorption-desorption isotherms and corresponding pore size distribution curves of as-prepared composites are exhibited in Fig. 6. All the composites have the type IV isotherms with a hysteresis loop at the high-relative-pressure(P/P0) region, suggesting the existence of mesopores [58]. And their hysteresis loops could be regarded as the type H3, implying the presence of slit-shaped pores aggregated from gC3N4 nanosheets [34]. Additionally, the inset in Fig. 6 confirmed that 8

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 9. (a) PL spectra of g-C3N4, Ag2WO4, g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/AgBr, g-C3N4/Ag2WO4/GO, and g-C3N4/Ag2WO4/AgBr/GO; (b) EIS spectra of g-C3N4, gC3N4/Ag2WO4, g-C3N4/Ag2WO4/AgBr, and g-C3N4/Ag2WO4/AgBr/GO.

g-C3N4, respectively. The content of TC only shows a slight decrease under 30 min dark conduction, implying that the absorption capacity of as-prepared samples does not have an obvious impact on their photocatalytic performance. The results indicate that coupling with AgBr and GO, the g-C3N4/Ag2WO4/AgBr/GO composite possesses the superior photocatalytic performance regarding TC degradation. The PL spectra could reveal the recombination rate of photoexcited charge carriers directly, and quenching indicates the prolong lifetime of photoexcited charge carriers in the composite. Fig. 9a shows the PL spectra of the pristine g-C3N4, g-C3N4/Ag2WO4, and other g-C3N4 based composites excited by 325 nm, and PL intensities decrease in the following order: g-C3N4 > g-C3N4/Ag2WO4 > g-C3N4/Ag2WO4/AgBr > g-C3N4/Ag2WO4/AgBr/GO. Above results confirm that coupling with GO and AgBr are helpful to further suppress photogenerated charge carrier recombination of g-C3N4/Ag2WO4. As-prepared g-C3N4/ Ag2WO4/AgBr/GO composite exhibits the lowest peak intensity, indicating that the sufficient separation of charge carriers. To further understand the transfer and separation efficiency of charge carriers, the as-prepared samples were used as electrode materials to analyze their EIS (Fig. 9b). The arc radius in the high-frequency region could be ascribed to the charge-transfer resistance (Rct), suggesting the transfer capability of charge [64]. Obviously, the arc radius increased in the following order: g-C3N4/Ag2WO4/AgBr/GO < g-C3N4/Ag2WO4/ AgBr < g-C3N4/Ag2WO4 < g-C3N4, indicating that g-C3N4/Ag2WO4/ AgBr/GO composite process the efficient charge transfer. These results demonstrate that effective separation and faster transfer of interfacial charge carriers occur in the g-C3N4/Ag2WO4/AgBr/GO composite, which is consistent with its photocatalytic performance. The optimal contents of CTAB (50.0 wt%) and GO (3.0 wt%) in the g-C3N4/Ag2WO4/ AgBr/GO composites were also confirmed by photocatalytic degradation.

based Ag-containing composites, which can provide more active sites and improve the efficiency of the heterogeneous photocatalytic process. As the DRS spectra (Fig. 7a) of g-C3N4, Ag2WO4, and AgBr illustrates, they hold the absorption onsets at 456, 410 and 490 nm, and the band-gap energies (Eg) are calculated to be 2.71, 3.02 and 2.53 eV, by the Kubelka-Munk function. As given by Eq. (2):

F (R) =

(1 − R)2 2R

(2)

where R is the diffuse reflectance. Fig. 7c shows the light absorption of g-C3N4/Ag2WO4/AgBr/GO, compared with those from pristine g-C3N4, g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/GO, and g-C3N4/Ag2WO4/AgBr. Pristine g-C3N4 demonstrated the spectrum with the absorption edge located around 456 nm. After the introduction of Ag2WO4 and AgBr, the DRS curves of g-C3N4/Ag2WO4 and g-C3N4/Ag2WO4/AgBr exhibit enhanced absorption from 200 to 800 nm, and the absorption edge of gC3N4/Ag2WO4/AgBr composite red shifts. Since AgBr could serve as a photosensitizer, which extends the light utilization range of g-C3N4/ Ag2WO4. Moreover, the incorporation of GO can further promote the light harvest of all the g-C3N4-based composites. Eventually, the absorption edge of g-C3N4/Ag2WO4/AgBr/GO has been tuned to 485 nm from 461 nm, compared with g-C3N4/Ag2WO4. After calculation, the Eg of g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/GO, and g-C3N4/Ag2WO4/AgBr/ GO are 2.70, 2.65, and 2.58 eV, respectively. The enhancement of GOmodified composites could be ascribed to the increase of the surface electric charge of the composites, which may occur due to the possible electron transition of π → π* of GO as well as π → π* between semiconductors and GO [62,63]. It indicates that the co-decoration of AgBr and GO provides great potential in tuning the light absorption of gC3N4/Ag2WO4. 3.2. Photocatalytic activity

3.3. Recycling experiments The photocatalytic degradation curves of as-prepared samples towards antibiotic TC are displayed in Fig. 8a. The TC degrading efficiencies over g-C3N4, g-C3N4/Ag2WO4, g-C3N4/Ag2WO4/AgBr and gC3N4/Ag2WO4/AgBr/GO composites are 17.45, 35.45, 66.57 and 91.64%, within 100 min irradiation. The photocatalytic organic contaminants degradation is a pseudo-first-order reaction and its kinetics can be expressed as follows:

− ln(

C ) = kt C0

The stability and recyclability of photocatalyst is another major issue of the photocatalytic process. As shown in Fig. 10a, the g-C3N4/ Ag2WO4/AgBr/GO composite was used for RhB degrading experiments repeatedly for five times, and its degradation efficiency was reduced only by 8.91% after the fourth run (further decreased to 19.28% in the next run), indicating a good degrading activity and recyclability. As for the g-C3N4/Ag2WO4 composite (Fig. 10c), its degradation efficiency was reduced obviously by 36.43% after the second run. To further understand their stability and recyclability, the samples after recycling experiments were investigated by XRD, and the results were presented in Fig. 10b, d. There are two identified diffraction peaks of Ag0 appeared in the pattern of the used g-C3N4/Ag2WO4 composite (Fig. 10d),

(3)

where k is the apparent rate constant. And the k of TC degradation with g-C3N4/Ag2WO4/AgBr/GO is 0.0245 min−1, which is 2.3, 5.9 and 13.7folds greater than that of g-C3N4/Ag2WO4/AgBr, g-C3N4/Ag2WO4, and 9

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Fig. 10. Recycling experiments (a, c, e) and XRD patterns of the fresh and used composites (b, d, f) of g-C3N4/Ag2WO4/AgBr/GO (a, b), g-C3N4/Ag2WO4 (c, d) and gC3N4/AgBr/GO (e, f).

composite. Then the peaks of g-C3N4, Ag0, and AgBr coexisted in the pattern of used g-C3N4/Ag2WO4/AgBr/GO composite, and its XRD pattern exhibits no obvious difference in phase and crystalline structure after the next four times run. This phenomenon could be explained by the following reasons. Firstly, the enhanced characteristic peak of AgBr mainly ascribes to the formation of more AgBr (Ksp AgBr = 5.0 × 10‒13, Ksp Ag2WO4 = 5.5 × 10−12) on the surface, because the Ag2WO4 of the composite surface would further react with the residual CTAB and form an AgBr shelter on the composite. Secondly, the Ag nanoparticles (NPs) on the Ag2WO4 and AgBr would accept the excessive electrons rather than Ag+ ions of Ag-containing compounds receive the electrons, resulting in Ag NPs will not be produced continuously [65]. It suggests that the surface of g-C3N4/Ag2WO4/AgBr/GO composite would self-

and the diffraction peaks of Ag2WO4 almost disappear. It suggests that the chemical composition of the g-C3N4/Ag2WO4 surface has been destroyed after two cycles, resulting in a decrease of degradation efficiency. The photo-corrosion of Ag2WO4 could be ascribed to the accumulation of electron on the Ag2WO4 and most importantly its solubility (Ksp Ag2WO4 = 5.5 × 10−12) could not be neglected. Besides, as the light irradiation further increasing, excessive Ag particles are reduced on the surface of the g-C3N4/Ag2WO4 composite, which might present a shielding effect for the incidence of light. As for the g-C3N4/Ag2WO4/ AgBr/GO composite (Fig. 10b), its XRD pattern presented a different phenomenon that the peak intensity of AgBr in the used composite became stronger than those in the fresh composite. And the characteristic peaks of Ag2WO4 cannot be seen in the pattern of the used 10

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

ECB = EVB − Eg

where EVB and ECB are the VB and CB edge potential, Eg represents the band gap of the semiconductor. The X values (the electronegativity of the semiconductor) for g-C3N4, Ag2WO4, and AgBr are 4.67, 6.00, and 5.81 eV [29,59], respectively. And Ee is the energy of free electrons on the hydrogen sale (about 4.5 eV). According to the DRS results, the EVB of g-C3N4, Ag2WO4, and AgBr can be estimated by Eq. (4), which are approximately 1.53, 3.01, and 2.58 V, respectively. Moreover, the ECB can be estimated to be about ‒1.19, ‒0.01, and 0.045 V, respectively. And a reasonable photocatalytic mechanism for the g-C3N4/Ag2WO4/AgBr/GO composite is proposed (Scheme 2). Based on the previous studies, g-C3N4, Ag2WO4 and AgBr are n-type semiconductors [35,66]. Prior to the irradiation (Scheme 2a), the electrons (e−) on the g-C3N4 near the g-C3N4-Ag2WO4 interface would transfer into the Ag2WO4, due to the more negative Fermi level potential. And the positively charged species left at the g-C3N4 side would form an e− depletion layer, whereas numerous e− accumulated at the Ag2WO4 side would form an e− accumulation layer. As a result, an internal electric field in the g-C3N4-Ag2WO4 interface is formed (Scheme 2b). The diffusion of e− will continue until their Fermi levels become coincident. As for the interface between Ag2WO4 and AgBr, a similar process takes place. After their Fermi level equilibrium is achieved, an internal electric field in the Ag2WO4-AgBr interface is formed, either. Then, under the illumination of incident light with enough energy, g-C3N4, Ag2WO4, and AgBr are excited to form e−/h+ pairs, respectively. Under the influence of the internal electric field (Scheme 2b), the e− on the CB of Ag2WO4 and AgBr would migrate to the CB of g-C3N4, and then shift rapidly to the GO, because the Fermi level potential of GO (-0.49 V) is lower than the CB of g-C3N4 [67]. Simultaneously, the h+ in the VB of Ag2WO4 would transfer to the VB of AgBr. As a consequence, the e− and h+ would be remained at the CB of GO and the VB of AgBr and g-C3N4, respectively, hindering the e−/h+ pair recombination [68–70]. Thus, an n-n heterojunction-based charge carrier separation pathway could be achieved in the g-C3N4/Ag2WO4/AgBr/ GO composite, which is more efficient than that of Type-I and Type-II heterojunction composites, mainly benefited from the synergy effect between the internal electric filed and the energy band alignment [35,71]. Besides, if the as-prepared g-C3N4/Ag2WO4/AgBr/GO composite follows a Type-I and Type-II heterojunction-based charge carrier separation pathway (Scheme 3), the excessive e− would diffuse into the

Fig. 11. Trapping experiments of active species of g-C3N4/Ag2WO4/AgBr/GO.

assemble and form a stable one, which was responsible for its enhanced recycle photocatalytic activity compared with g-C3N4/Ag2WO4. To further prove the above explanation, the recycling experiments of gC3N4/AgBr/GO were also carried out, and its results (Fig. 10e, f) showed the g-C3N4/AgBr/GO possesses good stability in five runs due to its stable crystalline structure confirmed by the XRD patterns. 3.4. Photocatalytic mechanism To investigate the dominant active species in the processes of TC degradation, the active species trapping experiments of as-prepared gC3N4/Ag2WO4/AgBr/GO composite were carried out under simulated sunlight irradiation. As the results shown in Fig. 11, the introduction of EDTA-2Na (1 mmol L−1) and BQ (1 mmol L−1) that could trap h+ and % O2‒ obviously suppress the photocatalytic efficiency towards TC, suggesting that %O2‒ and h+ were the primary and secondary active species, respectively. Whereas, when IPA (1 mmol L−1) that could trap % OH was introduced, the degradation efficiency slightly declined, revealing that %OH was not mainly responsible for the TC degradation. The energy band configuration of composites has essential impact on their properties, including the excitation wavelength, the photoinduced carriers redox capacity and the charge carrier separation efficiency. The band energy position of semiconductors could be calculated through the following equations:

EVB = X + 0.5Eg − Ee

(5)

(4)

Scheme 2. A schematic diagram for the n-n heterojunction-based separation of charge carriers in the g-C3N4/Ag2WO4/AgBr/GO. 11

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

Scheme 3. A schematic diagram for Type-I/II heterojunction-based separation of charge carriers in the g-C3N4/Ag2WO4/AgBr/GO.

Fig. 12. Photocatalytic mechanism for g-C3N4/Ag2WO4/AgBr/GO under simulated sunlight irradiation.

are regarded as the main active species in this photocatalytic process. At the same times, the h+ in the CB of AgBr shift to the surface of AgBr, reacting with the interstitial Br− to produce Br0 with strong oxidizing capacity. Then Br0, %O2− and h+ are the dominant species that are responsible for degradation of TC. Based on previous researches of TC degradation pathways [72,73], the TC molecules would transform into intermediates via N-demethylation, hydroxylation, and dehydration processes. Then some of intermediates might further decompose to small molecules through the ringopening and oxidative reaction. And ethylamine, ethanedioic acid, 3hydroxybutyrate, etc., are the potential products of the TC degradation.

CB of Ag2WO4 and AgBr, resulting in the photoreduction of metallic Ag0. However, the continuous formation of Ag NPs in the composites is a bad factor for its photocatalytic stability, which is not in accordance with the results of recycling experiments. With the assistance of the internal electric filed provided by n-n heterojunction, the photoexcited e− would be remain on the g-C3N4 and GO side, decreasing the accumulation of e− on the Ag2WO4 and AgBr side. Therefore, the photocorrosion of Ag2WO4 and AgBr could be mitigated. According to the above discussion, the lifetime of e−/h+ pairs in the as-prepared g-C3N4/Ag2WO4/AgBr/GO composite would increase, confirmed by the results of PL spectra. Afterward, the separated e− and h+ would shift to the surface of composite and initiate the degradation reactions, implying that the enhanced photocatalytic activity relative to its counterparts. In brief, the photocatalytic degradation mechanism can be described as follow (Fig. 12): The e− accumulated on the CB of GO would be trapped by O2 molecules, generating %O2− radicals that

4. Conclusion A novel g-C3N4/Ag2WO4/AgBr/GO composite was successfully synthesized through in-situ deposition and CTAB-assisted co-deposition 12

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

method. The photocatalytic efficiency for TC degradation over the asprepared g-C3N4/Ag2WO4/AgBr/GO composite is 91.64% within 100 min simulated sunlight irradiation, which is nearly 2.59 and 5.25folds greater than those of the g-C3N4/Ag2WO4 and g-C3N4 samples. This enhancement is mainly benefited from face-to-face hierarchical heterostructure, efficient light utilization, faster charge carriers transfer and efficient charge carrier separation, for which AgBr and GO could serve as a photosensitizer and an e− acceptor, respectively. And the gC3N4/Ag2WO4/AgBr/GO composite remained good photocatalytic efficiency after 5 times of cycle. Additionally, %O2− and h+ are the dominant species that are responsible for the degradation of TC, confirmed by the active species trapping experiments. Further, a novel n-n heterojunction mechanism for g-C3N4/Ag2WO4/AgBr/GO composite was proposed as follows: Benefited from an internal electric field provided by the n-n heterojunction, the transfer and separation efficiency of charge carriers were improved and the e− accumulation on the Ag2WO4 and AgBr side were mitigated. Thus, CTAB-assisted co-deposition method could be regarded as an effective approach to fabricate the g-C3N4 based Ag-containing composites for enhanced photocatalytic activity and good recycling degrading efficiency.

(2017) 53–62. [17] X.Y. Qian, J.F. Ding, J.L. Zhang, Y. Zhang, Y.N. Zhang, E. Kan, X. Wang, J.W. Zhu, Ultrathin molybdenum disulfide/carbon nitride nanosheets with abundant active sites for enhanced hydrogen evolution, Nanoscale 10 (2018) 1766–1773. [18] P.F. Xia, B.C. Zhu, B. Cheng, J.G. Yu, J.S. Xu, 2D/2D g-C3N4/MnO2 nanocomposite as a direct Z-scheme photocatalyst for enhanced photocatalytic activity, ACS Sustain. Chem. Eng. 6 (2017) 965–973. [19] J.W. Fu, B.C. Zhu, C.J. Jiang, B. Cheng, W. You, J.G. Yu, Hierarchical porous Odoped g-C3N4 with enhanced photocatalytic CO2 reduction activity, Small 13 (2017) 1603938. [20] W. Guo, K. Fan, J.J. Zhang, C.J. Xu, 2D/2D Z-scheme Bi2WO6/Porous-g-C3N4 with synergy of adsorption and visible-light-driven photodegradation, Appl. Surf. Sci. 447 (2018) 125–134. [21] H. Diarmand-Khalilabad, A. Habibi-Yangjeh, D. Seifzadeh, S. AsadzadehKhaneghah, E. Vesali-Kermani, g-C3N4 nanosheets decorated with carbon dots and CdS nanoparticles: novel nanocomposites with excellent nitrogen photofixation ability under simulated solar irradiation, Ceram. Int. 45 (2019) 2542–2555. [22] W.L. Yu, J.X. Chen, T.T. Shang, L.F. Chen, L. Gu, T.Y. Peng, Direct Z-scheme gC3N4/WO3 photocatalyst with atomically defined junction for H2 production, Appl. Catal. B: Environ. 219 (2017) 693–704. [23] S. Asadzadeh‐Khaneghah, A. Habibi‐Yangjeh, K. Yubuta, Novel g-C3N4 nanosheets/ CDs/BiOCl photocatalysts with exceptional activity under visible light, J. Am. Ceram. Soc. 102 (2019) 1435–1453. [24] M. Pandiri, R. Velchuri, R. Gundeboina, V. Muga, A facile in-situ hydrothermal route to construct a well-aligned β-Ag2MoO4/g-C3N4 heterojunction with enhanced visible light photodegradation: mechanistic views, J. Photochem. Photobiol. A: Chem. 360 (2018) 231–241. [25] M. Mousavi, A. Habibi-Yangjeh, Decoration of Fe3O4 and CoWO4 nanoparticles over graphitic carbon nitride: novel visible-light-responsive photocatalysts with exceptional photocatalytic performances, Mater. Res. Bull. 105 (2018) 159–171. [26] M. Mousavi, A. Habibi-Yangjeh, D. Seifzadeh, Novel ternary g-C3N4/Fe3O4/MnWO4 nanocomposites: synthesis, characterization, and visible-light photocatalytic performance for environmental purposes, J. Mater. Sci. Technol. 34 (2018) 1638–1651. [27] M. Mousavi, A. Habibi-Yangjeh, Integration of NiWO4 and Fe3O4 with graphitic carbon nitride to fabricate novel magnetically recoverable visible-light-driven photocatalysts, J. Mater. Sci. 53 (2018) 9046–9063. [28] S. Qu, Y. Xiong, Environment-friendly Ag/CDots/g-C3N4 photocatalysts: remarkably enhanced photocatalytic tetracycline degradation in visible light, J. Photochem. Photobiol. A: Chem. 365 (2018) 23–31. [29] W.J. Ong, L.K. Putri, L.L. Tan, S.P. Chai, S.T. Yong, Heterostructured AgX/g-C3N4 (X=Cl and Br) nanocomposites via a sonication-assisted deposition-precipitation approach: emerging role of halide ions in the synergistic photocatalytic reduction of carbon dioxide, Appl. Catal. B: Environ. 180 (2016) 530–543. [30] Y. Li, Y. Li, S.L. Ma, P.F. Wang, Q.L. Hou, J.J. Han, Efficient water disinfection with Ag2WO4-doped mesoporous g-C3N4 under visible light, J. Hazard. Mater. 338 (2017) 33–46. [31] J.S. Shayeh, H. Salari, A. Daliri, M. Omidi, Decorative reduced graphene oxide/ C3N4/Ag2O/conductive polymer as a high performance material for electrochemical capacitors, Appl. Surf. Sci. 447 (2018) 374–380. [32] L. Zhou, W. Zhang, L. Chen, H.P. Deng, Z-scheme mechanism of photogenerated carriers for hybrid photocatalyst Ag3PO4/g-C3N4 in degradation of sulfamethoxazole, J. Colloid Interf. Sci. 487 (2017) 410–417. [33] H. Tang, S.F. Chang, G.G. Tang, W. Liang, AgBr and g-C3N4 co-modified Ag2CO3 photocatalyst: a novel multi-heterostructured photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 391 (2017) 440–448. [34] B.C. Zhu, P.F. Xia, Y. Li, W.K. Ho, J.G. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175–183. [35] S. Feizpoor, A. Habibi-Yangjeh, Integration of Ag2WO4 and AgBr with TiO2 to fabricate ternary nanocomposites: novel plasmonic photocatalysts with remarkable activity under visible light, Mater. Res. Bull. 99 (2018) 93–102. [36] B.Y. Wang, G.Y. Zhang, G.W. Cui, Y.Y. Xu, Y. Liu, C.Y. Xing, Controllable fabrication of α-Ag2WO4 nanorod-clusters with superior simulated sunlight photocatalytic performance, Inorg. Chem. Front. 6 (2019) 209–219. [37] J. Li, F. Liu, Y.R. Li, Fabrication of Ag/Ag2MoO4 plasmonic photocatalyst with enhanced photocatalytic performance for degradation of ciprofloxacin, New J. Chem. 42 (2018) 12054–12061. [38] Z.G. Zhang, W.Q. Huang, Z. Xie, W.Y. Hu, P. Peng, G.F. Huang, Simultaneous covalent and noncovalent carbon nanotube/Ag3PO4 hybrids: new insights into the origin of enhanced visible light photocatalytic performance, Phys. Chem. Chem. Phys. 19 (2017) 7955–7963. [39] X.L. Miao, X.P. Shen, J.J. Wu, Z.Y. Ji, J.H. Wang, L.R. Kong, M.M. Liu, C.S. Song, Fabrication of an all solid Z-scheme photocatalyst g-C3N4/GO/AgBr with enhanced visible light photocatalytic activity, Appl. Catal. A: Gen. 539 (2017) 104–113. [40] S.H. Zhang, Z.Y. Liu, Y.Q. Zhang, S.M. Gao, R.C. Jin, Q.Y. Wang, Highly effective photoelectrochemical performance of solar energy materials based on Ag2WO4-AgX (X=Cl, Br, I) sensitized TiO2 nanotube arrays, Ceram. Int. 44 (2018) 6659–6665. [41] Z. Cui, Y.G. Sun, Z.D. Zhang, M.L. Xu, B.J. Xin, Facile synthesis and photocatalytic activity of Ag3PO4 decorated MoS2 nanoflakes on carbon fiber cloth, Mater. Res. Bull. 100 (2018) 345–352. [42] J.X. Low, J.G. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [43] D. Ma, J. Wu, M.C. Gao, Y.J. Xin, Y.Y. Sun, T.J. Ma, Hydrothermal synthesis of an artificial Z-scheme visible light photocatalytic system using reduced graphene oxide as the electron mediator, Chem. Eng. J. 313 (2017) 1567–1576.

Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos.21076092, 21376099, 21546002, 21878115). References [1] Y. Miseki, K. Sayama, Highly efficient Fe(III) reduction and solar-energy accumulation over a BiVO4 photocatalyst, Chem. Commun. 54 (2018) 2670–2673. [2] Y.N. Ma, J. Li, E. Liu, J. Wan, X.Y. Hu, J. Fan, High efficiency for H2 evolution and NO removal over the Ag nanoparticles bridged g-C3N4 and WS2 heterojunction photocatalysts, Appl. Catal. B: Environ. 219 (2017) 467–478. [3] R. Yadav, A.K. Sinha, Titania cowrapped α-sulfur composite as a visible light active photocatalyst for hydrogen evolution using in situ methanol from CO2 as a sacrificial agent, ACS Sustain. Chem. Eng. 5 (2017) 6736–6745. [4] Y.Z. Hong, C.S. Li, B.X, D. Li, Z.Y. Zhang, B.D. Mao, W.Q. Fan, W. Gu, W.D. Shi, Promoting visible-light-induced photocatalytic degradation of tetracycline by an efficient and stable beta-Bi2O3@g-C3N4 core/shell nanocomposite, Chem. Eng. J. 338 (2018) 137–146. [5] Y.S. Zhou, G. Chen, E.H. Sargent, T.T. Zhuang, C.T. Dinh, F. He, Freestanding nanophotoelectrode as a highly efficient and visible-light-driven photocatalyst for watersplitting, J. Mater. Chem. A 5 (2017) 10651–10657. [6] J.W. Fu, J.G. Yu, C.J. Jiang, B. Cheng, g-C3N4-Based heterostructured photocatalysts, Adv. Energy Mater. 8 (2018) 1701503. [7] A. Nikokavoura, C. Trapalis, Graphene and g-C3N4 based photocatalysts for NOx removal: a review, Appl. Surf. Sci. 430 (2018) 18–52. [8] M. Mousavi, A. Habibi-Yangjeh, S.R. Pouran, Review on magnetically separable graphitic carbon nitride-based nanocomposites as promising visible-light-driven photocatalysts, J. Mater. Sci-Mater. El. 29 (2018) 1719–1747. [9] M. Shekofteh-Gohari, A. Habibi-Yangjeh, M. Abitorabi, A. Rouhi, Magnetically separable nanocomposites based on ZnO and their applications in photocatalytic processes: a review, Crit. Rev. Environ. Sci. Technol. 48 (2018) 806–857. [10] M. Pirhashemi, A. Habibi-Yangjeh, S.R. Pouran, Review on the criteria anticipated for the fabrication of highly efficient ZnO-based visible-light-driven photocatalysts, J. Ind. Eng. Chem. 62 (2018) 1–25. [11] H. Yi, D.L. Huang, L. Qin, G.M. Zeng, C. Lai, M. Cheng, S.J.S.J. Ye, B. Song, X.Y. Ren, X.Y. Guo, Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production, Appl. Catal. B: Environ. 239 (2018) 408–424. [12] Z.F. Hu, Z.R. Shen, J.C. Yu, Converting carbohydrates to carbon-based photocatalysts for environmental treatment, Environ. Sci. Technol. 51 (2017) 7076–7083. [13] W.L. Yu, J.X. Chen, T.T. Shang, L.F. Chen, L. Gu, T.Y. Peng, Converting carbohydrates to carbon-based photocatalysts for environmental treatment, Appl. Catal. B: Environ. 219 (2017) 693–704. [14] W.J. Wang, G.Y. Li, T.C. An, D.K.L. Chan, J.C. Yu, P.K. Wong, Photocatalytic hydrogen evolution and bacterial inactivation utilizing sonochemical-synthesized gC3N4/red phosphorus hybrid nanosheets as a wide-spectral-responsive photocatalyst: the role of type I band alignment, Appl. Catal. B: Environ. 238 (2018) 126–135. [15] W. Cui, J.Y. Li, W.L. Cen, Y.J. Sun, S.C. Lee, F. Dong, Steering the interlayer energy barrier and charge flow via bioriented transportation channels in g-C3N4: enhanced photocatalysis and reaction mechanism, J. Catal. 352 (2017) 351–360. [16] X. Wang, Y.H. Liang, W.J. An, J.S. Zhu, W.Q. Cui, Removal of chromium (VI) by a self-regenerating and metal free g-C3N4/graphene hydrogel system via the synergy of adsorption and photo-catalysis under visible light, Appl. Catal. B: Environ. 219

13

Journal of Photochemistry & Photobiology A: Chemistry 382 (2019) 111957

Y. Wang, et al.

[59] K. Vignesh, M. Kang, Facile synthesis, characterization and recyclable photocatalytic activity of Ag2WO4@g-C3N4, Mater. Sci. Eng. B: Adv. 199 (2015) 30–36. [60] Y.X. Yang, W. Guo, Y.N. Guo, Y.H. Zhao, X. Yuan, Y.H. Guo, Fabrication of Zscheme plasmonic photocatalyst Ag@AgBr/g-C3N4 with enhanced visible-light photocatalytic activity, J. Hazard. Mater. 271 (2014) 150–159. [61] H. Xu, J. Yan, Y.G. Xu, Y.H. Song, H.M. Li, J.X. Xia, C.J. Huang, H.L. Wan, Novel visible-light-driven AgX/graphite-like C3N4 (X= Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal. B: Environ. 129 (2013) 182–193. [62] T.Y. Peng, K. Li, P. Zeng, Q.G. Zhang, X.G. Zhang, Enhanced photocatalytic hydrogen production over graphene oxide–cadmium sulfide nanocomposite under visible light irradiation, J. Phys. Chem. C 116 (2012) 22720–22726. [63] Y.Y. Bai, F.R. Wang, J.K. Liu, A new complementary catalyst and catalytic mechanism: Ag2MoO4/Ag/AgBr/GO heterostructure, Ind. Eng. Chem. Res. 55 (2016) 9873–9879. [64] X.J. Bai, L. Wang, Y.J. Wang, W.Q. Yao, Y.F. Zhu, Enhanced oxidation ability of gC3N4 photocatalyst via C60 modification, Appl. Catal. B: Environ. 152 (2014) 262–270. [65] J. Cao, Y.J. Zhao, H.L. Lin, B.Y. Xu, S.F. Chen, Ag/AgBr/g-C3N4: a highly efficient and stable composite photocatalyst for degradation of organic contaminants under visible light, Mater. Res. Bull. 48 (2013) 3873–3880. [66] W.J. Yin, L.J. Bai, Y.Z. Zhu, S.X. Zhong, L.H. Zhao, Z.Q. Li, S. Bai, Embedding metal in the interface of a p-n heterojunction with a stack design for superior Z-scheme photocatalytic hydrogen evolution, ACS Appl. Mater. Interface 8 (2016) 23133–23142. [67] D.F. Xu, B. Cheng, W.K. Wang, C.J. Jiang, J.G. Yu, Ag2CrO4/g-C3N4/graphene oxide ternary nanocomposite Z-scheme photocatalyst with enhanced CO2 reduction activity, Appl. Catal. B: Environ. 231 (2018) 368–380. [68] M. Pirhashemi, A. Habibi-Yangjeh, Photosensitization of ZnO by AgBr and Ag2CO3: nanocomposites with tandem n-n heterojunctions and highly enhanced visible-light photocatalytic activity, J. Colloid Interface Sci. 474 (2016) 103–113. [69] M. Pirhashemi, A. Habibi-Yangjeh, Ternary ZnO/AgBr/Ag2CrO4 nanocomposites with tandem n–n heterojunctions as novel visible-light-driven photocatalysts with excellent activity, Ceram. Int. 41 (2015) 14383–14393. [70] M. Shekofteh-Gohari, A. Habibi-Yangjeh, Novel magnetically separable ZnO/AgBr/ Fe3O4/Ag3VO4 nanocomposites with tandem n–n heterojunctions as highly efficient visible-light-driven photocatalysts, RSC Adv. 6 (2016) 2402–2413. [71] K. Zhang, J. Wang, W.J. Jiang, W.Q. Yao, H.P. Yang, Y.F. Zhu, Self-assembled perylene diimide based supramolecular heterojunction with Bi2WO6 for efficient visible-light-driven photocatalysis, Appl. Catal. B: Environ. 232 (2018) 175–181. [72] J.Y. Chen, X.Y. Xiao, Y. Wang, Z.H. Ye, Ag nanoparticles decorated WO3/g-C3N4 2D/2D heterostructure with enhanced photocatalytic activity for organic pollutants degradation, Appl. Surf. Sci. 467 (2019) 1000–1010. [73] Y. Liu, J.J. Kong, J.L. Yuan, W. Zhao, X. Zhu, C. Sun, J.M. Xie, Enhanced photocatalytic activity over flower-like sphere Ag/Ag2CO3/BiVO4 plasmonic heterojunction photocatalyst for tetracycline degradation, Chem. Eng. J. 331 (2018) 242–254.

[44] J.H. Luo, Z. Yan, R.Q. Liu, J.G. Xu, X. Wang, Synthesis and excellent visible light photocatalysis performance of magnetic reduced graphene oxide/ZnO/ZnFe2O4 composites, RSC Adv. 7 (2017) 23246–23254. [45] Y.H. Li, X.F. Wu, W.K. Ho, K.L. Lv, Q. Li, M. Li, S.C. Lee, Graphene-induced formation of visible-light-responsive SnO2-Zn2SnO4 Z-scheme photocatalyst with surface vacancy for the enhanced photoreactivity towards NO and acetone oxidation, Chem. Eng. J. 336 (2018) 200–210. [46] A.V. Thomas, B.C. Andow, S. Suresh, O. Eksik, J. Yin, A.H. Dyson, N. Koratkar, Controlled crumpling of graphene oxide films for tunable optical transmittance, Adv. Mater. 27 (2015) 3256–3265. [47] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z.Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. [48] X.J. Bai, R.L. Zong, C.X. Li, D. Liu, Y.F. Zhu, Enhancement of visible photocatalytic activity via Ag@C3N4 core–shell plasmonic composite, Appl. Catal. B: Environ. 147 (2014) 82–91. [49] X.F. Wang, C. Fu, P. Wang, H.G. Yu, J.G. Yu, Hierarchically porous metastable βAg2WO4 hollow nanospheres: controlled synthesis and high photocatalytic activity, Nanotechnology 24 (2013) 165602. [50] J.X. Low, S.W. Cao, J.G. Yu, S. Wageh, Two-dimensional layered composite photocatalysts, Chem. Commun. 50 (2014) 10768–10777. [51] S. Nayak, L. Mohapatra, K. Parida, Visible light-driven novel g-C3N4/NiFe-LDH composite photocatalyst with enhanced photocatalytic activity towards water oxidation and reduction reaction, J. Mater. Chem. A 3 (2015) 18622–18635. [52] S. Thangavel, S. Thangavel, N. Raghavan, R. Alagu, Efficient visible-light photocatalytic and enhanced photocorrosion inhibition of Ag2WO4 decorated MoS2 nanosheets, J. Phys. Chem. Solids 110 (2017) 266–273. [53] J.S. Zhang, M.W. Zhang, L.H. Lin, X.C. Wang, Sol processing of conjugated carbon nitride powders for thin‐film fabrication, Angew. Chem. Int. Ed. 127 (2015) 6395–6399. [54] F. Zhou, R. Shi, Y.F. Zhu, Significant enhancement of the visible photocatalytic degradation performances of γ-Bi2MoO6 nanoplate by graphene hybridization, J. Mol. Catal. A: Chem. 340 (2011) 77–82. [55] G.M. Veith, L. Baggetto, L.A. Adamczyk, B.K. Guo, S.S. Brown, X.G. Sun, A.A. Albert, J.R. Humble, C.E. Barnes, M.J. Bojdys, S. Dai, N.J. Dudney, Electrochemical and solid-state lithiation of graphitic C3N4, Chem. Mater. 25 (2013) 503–508. [56] Y.J. Cui, J.S. Zhang, G.G. Zhang, J.H. Huang, P. Liu, M. Antoniettib, X.C. Wang, Synthesis of bulk and nanoporous carbon nitride polymers from ammonium thiocyanate for photocatalytic hydrogen evolution, J. Mater. Chem. 21 (2011) 13032–13039. [57] Y.F. Li, R.X. Jin, X. Fang, Y. Yang, M. Yang, X.C. Liu, Y. Xing, A.Y. Song, In situ loading of Ag2WO4 on ultrathin g-C3N4 nanosheets with highly enhanced photocatalytic performance, J. Hazard. Mater. 313 (2016) 219–228. [58] J.G. Yu, S.H. Wang, B. Cheng, L. Zhang, F. Huang, Noble metal-free Ni(OH)2–g-C3N4 composite photocatalyst with enhanced visible-light photocatalytic H2-production activity, Catal. Sci. Technol. 3 (2013) 1782–1789.

14