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Ag ternary heterojunction visible-light photocatalyst
Materials Chemistry and Physics 177 (2016) 529e537
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A facile fabrication of plasmonic g-C3N4/Ag2WO4/Ag ternary heterojunction visible-light photocatalyst Kai Dai a, *, Jiali Lv a, Luhua Lu b, **, Changhao Liang a, c, Lei Geng a, Guangping Zhu a a
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, PR China Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan, 430074, PR China c Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
g-C3N4/Ag2WO4/Ag ternary nanocomposite photocatalyst was prepared. g-C3N4/Ag2WO4/Ag showed high photocatalytic activity. g-C3N4/Ag2WO4/Ag showed long reusable life.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2015 Received in revised form 17 March 2016 Accepted 2 April 2016 Available online 23 April 2016
It's important to reduce recombination of electrons and holes and enhance charge transfer through fine controlled interfacial structure. In this work, novel graphitic-C3N4 (g-C3N4)/Ag2WO4/Ag ternary photocatalyst has been synthesized by deposition of Ag2WO4 onto g-C3N4 template and followed by sun light reduction of Ag2WO4 into Ag2WO4/Ag. As-prepared g-C3N4/Ag2WO4/Ag presented significantly enhanced photocatalytic performance in degrading methylene blue (MB) under 410 nm LED light irradiation. Metallic Ag0 is used as plasmonic hot spots to generate high energy charge carriers. Optimal g-C3N4 content has been confirmed to be 40 wt%, corresponding to apparent pseudo-first-order rate constant kapp of 0.0298 min1, which is 3.3 times and 37.3 times more than that of pure g-C3N4 and Ag2WO4, respectively. This novel ternary g-C3N4/Ag2WO4/Ag structure material is an ideal candidate in environmental treatment and purifying applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Electron microscopy Crystal structure
1. Introduction The continuous increasing world's population, together with the substantial development of industry has brought about imperious
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Dai), [email protected] (L. Lu). http://dx.doi.org/10.1016/j.matchemphys.2016.04.065 0254-0584/© 2016 Elsevier B.V. All rights reserved.
attention for the environmental protection [1e5]. Many technologies, such as reverse osmosis [6,7], ultrafiltration [8,9], electrodeionization [10,11], ion exchange [12,13], vacuum evaporation [14,15], absorption [16,17], and biological water treatment [18,19], have been put forward to degrade toxic pollutants. But how to treat organic waste water with low price, unsurpassable efficiency and stability is still a challenge. Recently, semiconductor-based visible sunlight energy conversion materials have attracted sustained
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attention owing to their great potential in renewable energy systems and environmental applications [20e24]. However, the major challenge in conventional processes is the limitation in absorption of solar energy in the visible range or insufficient carrier separation ability [25e28]. To address this issue, substantial effort has been devoted to the exploration and fabrication of novel photocatalytic materials for expanding the energy utilization to the visible range of the solar spectrum [29e31], including dye-sensitization [32e34], development of new narrow-band gap semiconductors and band engineering of semiconductor composites [35e38]. In recent years, graphitic carbon nitride (g-C3N4), a novel “sustainable” photocatalysis with superior electronic structure (2.7 eV), have aroused great attention due to their potential applications in photocatalytic field [39e44]. It possesses many excellent properties such as inexpensiveness, high photooxidative capabilities, highly thermal and chemical stability, abundancy, innocuity and easy preparation. Wang et al. reported that g-C3N4 has the photocatalytic activities for H2 or O2 production from water splitting under visible light excitation [45]. However, the photocatalytic performance of pure g-C3N4 is limited due to poor light absorption and fast recombination of electronehole pairs. Up to now, continuous attempts have been carried out to promote the photocatalytic efficiency of g-C3N4, such as doping metal, elements and construct the heterojunction between g-C3N4 and another semiconductor with suitable band potential such as Zn2GeO4 [46], TiO2 [47], Co3O4 [48], MoS2 [49], Bi2WO6 [50], Ag3VO4 [51], In2O3 [52], SnS2 [53], or even g-C3N4 itself [54]. The results showed that the photoactivity of nanocomposites was dramatically improved and g-C3N4 could be used as efficient cocatalyst to increase the photocatalytic activity of the semiconductor. At the same time, the layered structure of gC3N4 is beneficial to the transfer of electrons. However, since g-C3N4 was poorly dispersed in the solution, the obtained insufficient contact interface would limit the transfer of photogenerated charges. The obtained insufficient contact interface would limit the transfer of photogenerated charges, which need to be further explored and improved for practical application. Recently, the incorporation of Agþ ions into metal/non-metal oxides constructing Ag-based composite oxide photocatalysts, such as Ag3PO4 [55], Ag2Ta4O11 [56], Ag2CO3 [57], Ag2Mo2O7 [58], Ag2Nb4O11 [59], Ag3VO4 [60], have been proved to be an effective strategy for adjusting band structure and position of the semiconductor to improve photocatalytic activity. It mainly benefits from the uniquely filled d10 electronic configurations of Agþ ions taking part in composition and hybridization of the energy band in the majority of Ag-based compounds [61,62]. Consequently, designing Ag-based composite oxides based on this strategy can adjust electronic structure and light absorption property of photocatalysts. Currently, there are many studies for Ag-based composite photocatalysts, which display high-efficiency decomposing organic pollutants and O2 evolution ability under light irradiation and are believed to be a kind of promising photocatalytic materials. Furthermore, Ag-based photocatalysts are not stable, Ag0 metal always generates from Ag-based composite, and Ag metalsemiconductor materials have gained much interest because the presence of surface plasmon resonance (SPR) of Ag nanoparticles can also improve light harvesting [63,64]. However, the durable operation of Ag-based photocatalysts is difficult to realize owing to their high photocorrosion feature. In fact, for all kinds of photocatalysts, it is the stability and durability that are very significant for their actual application. Nowadays, avoiding and mitigating deactivation of Ag-based photocatalysts is still one major challenge. Although many methods, such as doping, loading and constructing heterojunction, have been developed to improve photocatalytic activity and stability of Ag-based photocatalysts, they also increased the difficulty and complexity in the
synthesis and can not solve deactivation problems completely. Therefore, if we can develop a method which can maintain the long-term stability of Ag-based photocatalyst, we will solve the problem fundamentally. Beside the importance of high heterojunction area for effective reducing recombination of free electron and holes, heterostructure photocatalysts design requires two semiconducting materials which has matching energy band, and it favours the charge separation. To the best of our knowledge, there are no reports which study coupling mode of g-C3N4/Ag2WO4/Ag with metallic Ag0 as plasmonic material. And the photocatalytic mechanism of those inorganic-organic hybrid photocatalyst remains far from clear. Inspired by the role of SPR to improve charge transfer in the structure design of noble metalesemiconductor photocatalysts [65, 66], we synthesized a ternary composite composed of g-C3N4, Ag2WO4 and Ag in this work. The reduced photoluminescence (PL) peak intensity and enhanced photocatalytic performance have shown the effectively improvement of the electron and hole transfer between Ag, g-C3N4 and Ag2WO4. This novel mechanism has open up new opportunities for the development of visible light driven photocatalysts with high efficiency and stability. 2. Experimental 2.1. Materials Commercial P25 TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa, Germany. Methylene blue (MB), sodium tungstate dihydrate (Na2WO4), melamine were provided by Shanghai Chemical Reagent Co. Ltd., China. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. 18 MU deionized water was used for solution preparation. 2.2. Preparation of g-C3N4 Pure g-C3N4 was synthesized by simply heating melamine. 5.0 g melamine powder was put into a muffle furnace and heated to 500 C for 4 h with heating rate of 2 C/min. After naturally cooling to room temperature, the raw g-C3N4 was obtained in a powder form. 2.3. Preparation of g-C3N4/Ag2WO4/Ag hybrid In the typical synthesis, a certain part of g-C3N4, 0.1 mol/L Na2WO4 and 0.2 mol/L AgNO3 were added to 40 mL water and then ultrasonicated at 20 C for 30 min to obtain a uniform mixture. Then, the mixture were transferred into 50 mL Teflon-lined autoclave and subsequently heated at 180 C for 12 h. After filtering with double distilled water and drying at 80 C for 5 h, g-C3N4/ Ag2WO4 composite photocatalysts was obtained. To probe the impact of g-C3N4 content on the photocatalytic performance rates of g-C3N4/Ag2WO4 composites, the as-synthesized photocatalysts were labelled as X%g-C3N4/Ag2WO4, where X refers to the weight ratio of g-C3N4 and Ag2WO4, and X was chosen as 0, 20, 40, 60 and 100. Ag2WO4 was also prepared by using the above mentioned method without g-C3N4. Finally, the resulting X%g-C3N4/Ag2WO4 was irradiated by sunlight for 4 h, X%g-C3N4/Ag2WO4/Ag was obtained. 2.4. Analytical and testing instruments The structure of g-C3N4, Ag2WO4 and g-C3N4/Ag2WO4/Ag heterostructure was observed by JOEL JSM 6610LV scanning electron microscope (SEM) with an INCA x-act energy dispersive spectrometer (EDS) at an accelerating voltage of 20 kV. The X-ray
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photoelectron spectra (XPS) of g-C3N4/Ag2WO4/Ag were measured using a Thermo ESCALAB 250 with an Al Ka X-ray photoelectron spectrometer at 150 W. X-ray diffraction (XRD) data for g-C3N4, Ag2WO4, g-C3N4/Ag2WO4/Ag were acquired using a Rigaku D/MAX 24000 diffractometer at room temperature with Cu Ka radiation (l ¼ 1.5406 Å) with the 2q range from 5 to 60 , operated at 40 kV and 30 mA, and a scanning speed of 0.02 /s. The Brunauer-EmmettTeller (BET) specific surface area values of different samples were determined by using N2 gas adsorption data at 77 K obtained by a Micromeritics ASAP 2010 system with multipoint BET method. UVvis diffuse reflectance spectroscopy (DRS) measurements were tested by using a Hitachi UV-3600 UV-vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 250 to 600 nm, and BaSO4 was used as a reflectance standard. The total organic carbon (TOC) was tested by Elementar Liqui TOC II analyzer. PL spectra of geC3N4ebased materials were recorded by FLS920 combined fluorescence lifetime and steady state spectrometer with 450 W Xe lamp as the excitation light source. 2.5. Photocatalytic experiment In all photocatalytic experiments, 0.03 g of as-prepared catalysts were dispersed in a glass reactor system containing 30 mL 10 mg/L methylene blue (MB) solution under magnetic stirring. 50 W 410 nm LED arrays light was used as visible light source. The distance between LED light and glass reactor is 5 cm. The concentration of MB was tested by the absorbance, and the degradation efficiency was calculated as follows:
h¼
C0 C 100% C0
(1)
where: C0 is the absorbance of original MB solution and C is the absorbance of the dye solution after LED light irradiation. According to the Langmuir-Hinshelwood kinetics model, the photocatalytic process of dyes can be expressed as the following apparent pseudo-first-order kinetics equation:
ln
C0 ¼ kapp t C
(2)
where kapp is the apparent pseudo-first-order rate constant, C0 is original MB concentration and C is MB concentration in aqueous solution at time t. 3. Results and discussion As indicated in Fig. 1, the formation of as-prepared g-C3N4/ Ag2WO4/Ag composited samples can be divided into three stages. At stage (I), when AgNO3 mixed with Na2WO4 and g-C3N4, initial Ag2WO4 nuclei were formed on g-C3N4 by the reaction of AgNO3 and Na2WO4. As illustrated in stage (II), Ag2WO4 nanorods grown directly on Ag2WO4 nuclei via a hydrothermal reaction may be simplified as follows:
2AgNO3 þ Na2 WO4 /Ag2 WO4 þ 2NaNO3
(3)
2Ag2 WO4 þ 2H2 O þ 4hþ þ 4e /4Ag þ 2H2 WO4 þ O2
(4)
þ
At stage (III), the Ag ions on the outer surface of the Ag2WO4 nanorods are reduced by sunlight irradiation when Ag-based nanorods oxidizes water automatically, and then a certain amount of Ag nanoparticles are in-situ produced naturally on the surface of Ag2WO4 nanoparticles, thus, g-C3N4/Ag2WO4/Ag ternary composite was obtained.
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Fig. 2 shows the XRD patterns of g-C3N4, Ag2WO4 and g-C3N4/ Ag2WO4/Ag hybrids. The very strong diffraction angles at 2q ¼ 11.04 , 16.76 , 30.32 , 31.70 , 33.14 , 45.49 , 54.70 and 58.19 , can be assigned to (1 1 0), (0 1 1), (0 0 2), (2 3 1), (4 0 0), (4 0 2), (3 6 1) and (3 3 3) crystal planes of a-Ag2WO4 with the orthorhombic phase and the lines match well with the value reported by JCPDS (No. 34-0061, the space group: pn2n, a ¼ 1.082 nm, b ¼ 1.201 nm and c ¼ 0.59 nm). For pure g-C3N4, the strongest XRD peak at 27.27, corresponding to 0.325 nm, was indexed as (0 0 2) diffraction plane (JCPDS 87-1526, the space group: P6m2, a ¼ 0.4742 nm, b ¼ 0.672 nm). When a-Ag2WO4/Ag composited with g-C3N4, it can be found that the peaks of Ag2WO4 and g-C3N4 are similar with the pure Ag2WO4 and g-C3N4, and the peaks at 38.12 can be assigned to (111) reflections of the cubic Ag (JCPDS file 65-2871, the space group: Fm3m, a ¼ 0.4086 nm). Fig. 3a, 3b and 3c show the SEM images of Ag2WO4, g-C3N4 and 40%g-C3N4/Ag2WO4/Ag hybrids. As indicated in Fig. 3a, the large amount of dispersed Ag2WO4 nanorods with 500e800 nm in diameter and 2e5 mm in length can be easily observed, and the corresponding model of Ag2WO4 nanorod was inserted in Fig. 3a. The g-C3N4 shows the wrinkle two-dimensional structure in Fig. 3b. As indicated in Fig. 3c, g-C3N4 nanoparticles are distributed on the surface of Ag2WO4/Ag nanorods. Fig. 3d, 3e and 3f show the EDS spectra of Ag2WO4, g-C3N4 and 40%g-C3N4/Ag2WO4/Ag hybrids, respectively. The g-C3N4/Ag2WO4/Ag sample contains elements of C, N, O, W and Ag without any other elements. Fig. 4a shows the XPS spectrum including signals for C1s, O1s, N1s, Ag3d and W4f of 40%g-C3N4/Ag2WO4 to probe the chemical environment of the elements in the near surface range. As indicated in Fig. 4b, the asymmetrical and broad features of the observed C1s peaks suggest the co-existence of distinguishable models. A deconvolution core level spectrum at about 284.73 and 288.28 eV has been presented. The major peak at 284.73 eV is exclusively assigned to carbon atoms (CeC bonding) in a pure carbon environment, i.e., graphitic or amorphous carbons either in our sample or adsorbed to the surface. The peak at 288.28 eV is identified as originating from carbon atoms bonded to three nitrogen atoms in the g-C3N4 lattice [67]. The high resolution N1s XPS spectra in Fig. 4c show an asymmetrical feature indicating the co-existence of a number of distinguishable nitrogen environments; fitting with three results in binding energies of 398.83, 399.96 and 401.13 eV. The two peaks at 399.96 and 401.13 eV can be assigned to tertiary nitrogen (N-(C)3) and amino functional groups having a hydrogen atom (CeNeH) [68,69]. The peak at 398.83 eV is typically attributed to N atoms sp2-bonded to two carbon atoms (C]NeC) [70,71], thus confirming the presence of graphite-like sp2-bonded g-C3N4. A typical high-resolution XPS spectrum of Ag3d is shown in Fig. 4d. The peaks at about 367.84 and 373.91 eV are assigned to Ag3d5/2 and 3d3/2, respectively. The Ag3d3/2 is further divided into two different peaks at 373.62 and 374.18 eV and the Ag 3d5/2 peak is also divided into two different peaks at 367.66 and 368.24 eV. The peaks at 374.18 and 368.24 eV can be attributed to metal Ag0, whereas the peaks at 373.62 and 367.66 eV can be attributed to the Agþ of Ag2WO4. According to Fig. 4d, metal Ag0 is produced on the surface of g-C3N4/Ag2WO4/Ag composite, which is consistent with the result of XRD. Fig. 4e and f shows the binding energies of W4f7/ 2 and 4f5/2 are centred at 35.27 and 37.43 eV, and high-resolution spectra of O1s centred at 529.96 eV, respectively, which in agreement with those of pure Ag2WO4 [72]. UV-Vis DRS of the samples have been shown in Fig. 5. The fundamental absorption edge of g-C3N4 is about 463 nm, whereas Ag2WO4 exhibits absorption edge in 391 nm. The band gap energy of the prepared catalysts can be calculated by the following equation:
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Fig. 1. Schematic diagram of the formation of the g-C3N4/Ag2WO4/Ag composite.
Fig. 2. XRD patterns of g-C3N4, Ag2WO4 and g-C3N4/Ag2WO4/Ag hybrids.
ahn hnEg
1=2 . hn
(5)
where: a and Eg are the absorption coefficient and energy band gap (at wave vector k¼0) of the semiconductor, respectively. According to Eq. (3), plots of (ahy)2 versus energy (hy) for photocatalysts were shown in Fig. 5b. From the tangent line of the curve, extrapolated to the hy axis intercept, Eg of g-C3N4 and Ag2WO4 were estimated as 2.68 eV and 3.17 eV. As for the g-C3N4/Ag2WO4/Ag composite, the continuous absorbance in the visible light range was obtained. Compared with the Ag2WO4 and Bi2MoO6, the tail (range from 380 to 600 nm) of the UV-vis DRS curve of Ag2WO4/Ag/Bi2MoO6 composites inclines upward obviously. The particular appearance is likely to SPR effect of the metal nanoparticles on the surface of the composites [73]. Furthermore, with increasing g-C3N4 contents, the absorption edge of g-C3N4/Ag2WO4/Ag hybrid has a clear red shift. It suggests that the heterostructured g-C3N4/Ag2WO4/Ag photocatalyst could be used for visible light photocatalytic reactions. The photoactivity of g-C3N4/Ag2WO4/Ag photocatalysts was studied by degradation of MB under 410 nm LED light irradiation sources. As a comparison, MB degradation with pure Ag2WO4, g-
C3N4, Degussa P25 and no catalyst was also carried out under identical conditions. As shown in Fig. 6a, the degradation of MB in Degussa P25, Ag2WO4, 20%g-C3N4/Ag2WO4/Ag, 40%g-C3N4/ Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4 was 16%, 6%, 70%, 90%, 79% and 53%, respectively. Fig. 6b shows that there is a linear relationship between lnC0/C and t, confirming that the photodegradation reaction is indeed pseudo-first-order. According to Eq. (2) and Fig. 6b, c shows kapp constant with different catalysts. kapp of the photodegradation of MB are 0.0023 min1, 0.0008 min1, 0.0167 min1, 0.0298 min1, 0.0207 min1, and 0.0091 min1 for Degussa P25, Ag2WO4, 20%g-C3N4/Ag2WO4/Ag, 40%g-C3N4/ Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4, respectively. An optimal degradation performance of 90% MB was found for 40%gC3N4/Ag2WO4/Ag, 40%g-C3N4/Ag2WO4/Ag showing the highest photocatalytic activity among commercial Degussa P25, pure gC3N4 and other Ag2WO4-based composites. The reduction of the TOC was also presented in Fig. 6d to show the complete mineralization efficiency of MB by Degussa P25, Ag2WO4, 20%g-C3N4/ Ag2WO4/Ag, 40%g-C3N4/Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4, respectively. Fig. S1a shows the N2 gas adsorptiondesorption isotherms for Ag2WO4, g-C3N4 and g-C3N4/Ag2WO4/Ag composites. According to Fig. 6a and Fig. S1a, the absorption capacity of g-C3N4/Ag2WO4/Ag composites is not a major factor that obviously influences their photocatalytic performance. In order to investigate the influence of the Ag2WO4/Ag on photocatalytic performance of g-C3N4/Ag2WO4/Ag hybrid, PL analysis was carried out to discuss the separation efficiency of photogenerated electrons and holes in g-C3N4/Ag2WO4/Ag hybrids. Fig. S2 shows the PL spectra of the pure g-C3N4 and g-C3N4/ Ag2WO4/Ag materials excited by 325 nm. The main emission peak was centred at 461 nm for the pure g-C3N4 sample, which was similar to the former reports [74]. For g-C3N4/Ag2WO4/Ag hybrids, the position of the emission peak in the PL spectrum was slightly blue shift to that of the pure g-C3N4, but the emission intensity significantly decreased, which indicated that the g-C3N4/Ag2WO4/ Ag hybrids had much lower recombination rate of photo-generated charge carriers. This demonstrated that the recombination of photogenerated charge carriers was greatly inhibited by the introduction of Ag2WO4, showing that the photogenerated electrons and holes in g-C3N4/Ag2WO4/Ag hybrid materials had higher separation efficiency than those in the pure g-C3N4. It's known to all that the enhancement of photocatalytic activity of hybrid photocatalysts was mainly attributed to electrons and holes transfer at the interfaces of photocatalysts. When g-C3N4
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Fig. 3. SEM images of (a) Ag2WO4, (b) g-C3N4 and (c) 40%g-C3N4/Ag2WO4/Ag and EDS spectra of (d) Ag2WO4, (e) g-C3N4 and (f) 40%g-C3N4/Ag2WO4/Ag.
coupled with Ag2WO4 and Ag, the band edge potential value of gC3N4 and g-C3N4/Ag2WO4/Ag materials played an important role in studying the efficient generation and separation process of the electrons and holes pairs. The valence band (VB) potentials of a semiconductor at the point of zero charge can be theoretically predicted by the following empirical equation:
EVB ¼ X Ec þ 0:5Eg
(6)
where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ec is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Moreover, the conduction band (CB) edge potential (ECB) can be obtained by ECB ¼ EVBEg. The CB and VB edge potentials of Ag2WO4 were calculated at þ0.02 eV and þ3.19 eV, respectively. The CB and VB edge potentials of g-C3N4 were calculated at 1.12 eV and þ1.56 eV, respectively. The potentials of CB and VB of g-C3N4 are negative compared with those of Ag2WO4. The major photocatalytic reaction procedures in this research can be explained as the following equations:
Ag0 þ hn/Ag*
(7)
Ag* þ C3 N4 =Ag2 WO4 þ hn/Ag2 WO4 e þ C3 N4 hþ þ Ag (8) 2e þ 2Hþ þ O2 þ 2Hþ /H2 O2
(9)
e þ O2 /$O 2
(10)
H2 O2 þ O2 /$OH þ OH þ O2
(11)
hþ þ H2 O/$OH þ H þ
(12)
$OH þ MB/deg radation product
(13)
As far as g-C3N4 is concerned, the photogenerated electron-hole pairs quickly recombine and only few amounts of electron and hole can work in photocatalytic process. When the as-prepared g-C3N4/
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Fig. 4. The overview (a) and the corresponding high-resolution XPS spectra (b) C1s, (c) N1s, (d) Ag3d, (e) W4f, and (f) O1s of the as-prepared 40%g-C3N4/Ag2WO4/Ag.
Fig. 5. (a) UV-vis DRS spectra and (b) plots of (ahy)2 versus energy (hy) for pure Ag2WO4, g-C3N4 and g-C3N4/Ag2WO4/Ag hybrids.
Ag2WO4/Ag samples were irradiated by visible LED light, as indicated in Fig. 7, the energy of LED light exceeds band gap of g-C3N4, the electrons of g-C3N4 VB are excited to the CB, leaving holes in the VB. At the same time, Ag0 metal can absorb the visible light and
irradiate electronehole pairs due to SPR effect. Plasmon activation of Ag particles transfers electrons to CB of g-C3N4 and Ag2WO4, the similar with the literature [75]. And also the excited electrons of CB of g-C3N4 can transfer to CB of Ag2WO4. And the holes can stay in g-
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Fig. 6. (a) Photocatalytic degradation of MB under 410 nm LED light irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB degradation, (c) the apparent pseudo-firstorder rate constant kapp with different catalysts and (d) TOC degradation rate.
Fig. 7. A proposed visible LED light photodegradation mechanism of g-C3N4/Ag2WO4/Ag hybrid photocatalyst.
C3N4 VB. Therefore, electron and hole pairs can be easily and effectively separated. At almost the same time, electron can react with O2 or H2O molecular in the MB solution and $O 2 radicals will appear. As a result, more highly stable g-C3N4/Ag2WO4/Ag composite is obtained and displays better photocatalytic performance on MB treatment than pure g-C3N4 or Ag2WO4. However, when the content of Ag2WO4 further increased above its optimum value, the photocatalytic performance deteriorates, that is because some
Ag3WO4 and Ag will act as a kind of recombination center. Thus, 40%g-C3N4/Ag2WO4/Ag is the most suitable material in this study. Recycling stability of photocatalyst sample is particularly important for practical applications. To evaluate the stability of 40% g-C3N4/Ag2WO4/Ag hybrids, the recycled experiment was carried out under the same time. As shown in Fig. 8, the photocatalytic activity of the as-prepared 40%g-C3N4/Ag2WO4/Ag hybrids photocatalyst exhibited almost no decrease even after 5 times cycling.
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Fig. 8. Photodegradation performance within five cycles for 40%g-C3N4/Ag2WO4/Ag.
4. Conclusions In summary, a novel g-C3N4/Ag2WO4/Ag ternary composite with well-designed structure has been synthesized and showed higher visible-light photocatalytic activity for degradation of MB than those of pure g-C3N4 and Ag2WO4. The experiment results showed that the 40%g-C3N4/Ag2WO4/Ag had the highest photoactivity. The special plasmonic ternary composite favor the enhancement of light absorption and effective electron-hole separation, which exhibit wide spectral response and provide a new strategy for advanced photocatalyst design. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572103, 51302101 and 21303129), the Natural Science Foundation of Anhui Province (1408085QE78), the Foundation for Young Talents in College of Anhui Province (12600941) and Collaborative Innovation Center of Advanced Functional Materials (XTZX103732015008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.04.065. References [1] M.J. Damasiewicz, K.R. Polkinghorne, P.G. Kerr, Nat. Rev. Nephrol. 8 (2012) 725e734. [2] T.J. Qiao, Z.R. Yu, X.H. Zhang, D.W.T. Au, J. Environ. Monit. 13 (2011) 3097e3103. [3] D. Serpa, J.J. Keizer, J. Cassidy, A. Cuco, V. Silva, F. Gonçalves, M. Cerqueira, N. Abrantes, Environ. Sci. Process. Impacts 16 (2014) 1434e1444. [4] J.G. Liu, J. Diamond, Nature 435 (2005) 1179e1186. [5] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 749e750. [6] X.H. Lin, D. Sriramulu, S. Fong, Y. Li, Water Res. 68 (2015) 831e838. [7] B.E. Logan, M. Elimelech, Nature 488 (2012) 313e319. [8] R. Ghosh, Lab. Chip 14 (2014) 4559e4566. [9] M. Katarzyna, E. Goralska, A. Sobczynska, J. Szymanowski, Green Chem. 6 (2004) 176e182. € [10] A. Ozgür, Y. Ümran, K. Nalan, Y. Mithat, Desalination 342 (2014) 16e22. [11] X.L. Shen, T.J. Li, X.P. Jiang, X.M. Chen, Sep. Purif. Technol. 128 (2014) 39e44. [12] A.A. Moya, E. Belashova, P. Sistat, J. Membr. Sci. 474 (2015) 215e223. [13] N.P.G.N. Chandrasekara, R.M. Pashley, Desalination 357 (2015) 131e139. rrez, L. Alberto, J.M. Benito, J. Coca, C. Pazos, J. Hazard. Mater. 185 [14] G. Gutie (2011) 1569e1574. rrez, J.M. Benito, J. Coca, C. Pazos, Chem. Eng. J. 162 (2010) 201e207. [15] G. Gutie [16] W.D. Liang, Y. Liu, H.X. Sun, Z.Q. Zhu, X.H. Zhao, A. Li, W.Q. Deng, RSC Adv. 4
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A facile fabrication of plasmonic g-C3N4/Ag2WO4/Ag ternary heterojunction visible-light photocatalyst Kai Dai a, *, Jiali Lv a, Luhua Lu b, **, Changhao Liang a, c, Lei Geng a, Guangping Zhu a a
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, PR China Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan, 430074, PR China c Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
g-C3N4/Ag2WO4/Ag ternary nanocomposite photocatalyst was prepared. g-C3N4/Ag2WO4/Ag showed high photocatalytic activity. g-C3N4/Ag2WO4/Ag showed long reusable life.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2015 Received in revised form 17 March 2016 Accepted 2 April 2016 Available online 23 April 2016
It's important to reduce recombination of electrons and holes and enhance charge transfer through fine controlled interfacial structure. In this work, novel graphitic-C3N4 (g-C3N4)/Ag2WO4/Ag ternary photocatalyst has been synthesized by deposition of Ag2WO4 onto g-C3N4 template and followed by sun light reduction of Ag2WO4 into Ag2WO4/Ag. As-prepared g-C3N4/Ag2WO4/Ag presented significantly enhanced photocatalytic performance in degrading methylene blue (MB) under 410 nm LED light irradiation. Metallic Ag0 is used as plasmonic hot spots to generate high energy charge carriers. Optimal g-C3N4 content has been confirmed to be 40 wt%, corresponding to apparent pseudo-first-order rate constant kapp of 0.0298 min1, which is 3.3 times and 37.3 times more than that of pure g-C3N4 and Ag2WO4, respectively. This novel ternary g-C3N4/Ag2WO4/Ag structure material is an ideal candidate in environmental treatment and purifying applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Electron microscopy Crystal structure
1. Introduction The continuous increasing world's population, together with the substantial development of industry has brought about imperious
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Dai), [email protected] (L. Lu). http://dx.doi.org/10.1016/j.matchemphys.2016.04.065 0254-0584/© 2016 Elsevier B.V. All rights reserved.
attention for the environmental protection [1e5]. Many technologies, such as reverse osmosis [6,7], ultrafiltration [8,9], electrodeionization [10,11], ion exchange [12,13], vacuum evaporation [14,15], absorption [16,17], and biological water treatment [18,19], have been put forward to degrade toxic pollutants. But how to treat organic waste water with low price, unsurpassable efficiency and stability is still a challenge. Recently, semiconductor-based visible sunlight energy conversion materials have attracted sustained
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attention owing to their great potential in renewable energy systems and environmental applications [20e24]. However, the major challenge in conventional processes is the limitation in absorption of solar energy in the visible range or insufficient carrier separation ability [25e28]. To address this issue, substantial effort has been devoted to the exploration and fabrication of novel photocatalytic materials for expanding the energy utilization to the visible range of the solar spectrum [29e31], including dye-sensitization [32e34], development of new narrow-band gap semiconductors and band engineering of semiconductor composites [35e38]. In recent years, graphitic carbon nitride (g-C3N4), a novel “sustainable” photocatalysis with superior electronic structure (2.7 eV), have aroused great attention due to their potential applications in photocatalytic field [39e44]. It possesses many excellent properties such as inexpensiveness, high photooxidative capabilities, highly thermal and chemical stability, abundancy, innocuity and easy preparation. Wang et al. reported that g-C3N4 has the photocatalytic activities for H2 or O2 production from water splitting under visible light excitation [45]. However, the photocatalytic performance of pure g-C3N4 is limited due to poor light absorption and fast recombination of electronehole pairs. Up to now, continuous attempts have been carried out to promote the photocatalytic efficiency of g-C3N4, such as doping metal, elements and construct the heterojunction between g-C3N4 and another semiconductor with suitable band potential such as Zn2GeO4 [46], TiO2 [47], Co3O4 [48], MoS2 [49], Bi2WO6 [50], Ag3VO4 [51], In2O3 [52], SnS2 [53], or even g-C3N4 itself [54]. The results showed that the photoactivity of nanocomposites was dramatically improved and g-C3N4 could be used as efficient cocatalyst to increase the photocatalytic activity of the semiconductor. At the same time, the layered structure of gC3N4 is beneficial to the transfer of electrons. However, since g-C3N4 was poorly dispersed in the solution, the obtained insufficient contact interface would limit the transfer of photogenerated charges. The obtained insufficient contact interface would limit the transfer of photogenerated charges, which need to be further explored and improved for practical application. Recently, the incorporation of Agþ ions into metal/non-metal oxides constructing Ag-based composite oxide photocatalysts, such as Ag3PO4 [55], Ag2Ta4O11 [56], Ag2CO3 [57], Ag2Mo2O7 [58], Ag2Nb4O11 [59], Ag3VO4 [60], have been proved to be an effective strategy for adjusting band structure and position of the semiconductor to improve photocatalytic activity. It mainly benefits from the uniquely filled d10 electronic configurations of Agþ ions taking part in composition and hybridization of the energy band in the majority of Ag-based compounds [61,62]. Consequently, designing Ag-based composite oxides based on this strategy can adjust electronic structure and light absorption property of photocatalysts. Currently, there are many studies for Ag-based composite photocatalysts, which display high-efficiency decomposing organic pollutants and O2 evolution ability under light irradiation and are believed to be a kind of promising photocatalytic materials. Furthermore, Ag-based photocatalysts are not stable, Ag0 metal always generates from Ag-based composite, and Ag metalsemiconductor materials have gained much interest because the presence of surface plasmon resonance (SPR) of Ag nanoparticles can also improve light harvesting [63,64]. However, the durable operation of Ag-based photocatalysts is difficult to realize owing to their high photocorrosion feature. In fact, for all kinds of photocatalysts, it is the stability and durability that are very significant for their actual application. Nowadays, avoiding and mitigating deactivation of Ag-based photocatalysts is still one major challenge. Although many methods, such as doping, loading and constructing heterojunction, have been developed to improve photocatalytic activity and stability of Ag-based photocatalysts, they also increased the difficulty and complexity in the
synthesis and can not solve deactivation problems completely. Therefore, if we can develop a method which can maintain the long-term stability of Ag-based photocatalyst, we will solve the problem fundamentally. Beside the importance of high heterojunction area for effective reducing recombination of free electron and holes, heterostructure photocatalysts design requires two semiconducting materials which has matching energy band, and it favours the charge separation. To the best of our knowledge, there are no reports which study coupling mode of g-C3N4/Ag2WO4/Ag with metallic Ag0 as plasmonic material. And the photocatalytic mechanism of those inorganic-organic hybrid photocatalyst remains far from clear. Inspired by the role of SPR to improve charge transfer in the structure design of noble metalesemiconductor photocatalysts [65, 66], we synthesized a ternary composite composed of g-C3N4, Ag2WO4 and Ag in this work. The reduced photoluminescence (PL) peak intensity and enhanced photocatalytic performance have shown the effectively improvement of the electron and hole transfer between Ag, g-C3N4 and Ag2WO4. This novel mechanism has open up new opportunities for the development of visible light driven photocatalysts with high efficiency and stability. 2. Experimental 2.1. Materials Commercial P25 TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa, Germany. Methylene blue (MB), sodium tungstate dihydrate (Na2WO4), melamine were provided by Shanghai Chemical Reagent Co. Ltd., China. Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co. Ltd., China. 18 MU deionized water was used for solution preparation. 2.2. Preparation of g-C3N4 Pure g-C3N4 was synthesized by simply heating melamine. 5.0 g melamine powder was put into a muffle furnace and heated to 500 C for 4 h with heating rate of 2 C/min. After naturally cooling to room temperature, the raw g-C3N4 was obtained in a powder form. 2.3. Preparation of g-C3N4/Ag2WO4/Ag hybrid In the typical synthesis, a certain part of g-C3N4, 0.1 mol/L Na2WO4 and 0.2 mol/L AgNO3 were added to 40 mL water and then ultrasonicated at 20 C for 30 min to obtain a uniform mixture. Then, the mixture were transferred into 50 mL Teflon-lined autoclave and subsequently heated at 180 C for 12 h. After filtering with double distilled water and drying at 80 C for 5 h, g-C3N4/ Ag2WO4 composite photocatalysts was obtained. To probe the impact of g-C3N4 content on the photocatalytic performance rates of g-C3N4/Ag2WO4 composites, the as-synthesized photocatalysts were labelled as X%g-C3N4/Ag2WO4, where X refers to the weight ratio of g-C3N4 and Ag2WO4, and X was chosen as 0, 20, 40, 60 and 100. Ag2WO4 was also prepared by using the above mentioned method without g-C3N4. Finally, the resulting X%g-C3N4/Ag2WO4 was irradiated by sunlight for 4 h, X%g-C3N4/Ag2WO4/Ag was obtained. 2.4. Analytical and testing instruments The structure of g-C3N4, Ag2WO4 and g-C3N4/Ag2WO4/Ag heterostructure was observed by JOEL JSM 6610LV scanning electron microscope (SEM) with an INCA x-act energy dispersive spectrometer (EDS) at an accelerating voltage of 20 kV. The X-ray
K. Dai et al. / Materials Chemistry and Physics 177 (2016) 529e537
photoelectron spectra (XPS) of g-C3N4/Ag2WO4/Ag were measured using a Thermo ESCALAB 250 with an Al Ka X-ray photoelectron spectrometer at 150 W. X-ray diffraction (XRD) data for g-C3N4, Ag2WO4, g-C3N4/Ag2WO4/Ag were acquired using a Rigaku D/MAX 24000 diffractometer at room temperature with Cu Ka radiation (l ¼ 1.5406 Å) with the 2q range from 5 to 60 , operated at 40 kV and 30 mA, and a scanning speed of 0.02 /s. The Brunauer-EmmettTeller (BET) specific surface area values of different samples were determined by using N2 gas adsorption data at 77 K obtained by a Micromeritics ASAP 2010 system with multipoint BET method. UVvis diffuse reflectance spectroscopy (DRS) measurements were tested by using a Hitachi UV-3600 UV-vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 250 to 600 nm, and BaSO4 was used as a reflectance standard. The total organic carbon (TOC) was tested by Elementar Liqui TOC II analyzer. PL spectra of geC3N4ebased materials were recorded by FLS920 combined fluorescence lifetime and steady state spectrometer with 450 W Xe lamp as the excitation light source. 2.5. Photocatalytic experiment In all photocatalytic experiments, 0.03 g of as-prepared catalysts were dispersed in a glass reactor system containing 30 mL 10 mg/L methylene blue (MB) solution under magnetic stirring. 50 W 410 nm LED arrays light was used as visible light source. The distance between LED light and glass reactor is 5 cm. The concentration of MB was tested by the absorbance, and the degradation efficiency was calculated as follows:
h¼
C0 C 100% C0
(1)
where: C0 is the absorbance of original MB solution and C is the absorbance of the dye solution after LED light irradiation. According to the Langmuir-Hinshelwood kinetics model, the photocatalytic process of dyes can be expressed as the following apparent pseudo-first-order kinetics equation:
ln
C0 ¼ kapp t C
(2)
where kapp is the apparent pseudo-first-order rate constant, C0 is original MB concentration and C is MB concentration in aqueous solution at time t. 3. Results and discussion As indicated in Fig. 1, the formation of as-prepared g-C3N4/ Ag2WO4/Ag composited samples can be divided into three stages. At stage (I), when AgNO3 mixed with Na2WO4 and g-C3N4, initial Ag2WO4 nuclei were formed on g-C3N4 by the reaction of AgNO3 and Na2WO4. As illustrated in stage (II), Ag2WO4 nanorods grown directly on Ag2WO4 nuclei via a hydrothermal reaction may be simplified as follows:
2AgNO3 þ Na2 WO4 /Ag2 WO4 þ 2NaNO3
(3)
2Ag2 WO4 þ 2H2 O þ 4hþ þ 4e /4Ag þ 2H2 WO4 þ O2
(4)
þ
At stage (III), the Ag ions on the outer surface of the Ag2WO4 nanorods are reduced by sunlight irradiation when Ag-based nanorods oxidizes water automatically, and then a certain amount of Ag nanoparticles are in-situ produced naturally on the surface of Ag2WO4 nanoparticles, thus, g-C3N4/Ag2WO4/Ag ternary composite was obtained.
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Fig. 2 shows the XRD patterns of g-C3N4, Ag2WO4 and g-C3N4/ Ag2WO4/Ag hybrids. The very strong diffraction angles at 2q ¼ 11.04 , 16.76 , 30.32 , 31.70 , 33.14 , 45.49 , 54.70 and 58.19 , can be assigned to (1 1 0), (0 1 1), (0 0 2), (2 3 1), (4 0 0), (4 0 2), (3 6 1) and (3 3 3) crystal planes of a-Ag2WO4 with the orthorhombic phase and the lines match well with the value reported by JCPDS (No. 34-0061, the space group: pn2n, a ¼ 1.082 nm, b ¼ 1.201 nm and c ¼ 0.59 nm). For pure g-C3N4, the strongest XRD peak at 27.27, corresponding to 0.325 nm, was indexed as (0 0 2) diffraction plane (JCPDS 87-1526, the space group: P6m2, a ¼ 0.4742 nm, b ¼ 0.672 nm). When a-Ag2WO4/Ag composited with g-C3N4, it can be found that the peaks of Ag2WO4 and g-C3N4 are similar with the pure Ag2WO4 and g-C3N4, and the peaks at 38.12 can be assigned to (111) reflections of the cubic Ag (JCPDS file 65-2871, the space group: Fm3m, a ¼ 0.4086 nm). Fig. 3a, 3b and 3c show the SEM images of Ag2WO4, g-C3N4 and 40%g-C3N4/Ag2WO4/Ag hybrids. As indicated in Fig. 3a, the large amount of dispersed Ag2WO4 nanorods with 500e800 nm in diameter and 2e5 mm in length can be easily observed, and the corresponding model of Ag2WO4 nanorod was inserted in Fig. 3a. The g-C3N4 shows the wrinkle two-dimensional structure in Fig. 3b. As indicated in Fig. 3c, g-C3N4 nanoparticles are distributed on the surface of Ag2WO4/Ag nanorods. Fig. 3d, 3e and 3f show the EDS spectra of Ag2WO4, g-C3N4 and 40%g-C3N4/Ag2WO4/Ag hybrids, respectively. The g-C3N4/Ag2WO4/Ag sample contains elements of C, N, O, W and Ag without any other elements. Fig. 4a shows the XPS spectrum including signals for C1s, O1s, N1s, Ag3d and W4f of 40%g-C3N4/Ag2WO4 to probe the chemical environment of the elements in the near surface range. As indicated in Fig. 4b, the asymmetrical and broad features of the observed C1s peaks suggest the co-existence of distinguishable models. A deconvolution core level spectrum at about 284.73 and 288.28 eV has been presented. The major peak at 284.73 eV is exclusively assigned to carbon atoms (CeC bonding) in a pure carbon environment, i.e., graphitic or amorphous carbons either in our sample or adsorbed to the surface. The peak at 288.28 eV is identified as originating from carbon atoms bonded to three nitrogen atoms in the g-C3N4 lattice [67]. The high resolution N1s XPS spectra in Fig. 4c show an asymmetrical feature indicating the co-existence of a number of distinguishable nitrogen environments; fitting with three results in binding energies of 398.83, 399.96 and 401.13 eV. The two peaks at 399.96 and 401.13 eV can be assigned to tertiary nitrogen (N-(C)3) and amino functional groups having a hydrogen atom (CeNeH) [68,69]. The peak at 398.83 eV is typically attributed to N atoms sp2-bonded to two carbon atoms (C]NeC) [70,71], thus confirming the presence of graphite-like sp2-bonded g-C3N4. A typical high-resolution XPS spectrum of Ag3d is shown in Fig. 4d. The peaks at about 367.84 and 373.91 eV are assigned to Ag3d5/2 and 3d3/2, respectively. The Ag3d3/2 is further divided into two different peaks at 373.62 and 374.18 eV and the Ag 3d5/2 peak is also divided into two different peaks at 367.66 and 368.24 eV. The peaks at 374.18 and 368.24 eV can be attributed to metal Ag0, whereas the peaks at 373.62 and 367.66 eV can be attributed to the Agþ of Ag2WO4. According to Fig. 4d, metal Ag0 is produced on the surface of g-C3N4/Ag2WO4/Ag composite, which is consistent with the result of XRD. Fig. 4e and f shows the binding energies of W4f7/ 2 and 4f5/2 are centred at 35.27 and 37.43 eV, and high-resolution spectra of O1s centred at 529.96 eV, respectively, which in agreement with those of pure Ag2WO4 [72]. UV-Vis DRS of the samples have been shown in Fig. 5. The fundamental absorption edge of g-C3N4 is about 463 nm, whereas Ag2WO4 exhibits absorption edge in 391 nm. The band gap energy of the prepared catalysts can be calculated by the following equation:
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Fig. 1. Schematic diagram of the formation of the g-C3N4/Ag2WO4/Ag composite.
Fig. 2. XRD patterns of g-C3N4, Ag2WO4 and g-C3N4/Ag2WO4/Ag hybrids.
ahn hnEg
1=2 . hn
(5)
where: a and Eg are the absorption coefficient and energy band gap (at wave vector k¼0) of the semiconductor, respectively. According to Eq. (3), plots of (ahy)2 versus energy (hy) for photocatalysts were shown in Fig. 5b. From the tangent line of the curve, extrapolated to the hy axis intercept, Eg of g-C3N4 and Ag2WO4 were estimated as 2.68 eV and 3.17 eV. As for the g-C3N4/Ag2WO4/Ag composite, the continuous absorbance in the visible light range was obtained. Compared with the Ag2WO4 and Bi2MoO6, the tail (range from 380 to 600 nm) of the UV-vis DRS curve of Ag2WO4/Ag/Bi2MoO6 composites inclines upward obviously. The particular appearance is likely to SPR effect of the metal nanoparticles on the surface of the composites [73]. Furthermore, with increasing g-C3N4 contents, the absorption edge of g-C3N4/Ag2WO4/Ag hybrid has a clear red shift. It suggests that the heterostructured g-C3N4/Ag2WO4/Ag photocatalyst could be used for visible light photocatalytic reactions. The photoactivity of g-C3N4/Ag2WO4/Ag photocatalysts was studied by degradation of MB under 410 nm LED light irradiation sources. As a comparison, MB degradation with pure Ag2WO4, g-
C3N4, Degussa P25 and no catalyst was also carried out under identical conditions. As shown in Fig. 6a, the degradation of MB in Degussa P25, Ag2WO4, 20%g-C3N4/Ag2WO4/Ag, 40%g-C3N4/ Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4 was 16%, 6%, 70%, 90%, 79% and 53%, respectively. Fig. 6b shows that there is a linear relationship between lnC0/C and t, confirming that the photodegradation reaction is indeed pseudo-first-order. According to Eq. (2) and Fig. 6b, c shows kapp constant with different catalysts. kapp of the photodegradation of MB are 0.0023 min1, 0.0008 min1, 0.0167 min1, 0.0298 min1, 0.0207 min1, and 0.0091 min1 for Degussa P25, Ag2WO4, 20%g-C3N4/Ag2WO4/Ag, 40%g-C3N4/ Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4, respectively. An optimal degradation performance of 90% MB was found for 40%gC3N4/Ag2WO4/Ag, 40%g-C3N4/Ag2WO4/Ag showing the highest photocatalytic activity among commercial Degussa P25, pure gC3N4 and other Ag2WO4-based composites. The reduction of the TOC was also presented in Fig. 6d to show the complete mineralization efficiency of MB by Degussa P25, Ag2WO4, 20%g-C3N4/ Ag2WO4/Ag, 40%g-C3N4/Ag2WO4/Ag, 60%g-C3N4/Ag2WO4/Ag and g-C3N4, respectively. Fig. S1a shows the N2 gas adsorptiondesorption isotherms for Ag2WO4, g-C3N4 and g-C3N4/Ag2WO4/Ag composites. According to Fig. 6a and Fig. S1a, the absorption capacity of g-C3N4/Ag2WO4/Ag composites is not a major factor that obviously influences their photocatalytic performance. In order to investigate the influence of the Ag2WO4/Ag on photocatalytic performance of g-C3N4/Ag2WO4/Ag hybrid, PL analysis was carried out to discuss the separation efficiency of photogenerated electrons and holes in g-C3N4/Ag2WO4/Ag hybrids. Fig. S2 shows the PL spectra of the pure g-C3N4 and g-C3N4/ Ag2WO4/Ag materials excited by 325 nm. The main emission peak was centred at 461 nm for the pure g-C3N4 sample, which was similar to the former reports [74]. For g-C3N4/Ag2WO4/Ag hybrids, the position of the emission peak in the PL spectrum was slightly blue shift to that of the pure g-C3N4, but the emission intensity significantly decreased, which indicated that the g-C3N4/Ag2WO4/ Ag hybrids had much lower recombination rate of photo-generated charge carriers. This demonstrated that the recombination of photogenerated charge carriers was greatly inhibited by the introduction of Ag2WO4, showing that the photogenerated electrons and holes in g-C3N4/Ag2WO4/Ag hybrid materials had higher separation efficiency than those in the pure g-C3N4. It's known to all that the enhancement of photocatalytic activity of hybrid photocatalysts was mainly attributed to electrons and holes transfer at the interfaces of photocatalysts. When g-C3N4
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Fig. 3. SEM images of (a) Ag2WO4, (b) g-C3N4 and (c) 40%g-C3N4/Ag2WO4/Ag and EDS spectra of (d) Ag2WO4, (e) g-C3N4 and (f) 40%g-C3N4/Ag2WO4/Ag.
coupled with Ag2WO4 and Ag, the band edge potential value of gC3N4 and g-C3N4/Ag2WO4/Ag materials played an important role in studying the efficient generation and separation process of the electrons and holes pairs. The valence band (VB) potentials of a semiconductor at the point of zero charge can be theoretically predicted by the following empirical equation:
EVB ¼ X Ec þ 0:5Eg
(6)
where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ec is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Moreover, the conduction band (CB) edge potential (ECB) can be obtained by ECB ¼ EVBEg. The CB and VB edge potentials of Ag2WO4 were calculated at þ0.02 eV and þ3.19 eV, respectively. The CB and VB edge potentials of g-C3N4 were calculated at 1.12 eV and þ1.56 eV, respectively. The potentials of CB and VB of g-C3N4 are negative compared with those of Ag2WO4. The major photocatalytic reaction procedures in this research can be explained as the following equations:
Ag0 þ hn/Ag*
(7)
Ag* þ C3 N4 =Ag2 WO4 þ hn/Ag2 WO4 e þ C3 N4 hþ þ Ag (8) 2e þ 2Hþ þ O2 þ 2Hþ /H2 O2
(9)
e þ O2 /$O 2
(10)
H2 O2 þ O2 /$OH þ OH þ O2
(11)
hþ þ H2 O/$OH þ H þ
(12)
$OH þ MB/deg radation product
(13)
As far as g-C3N4 is concerned, the photogenerated electron-hole pairs quickly recombine and only few amounts of electron and hole can work in photocatalytic process. When the as-prepared g-C3N4/
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Fig. 4. The overview (a) and the corresponding high-resolution XPS spectra (b) C1s, (c) N1s, (d) Ag3d, (e) W4f, and (f) O1s of the as-prepared 40%g-C3N4/Ag2WO4/Ag.
Fig. 5. (a) UV-vis DRS spectra and (b) plots of (ahy)2 versus energy (hy) for pure Ag2WO4, g-C3N4 and g-C3N4/Ag2WO4/Ag hybrids.
Ag2WO4/Ag samples were irradiated by visible LED light, as indicated in Fig. 7, the energy of LED light exceeds band gap of g-C3N4, the electrons of g-C3N4 VB are excited to the CB, leaving holes in the VB. At the same time, Ag0 metal can absorb the visible light and
irradiate electronehole pairs due to SPR effect. Plasmon activation of Ag particles transfers electrons to CB of g-C3N4 and Ag2WO4, the similar with the literature [75]. And also the excited electrons of CB of g-C3N4 can transfer to CB of Ag2WO4. And the holes can stay in g-
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Fig. 6. (a) Photocatalytic degradation of MB under 410 nm LED light irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB degradation, (c) the apparent pseudo-firstorder rate constant kapp with different catalysts and (d) TOC degradation rate.
Fig. 7. A proposed visible LED light photodegradation mechanism of g-C3N4/Ag2WO4/Ag hybrid photocatalyst.
C3N4 VB. Therefore, electron and hole pairs can be easily and effectively separated. At almost the same time, electron can react with O2 or H2O molecular in the MB solution and $O 2 radicals will appear. As a result, more highly stable g-C3N4/Ag2WO4/Ag composite is obtained and displays better photocatalytic performance on MB treatment than pure g-C3N4 or Ag2WO4. However, when the content of Ag2WO4 further increased above its optimum value, the photocatalytic performance deteriorates, that is because some
Ag3WO4 and Ag will act as a kind of recombination center. Thus, 40%g-C3N4/Ag2WO4/Ag is the most suitable material in this study. Recycling stability of photocatalyst sample is particularly important for practical applications. To evaluate the stability of 40% g-C3N4/Ag2WO4/Ag hybrids, the recycled experiment was carried out under the same time. As shown in Fig. 8, the photocatalytic activity of the as-prepared 40%g-C3N4/Ag2WO4/Ag hybrids photocatalyst exhibited almost no decrease even after 5 times cycling.
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Fig. 8. Photodegradation performance within five cycles for 40%g-C3N4/Ag2WO4/Ag.
4. Conclusions In summary, a novel g-C3N4/Ag2WO4/Ag ternary composite with well-designed structure has been synthesized and showed higher visible-light photocatalytic activity for degradation of MB than those of pure g-C3N4 and Ag2WO4. The experiment results showed that the 40%g-C3N4/Ag2WO4/Ag had the highest photoactivity. The special plasmonic ternary composite favor the enhancement of light absorption and effective electron-hole separation, which exhibit wide spectral response and provide a new strategy for advanced photocatalyst design. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572103, 51302101 and 21303129), the Natural Science Foundation of Anhui Province (1408085QE78), the Foundation for Young Talents in College of Anhui Province (12600941) and Collaborative Innovation Center of Advanced Functional Materials (XTZX103732015008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.04.065. References [1] M.J. Damasiewicz, K.R. Polkinghorne, P.G. Kerr, Nat. Rev. Nephrol. 8 (2012) 725e734. [2] T.J. Qiao, Z.R. Yu, X.H. Zhang, D.W.T. Au, J. Environ. Monit. 13 (2011) 3097e3103. [3] D. Serpa, J.J. Keizer, J. Cassidy, A. Cuco, V. Silva, F. Gonçalves, M. Cerqueira, N. Abrantes, Environ. Sci. Process. Impacts 16 (2014) 1434e1444. [4] J.G. Liu, J. Diamond, Nature 435 (2005) 1179e1186. [5] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 749e750. [6] X.H. Lin, D. Sriramulu, S. Fong, Y. Li, Water Res. 68 (2015) 831e838. [7] B.E. Logan, M. Elimelech, Nature 488 (2012) 313e319. [8] R. Ghosh, Lab. Chip 14 (2014) 4559e4566. [9] M. Katarzyna, E. Goralska, A. Sobczynska, J. Szymanowski, Green Chem. 6 (2004) 176e182. € [10] A. Ozgür, Y. Ümran, K. Nalan, Y. Mithat, Desalination 342 (2014) 16e22. [11] X.L. Shen, T.J. Li, X.P. Jiang, X.M. Chen, Sep. Purif. Technol. 128 (2014) 39e44. [12] A.A. Moya, E. Belashova, P. Sistat, J. Membr. Sci. 474 (2015) 215e223. [13] N.P.G.N. Chandrasekara, R.M. Pashley, Desalination 357 (2015) 131e139. rrez, L. Alberto, J.M. Benito, J. Coca, C. Pazos, J. Hazard. Mater. 185 [14] G. Gutie (2011) 1569e1574. rrez, J.M. Benito, J. Coca, C. Pazos, Chem. Eng. J. 162 (2010) 201e207. [15] G. Gutie [16] W.D. Liang, Y. Liu, H.X. Sun, Z.Q. Zhu, X.H. Zhao, A. Li, W.Q. Deng, RSC Adv. 4
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