Bi2MoO6 nanocomposite for enhanced visible photocatalytic properties

Accepted Manuscript Title: Facile synthesis of Z-scheme graphitic-C3 N4 /Bi2 MoO6 nanocomposite for enhanced visible photocatalytic properties Author: Jiali Lv Kai Dai Jinfeng Zhang Lei Geng Changhao Liang Qiangchun Liu Guangping Zhu Chen Chen PII: DOI: Reference:

S0169-4332(15)01531-7 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.183 APSUSC 30704

To appear in:

APSUSC

Received date: Revised date: Accepted date:

28-5-2015 21-6-2015 28-6-2015

Please cite this article as: J. Lv, K. Dai, J. Zhang, L. Geng, C. Liang, Q. Liu, G. Zhu, C. Chen, Facile synthesis of Z-scheme graphitic-C3 N4 /Bi2 MoO6 nanocomposite for enhanced visible photocatalytic properties, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.183 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of Z-scheme graphitic-C3N4/Bi2MoO6 nanocomposite for enhanced visible photocatalytic properties Jiali Lva, Kai Dai a,*, Jinfeng Zhanga, Lei Genga, Changhao Lianga, b,*, Qiangchun Liua,

a

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Guangping Zhua, Chen Chena

College of Physics and Electronic Information, Huaibei Normal University, Huaibei,

Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials

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b

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235000, P.R. China

and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical

an

Science, Chinese Academy of Sciences, Hefei, 230031, P.R. China

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Abstract:

The band engineering of visible-light-driven photocatalysts is a promising route for

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harnessing of effective solar energy to perform high chemical reactions and to treat

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environmental pollution. In this study, two narrow band gap semiconductor nanomaterials, graphitic carbon nitride (g-C3N4) and Bi2MoO6, were selected and coupled to form series of g-C3N4/Bi2MoO6 photocatalysts. Their structure, light absorption wavelength range, charge transport properties and energy level were investigated. Through perfect manipulation of their composition, enhanced photocatalytic activity of the Z-scheme g-C3N4/Bi2MoO6 photocatalysts with efficient reduction of recombination of photogenerated electrons and holes was achieved. The optimized Z-scheme g-C3N4/Bi2MoO6 photocatalysts with 25wt% g-C3N4 showed apparent pseudo-first-order rate constant kapp as high as 0.0688 min−1, which was 4.8 * Corresponding authors. Fax: +86-561-3803394 Email address: [email protected] (K. Dai), [email protected] (C.H. Liang)

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times and 8.2 times higher than that of g-C3N4 and Bi2MoO6 photocatalyst, respectively. Keywords: C3N4; Bi2MoO6; Photocatalyst; Charge transfer; Methylene blue

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1. Introduction

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Over the past two decades, considerable public and scientific attention has been

focused on semiconductor photocatalysts in environmental pollution remediation field of

the

simple

fabrication

procedures,

environmental

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because

friendliness,

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transparency, and excellent plasticity [1-5]. Due to its high chemical stability, nontoxicity, long-term stability and low cost, titanium dioxide (TiO2) has been widely

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used in toxic pollutant degradation applications [6-10]. However, TiO2 can only work

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with UV light less than 387 nm irradiation because of its large band gap energy of 3.2

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eV, so most of solar energy can not play its full role for light irradiation. It is of great interest to fabricate the visible light response photocatalysts and improve the separation of photoinduced electron-hole pairs for further applications. In recent years, a typical metal-free polymeric graphitic-like carbon nitride (g-C3N4) made of earth abundant elements has attracted considerable attention because of its visible light photocatalytic response, specific electronic properties, inexpensive, chemical inertness, thermal stability and nontoxicity [11-14]. Wang et al. firstly used g-C3N4 as visible-light-driven photocatalyst to produce hydrogen and oxygen by water splitting with visible light irridation [15]. On the basis of this, many researchers prepared g-C3N4 by pyrolysis of nitrogen-rich precursors, such as trithiocyanuric acid [16], cyanamide [17], urea [18], triazine [19], melamine [20], 2 Page 2 of 34

thiourea [21], and so on. However, the practical application of g-C3N4 is restricted by its poor quantum yield which is caused by the fast recombination of photoinduced charge carriers. The Z-scheme semiconductor nanocomposites exhibit better

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photocatalytic activity than single semiconductor for the well separation of electrons and holes and also reduction of their recombination. To further improve the

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photoactivity of g-C3N4, researchers have coupled g-C3N4 with other inorganic

g-C3N4/MoO3[25],

SnO2-x/g-C3N4[26],

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semiconductors, such as, BiVO4/g-C3N4[22], g-C3N4/Bi2MoO6[23], g-C3N4/Bi2WO6[24], WO3/g-C3N4[27],

Bi2O3/g-C3N4[28],

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g-C3N4/Au/CdS[29]. As for most multiple component semiconductors, only one

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component of the compositions act as the light absorption element, the rest are usually used to transfer electrons or holes. Nowadays, the z-scheme principle photocatalysts

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have paid more attention because of their considerably stronger redox, higher efficient

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charge separation and higher reduction power and deeper oxidation power than that of the traditional band-band transfer photocatalysts. And many kinds of composites can be developed through z-scheme principle, for instance, Ag2CrO4/GO[30], TiO2/Ag/Cu2O[31],

TNT/GR/CdS[32],

Ag2PO4/Ag/SiC[33],

and

Ag2CO3/Ag/AgBr[34]. As similar with the heterojunction type system, making good use of the spatial isolation of photogenerated electrons and holes is still important for advanced

photocatalyst

design.

Therefore,

designing

advanced

Z-scheme

photocatalysts with co-light absorption and co-transporting electrons and holes for more efficient degradation is of great significance.

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Bi2MoO6, with narrow bandgap (~2.7eV) close to g-C3N4 [35, 36], performs as an excellent visible-light-driven photocatalyst for the destruction of dye pollutants with a high degree of mineralization [37-40]. In this study, graphitic carbon nitrides

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(g-C3N4) were used to compose with Bi2MoO6, with similar narrow bandgap but different band edge position to construct a series of Z-scheme photocatalysts. The

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photocatalytic activities were evaluated by the degradation of methylene blue (MB),

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and results show that the narrow bandgap of both materials allows sufficient visible light absorption while different band position effectively separates the electrons and

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holes for photocatalytic reaction, and 25%g-C3N4/Bi2MoO6 Z-scheme photocatalysts

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showed the highest apparent pseudo-first-order rate constant kapp of 0.0688 min−1, which was 8.2 times and 4.8 times more than that of pure Bi2MoO6 and g-C3N4

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photocatalyst, respectively. Moreover, the possible mechanisms of photocatalytic

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activity enhancement and the spatiallly synergic effect between g-C3N4 and Bi2MoO6 were also investigated systematically.

2. Experimental 2.1 Materials

Melamine was purchased from Aldrich. Analytic grade ammonium molybdate ((NH4)2MoO4), sodium hydroxide (NaOH), polyethylene glycol (PEG), bismuth nitrate pentahydrate (Bi(NO3)5·5H2O), ethanediol (C2H6O2) and MB were purchased from Sinopharm Chemical Reagent Corp (China). 18 MΩ deionized water was used for solution preparation.

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2.2 Preparation of g-C3N4 Pure g-C3N4 nanosheet was prepared by previously reported method. In typical preparation, 5.0 g raw melamine powder was put into a muffle furnace and heated to

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faint yellow g-C3N4 nanosheet was obtained in a powder form.

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500 oC for 4 h with a heating rate of 2 oC/min. After cooling to room temperature, the

2.3 Preparation of g-C3N4/Bi2MoO6

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A certain of g-C3N4, 0.6026 g of (NH4)2MoO4, 0.6548 g Bi(NO3)5·5H2O, 15 ml

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ethanediol and 25 mL distilled water were initially reacted to form a mixture. 10 M NaOH were added drop by drop into the mixture until the pH value is 9. And then the

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mixture was transferred into a 50 mL Teflon-lined autoclave and subsequently heated

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at 160 °C for 3 h. After hydrothermal reaction, the precipitates were harvested by

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centrifugation and through out washing with distilled water and ethanol subsequently. Finally it was dried in an oven at 60 °C for 4 h obtaining g-C3N4/Bi2MoO6 composites. To investigate the effect of g-C3N4 content on the photocatalytic performance of g-C3N4/Bi2MoO6 hybrids, the weight percentages of g-C3N4 to Bi2MoO6 varied by changing the weight of g-C3N4, and the samples were presented as 0%, 16%, 25%, and 50% g-C3N4/Bi2MoO6. 2.4 Analytical and testing instrument The X-ray diffraction (XRD) patterns of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 hybrids were measured using a Rigaku D/MAX 24000 diffractometer at room tempreture with Cu Kα radiation (λ=1.5406 Å) with the 2θ range from 10° to 70°,

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operated at 40 kV and 40 mA with the scanning speed of 8°/min. The morphologies and nanostructures of as-prepared samples were recorded on a JEOL JEM-(2010) high resolution transmission electron microscopy (HRTEM) at an accelerating voltage

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of 200 kV and Hitachi S5500 scanning electron microscopy (SEM) with an INCA x-act energy dispersive spectrometer (EDS) at an accelerating voltage of 30 kV. The

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specific surface area values of photocatalysts were measured by using nitrogen

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adsorption data on an ASAP2010 system with multipoint Brunauer-Emmett-Teller (BET) method. The X-ray photoelectron spectroscopy (XPS) was performed in a

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Thermo ESCALAB 250 with an Al Kα X-ray photoelectron spectrometer at 150 W.

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The infrared absorption spectra were obtained by using a Nicolet 6700 fourier transform infrared (FT-IR) in frequency range from 400 to 4000 cm-1. The spectra

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were measured after the spectrum scan of the blank pure KBr pellet. UV-vis diffuse

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reflectance spectroscopy (DRS) measurements were measured by Hitachi UV-3600 UV-vis spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 250 to 700 nm, and BaSO4 was used as a reflectance standard. PL spectra of g-C3N4-based materials were acquired by FLS920 combined fluorescence lifetime and steady state spectrometer using a 450 W Xe lamp as the excitation light source. The photoelectrochemical measurements were measured on a Shanghai Chenhua CHI-660D electrochemical system, using the three-electrode cell. The counter and the reference electrodes were Pt wire and saturated calomel electrode (SCE), respectively. The electrolyte solution was 1.0 M Na2SO4. The working electrodes were fabricated as follows: 0.1 g as-prepared photocatalyst was mixed with 6 Page 6 of 34

0.02 g PEG and 0.5 mL distilled water to make slurry. The slurry was then injected onto a 1.0 cm ×1.0 cm ITO conductive glass electrode and these electrolytes were dried at 60 °C for 2 h and then calcined at 250 °C for 4 h.

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2.5 Photocatalytic experiment The photocatalytic performance of catalysts was evaluated by the photocatalytic

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degradation of MB irradiated by 50 W LED light with the emission center wavelength

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of 410 nm. The photocatalytic experiments were carried out in a reactor containing

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the 30 mL 10 mg/L aqueous solution of MB and 0.03 g photocatalyst. The distance between the LED light and the reactor was 5 cm. Then the reactor was exposed to the

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LED light irradiation. At the given irradiation time, the absorbance wavelength used in the spectrophotometer analysis to determine MB concentration at 664 nm. The

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C0 − C × 100 % （1） C0

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degradation efficiency was calculated as follows:

η=

Where C0 is the concentration of original MB solution and C is the concentration of the MB solution after visible LED light irradiation. According to the Langmuir-Hinshelwood kinetics model, the photocatalytic process of MB can be expressed as the following apparent pseudo-first-order kinetics equation:

ln

C0 = k app 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.

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3. Result and discussion Fig. 1 shows the XRD patterns of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 hybrids. The very strong diffraction angles at 2θ=11.00o, 28.55o, 32.66o, 33.24o, 36.17o,

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47.42o, 55.61o, 56.43o and 58.54o, can be assigned to (0 2 0), (1 3 1), (2 0 0), (2 1 0), (1 5 1), (2 6 0), (3 3 1), (1 9 1) and (2 6 2) crystal planes of pure Bi2MoO6 with the

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orthorhombic and the lines match well with the value reported by JCPDS (No.

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84-0787, a=5.489 nm, b=16.22 nm and c=5.513 nm). For pure g-C3N4, the strongest XRD peak at 27.3o, corresponding to 0.325 nm, was indexed as (0 0 2) diffraction

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plane (JCPDS 87-1526). For g-C3N4/Bi2MoO6 hybrids, no other characteristic peaks

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are found, indicating the high purity of the as-prepared samples. Fig. 2a, 2b and 2c show the SEM images of g-C3N4, Bi2MoO6 and

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25%g-C3N4/Bi2MoO6. The g-C3N4 shows the wrinkle two-dimensional structure in

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Fig. 2a. As indicated in Fig. 2b, the large amount of Bi2MoO6 with side length of 60-300 nm can be easily observed. Bi2MoO6 nanoparticles are connected with g-C3N4 nanosheets and can be clearly observed by Fig. 2c. EDS in Fig. 2d, 2e and 2f show the presence of element on g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 hybrid, there are no other impurity elements in the synthesized samples, which is in accordance with the XRD results. TEM image is shown in Fig. 2g. Fig. 2h depicts the spacings of adjacent lattice planes, which are consistent with the interplanar distance (0.305 nm) of the (1 1 1) plane of orthorhombic phase of Bi2MoO6 and the interplanar distance (0.325 nm) of the (0 0 2) plane of g-C3N4 nanosheets, respectively, indicating the formation of nanocomposites of Bi2MoO6 and g-C3N4/Bi2MoO6, which can benefit better charge 8 Page 8 of 34

separation and efficient electron transfer within the hybrid structure compared with pure g-C3N4 and Bi2MoO6. The XPS spectrum including full-spectrum scanning signals for C1s, N1s, O1s,

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Mo3d and Bi4f of 25%g-C3N4/Bi2MoO6 to probe the chemical composition and element chemical status has shown in Fig. 3a. As indicated in Fig. 3b, the

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asymmetrical and broad features of the observed C1s peaks suggest the coexistence of

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distinguishable models. A deconvolution core level spectrum at about 284.76 and 288.38 eV has been presented. The major peak at 284.76 eV is exclusively assigned to

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carbon atoms (C–C bonding) in a pure carbon environment, i.e., graphitic or

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amorphous carbons either in our sample or adsorbed to the surface [41]. The peak at 288.38 eV is identified as originating from carbon atoms bonded to three nitrogen

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atoms in the g-C3N4 lattice [42]. The high resolution N1s XPS spectra in Fig. 3c

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shows an asymmetrical feature indicating the co-existence of a number of distinguishable nitrogen environments; fitting with four results in binding energies of 398.86, 399.80, 401.36 and 404.05 eV. The two peaks at 399.80 and 401.36 eV can be assigned to tertiary nitrogen (N-(C)3) and amino functional groups having a hydrogen atom (C–N–H) [43, 44]. The peak at 398.86 eV is typically attributed to N atoms sp2-bonded to two carbon atoms (C N–C), thus confirming the presence of graphite-like sp2-bonded g-C3N4 [45]. The peak at 404.05 eV is due to charging effects or positive charge localization in heterocycles. As indicated in Fig. 3d, O1s spectra can be deconvoluted into three component peaks of 529.2, 529.8 and 530.5 eV. Those at 529.2 and 529.8 for Bi2MoO6 are related to Bi–O, Mo–O, respectively 9 Page 9 of 34

[46]. The peak at 530.5 eV is hydroxyl radicals, implying that surface hydroxyl groups (O–H) formed in the hybrids which are active species in semiconductor photocatalysis. Fig. 3e shows the Mo 4d 3/2 and 4d 5/2 centered at 235.4eV and

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234.0eV respectively. As indicated in Fig. 3f, the asymmetrical feature is due to the weak energy of electron hole pair of Bi. The binding energies of Bi 4f 5/2 and 4f 7/2

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are centered at 164 eV and 159.2 eV, in agreement with those of Bi2MoO6 [47, 48].

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FT-IR spectra analysis of g-C3N4 and 25%g-C3N4/Bi2MoO6 are shown in Fig. 4. With regard to 25%g-C3N4/Bi2MoO6, the peaks at 1336 and 1413 cm-1 correspond to

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the typical stretching modes of the C-N heterocycles [49]. Additionally, a This

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characteristic breathing mode of s-triazine at 803 cm-1 is observed [50].

illustrates the existence of g-C3N4 in the 25%g-C3N4/Bi2MoO6 composites. It could be

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seen that the absorption peak of 25%g-C3N4/Bi2MoO6 at 668 cm-1 was ascribed to the

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Bi-O stretching mode [51]. The FT-IR and HRTEM results show that Bi2MoO6 and g-C3N4 have been coupled together successfully. Fig. 5a shows the N2 gas adsorption–desorption isotherms for g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 hybrids at the same condition of 77 K. Fig. 5b summarizes the BET surface area of g-C3N4/Bi2MoO6 hybrids with different loading amounts of g-C3N4. It is found that pure Bi2MoO6 (7.8 m2/g) have a smaller surface area than that of g-C3N4 (16.2 m2/g). When g-C3N4 is coupled with Bi2MoO6, the surface area of g-C3N4/Bi2MoO6 hybrids increased with the increased content of g-C3N4. Fig. 6 shows the UV-vis DRS of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 composite photocatalysts. The fundamental absorption edge of Bi2MoO6 is 467 nm, whereas 10 Page 10 of 34

g-C3N4 exhibits absorption edge in 458 nm. As the two different semiconductors are coupled, the g-C3N4/Bi2MoO6 composites exhibit absorption edges between 467 nm and 458 nm, and with increasing g-C3N4 contents, the absorption edge of

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g-C3N4/Bi2MoO6 hybrid has a slight blue shift. The result indicates that all of the as-prepared catalysts can be used as visible light-driven photocatalysts. The band gap

(3)

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α(hv) = A(hv-Eg)1/2

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energy of the prepared catalysts can be calculated by the following equation [52]:

Where: α and Eg are the absorption coefficient and energy band gap (at wave vector

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k = 0) of the semiconductor, respectively. According to eq. (3), plots of α2(hv) versus

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energy hv for photocatalysts are shown in Fig. 4b. From the tangent line of the curve, Eg of g-C3N4 and Bi2MoO6 were calculated as 2.70 and 2.65 eV, respectively.

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PL analysis was carried out to discuss the migration, transfer and recombination

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processes of photogenerated electrons and holes in g-C3N4/Bi2MoO6 composites. Fig. 7 shows the PL spectra of the pure g-C3N4 and g-C3N4/Bi2MoO6 nanomaterials excited by 325 nm. The main emission peak was centered at 460 nm for the pure g-C3N4 sample, which was similar to the former reports. For g-C3N4/Bi2MoO6 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/Bi2MoO6 hybrids had lower recombination rate of photo-generated charge carriers. This demonstrated that the recombination of photogenerated charge carriers was greatly inhibited by the introduction of Bi2MoO6,

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showing that the photogenerated electrons and holes in g-C3N4/Bi2MoO6 composite photocatalysts had higher separation efficiency than those in the pure g-C3N4. Photocurrent can be used to examine the total amount of charge carrier generation

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within the photocatalyst. A relationship is commonly recognized as follows: the higher the photocurrent, the higher the effectively separated electrons-holes in the

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materials [53, 54]. To give further evidence to support the mechanism suggested

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above, photocurrent-time measurements were performed in an on-and-off cycle mode. Fig. 8 shows the photocurrent-time curves of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6

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hybrid samples with two on-off intermittent irradiation cycles. The electrodes of the

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samples demonstrate a rapid photocurrent response when the visible light illumination is on an on-and-off mode. The 25%g-C3N4/Bi2MoO6 exhibits higher photocurrent

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intensity than g-C3N4, Bi2MoO6 and other g-C3N4/Bi2MoO6 hybrid composites, which

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is in well accordance with the effectively reduced recombination of electrons and holes of PL.

The photoactivity of g-C3N4/Bi2MoO6 photocatalysts was studied by degradation of MB under 410 nm LED light irradiation sources. As a comparison, MB degradation with pure Bi2MoO6, g-C3N4 and no catalyst was also carried out under identical conditions. As shown in Fig. 9a, the degradation of MB in pure Bi2MoO6 and g-C3N4 was 30% and 40% under 410nm LED light irradiation source, respectively. But the g-C3N4/Bi2MoO6 hybrid obtain more obvious degradation rate, and about 90% MB was removed by 25%g-C3N4/Bi2MoO6 in 40 min. Fig. 9b shows that there is a linear relationship between lnC0/C and t, comfirming that the photodegradation reaction is 12 Page 12 of 34

indeed pseudo-first-order. According to eq.(2), kapp of the photodegradation of MB are 0.0084min-1, 0.0143min-1, 0.0274min-1, 0.0497min-1, 0.0688min-1 for pure Bi2MoO6, g-C3N4, 16%g-C3N4/Bi2MoO6, 50%g-C3N4/Bi2MoO6, 25%g-C3N4/Bi2MoO6.

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It was well known that the enhancement of photocatalytic performance of composite photocatalysts was mainly attributed to electrons and holes transfer at the

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interfaces of photocatalysts [55]. As indicated in Fig. 10, g-C3N4 is only composed of

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C and N. Bi2MoO6 is consisted with [Bi2O2]2+ layers sandwiched and MoO42− slabs, it is a layered Aurivillius-related oxide. When Bi2MoO6 contacted with g-C3N4, the

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valence band (VB) potentials of a semiconductor at the point of zero charge can be

EVB = X − Ec + 0.5Eg

( 4)

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theoretically predicted by the following empirical equation [56]:

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Where: EVB is the VB edge potential, X the electronegativity of semiconductor, which

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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= EVB− Eg. The Eg of g-C3N4 and Bi2MoO6 were calculated as 2.70 and 2.65 eV, The X values for the g-C3N4 and Bi2MoO6 materials are 4.72 and 5.51 eV, respectively. Herein, the CB and VB edge potentials of g-C3N4 were determined at −1.13 and +1.57 eV, respectively. The CB and VB edge potentials of Bi2MoO6 were calculated at -0.32 and +2.33 eV, respectively. Based on the above information, Fig. 10 shows the proposed charge separation processes

for

the

photocatalytic

degradation

of

MB

molecules

by

the 13 Page 13 of 34

g-C3N4/Bi2MoO6 composite. Both Bi2MoO6 and g-C3N4 can be excited by 410 nm LED light and induce the generation of electrons and holes. Since the CB and VB edge potentials of g-C3N4 are negative than CB and VB edge potentials of Bi2MoO6

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(-0.32 eV), the photoinduced electrons in the CB of g-C3N4 will migrate to the CB of Bi2MoO6, and the photoinduced holes in the VB of Bi2MoO6 will migrate to the VB

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of g-C3N4 according to the traditional electron-hole separation process [57, 58]. By

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this channel, the holes in the VB of g-C3N4 cannot react with H2O or OH- near the surface of g-C3N4 to generate •OH because the VB edge potential of g-C3N4 is higher

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than the standard redox potential [59]. Thus, it is not sympathetic for engendering the

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main reactive species •O2- and •OH on the basis of traditional carriers transfer pattern, resulting in the lower photocatalytic performance for g-C3N4/Bi2MoO6, and the

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Bi2MoO6 and g-C3N4 can not form the conventional heterojunction according to the

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above discussion. However, the g-C3N4/Bi2MoO6 photocatalytic system can be explained by Z-scheme photocatalytic mechanism [60-62]. Both g-C3N4 and Bi2MoO6 can be easily excited to yield photogenerated electron-hole pairs under 410 nm LED light irradiation, and the photoinduced electrons from the VB of g-C3N4 and Bi2MoO6 can be easily transfered into their corresponding CB of g-C3N4 and Bi2MoO6 respectively. Then, photoinduced electrons would be easily injected from the CB of Bi2MoO6 to the VB of g-C3N4. As a result, the photogenerated carriers are spatially separated, which greatly inhibits the recombination of photogenerated electrons and holes.

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The stored photoinduced electrons in the CB of g-C3N4 can adsorb O2 near the g-C3N4 to yield reactive superoxide radical ion •O2-, which is a powerful oxidative specie for MB degradation. In the meantime, the h+ left in VB of Bi2MoO6 can

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directly oxide the organic compounds or react with H2O near the surface of Bi2MoO6 to form hydroxyl radicals •OH [63]. But the h+ in VB of g-C3N4 can not react with

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H2O/OH- near the surface of g-C3N4 to form •OH on the base of the VB position of

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g-C3N4 higher than the potential H2O/•OH couple. MB can be easily oxidized by •OH radicals species into H2O, CO2 and other simple inorganic molecules. Therefore, the

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efficient photodegradation progress of organic dye can smoothly proceed. And the process can be described as follows:

( 6)

(5)

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e− +O2 →⋅O2−

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C3 N 4 / Bi2 MoO6 + hv → C3 N 4 (e − ) + Bi2 MoO6 (h + )

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2e− + 2H + + O2 → H 2O2 (7)

(8)

H2O2 +⋅O2− →⋅OH + OH − + O2 h+ + H 2O → ⋅OH + H +

⋅OH + MB → deg radation

(9)

product

(10 )

As a result, the photocatalytic activity of 25%g-C3N4/Bi2MoO6 hybrid is much higher than that of pure g-C3N4 or Bi2MoO6. However, with the content of g-C3N4 in g-C3N4/Bi2MoO6 hybrid being in excess, numerous photoinduced electrons and holes would recombine easily on the surface of g-C3N4 and Bi2MoO6 as mentioned above. 25%g-C3N4/Bi2MoO6 showed the best photocatalytic performance among the different weight ratio of g-C3N4/Bi2MoO6 photocatalysts in this study. 15 Page 15 of 34

The reusable performance of catalyst is crucially important parameter for practical applications [64]. To evaluate the stability of 25%g-C3N4/Bi2MoO6, we carried out 5 times recycle experiment under identical conditions compared with pure g-C3N4 and

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Bi2MoO6. As shown in Fig. 11, there is almost no change in the photocatalytic activity of the as-prepared g-C3N4/Bi2MoO6 photocatalyst even after 5 times cycling,

cr

implying the long reusable life of the 25%g-C3N4/Bi2MoO6 composites.

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4. Conclusions

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In summary, g-C3N4/Bi2MoO6 Z-scheme photocatalysts exhibited excellent photocatalytic performance, largely reduced PL intensity and higher photocatalytic

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activity than single g-C3N4 or Bi2MoO6 under visible LED light irradiation was

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designed. And the optimal photodegradation performance was achieved for

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25%g-C3N4/Bi2MoO6, which is 8.2 times than that of Bi2MoO6 photocatalyst. After 5 cycles of repetition tests, the MB degradation efficiency of 25%g-C3N4/Bi2MoO6 still remained 88%. The bandgap of two components of nanomaterials are both narrow, and visible light absorption within the composite is believed to be much more effective than previously reported heterostructure nanomaterials coupled with wide bandgap semiconductors. Because the bandgap of these two nanomaterials are highly close (~2.7eV), we can clearly prove that g-C3N4/Bi2MoO6 nanocomposite is a direct Z-scheme photocatalyst for the effective separation of electrons and holes.

Acknowledgments This work was supported by the National Natural Science Foundation of China 16 Page 16 of 34

(51302101 and 51302100), The Foundation for Young Talents in College of Anhui Province (12600941), the Natural Science Foundation of Anhui Province (1408085QE78, 1508085ME100) and Collaborative Innovation Center of Advanced

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Functional Materials (XTZX103732015008).

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Fig. 1 XRD patterns of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 composites.

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ip t cr us an M d Ac ce pt e Fig. 2 SEM images of (a) g-C3N4, (b) Bi2MoO6, (c) 25%g-C3N4/Bi2MoO6, the EDS spectra of (d) g-C3N4, (e) Bi2MoO6, (f) 25%g-C3N4/Bi2MoO6, and (g) TEM and (h) HRTEM images of 25%g-C3N4/Bi2MoO6.

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Fig. 3 The overview (a) and the corresponding high-resolution XPS spectra (b) C1s (c) N1s, (d) O1s, (e) Mo 3d and (f) Bi 4f of the as-prepared 25%g-C3N4/Bi2MoO6.

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Fig. 4 FT-IR spectra of g-C3N4 and 25%g-C3N4/Bi2MoO6

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Fig. 5 (a) Isotherms for N2 adsorption–desorption and (b) BET surface area versus different g-C3N4 contents photocatalysts.

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Fig. 6 (a) UV–vis DRS spectra of g-C3N4, Bi2MoO6 and g-C3N4/Bi2MoO6 composite photocatalysts and (b) plots of (αhν ) 2 versus energy (hν ) for g-C3N4 and Bi2MoO6.

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Fig. 7 PL spectra of pure g-C3N4 and g-C3N4/Bi2MoO6 hybrid samples.

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Fig. 8 Photocurrent-time curves of different photocatalysts.

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Fig. 9 (a) Photocatalytic degradation of MB under 410 nm LED light irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB degradation.

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Fig. 10 The possible charge transfer process under LED light irradiation.

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Fig. 11 Comparison of photodegradation performance within five cycles for g-C3N4,

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Bi2MoO6 and 25% g-C3N4/Bi2MoO6.

Highlights：

>g-C3N4/Bi2MoO6 nanocomposite photocatalyst was prepared.> g-C3N4/Bi2MoO6 as a typical Z-scheme photocatalyst was proved.> g-C3N4/Bi2MoO6 showed long reusable life with irradiation of LED light.>

Graphical abstract

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