g-C3N4 composite photocatalysts

Accepted Manuscript Title: Enhanced visible-light-driven photocatalytic activities of Bi2 Fe4 O9 /g-C3 N4 composite photocatalysts Authors: Geming Wang, Shutong Liu, Tiancheng He, Xuan Liu, Quanrong Deng, Yangwu Mao, Shenggao Wang PII: DOI: Reference:

S0025-5408(17)34484-7 https://doi.org/10.1016/j.materresbull.2018.04.013 MRB 9951

To appear in:

MRB

Received date: Revised date: Accepted date:

2-12-2017 3-4-2018 8-4-2018

Please cite this article as: Wang G, Liu S, He T, Liu X, Deng Q, Mao Y, Wang S, Enhanced visible-light-driven photocatalytic activities of Bi2 Fe4 O9 /g-C3 N4 composite photocatalysts, Materials Research Bulletin (2010), https://doi.org/10.1016/j.materresbull.2018.04.013 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.

Enhanced visible-light-driven photocatalytic activities of Bi2Fe4O9/gC3N4 composite photocatalysts Geming Wanga, b,*, Shutong Liua, Tiancheng Hea, Xuan Liua, Quanrong Deng a, b, Yangwu Mao a, b Shenggao Wang a, b a School of Materials Science and Engineering，Wuhan Institute of Technology， Wuhan，P. R. China

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Institute of Technology，Wuhan，P. R. China

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b Hubei Key Laboratory of Plasma Chemistry and Advanced Materials，Wuhan

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Graphical abstract

The Z-scheme mechanism is proposed in Bi2Fe4O9/g-C3N4 composites. The

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photoinduced electrons in the CB of Bi2Fe4O9 will move to the VB of g-C3N4 and recombine with those holes in its VB. The holes of Bi2Fe4O9 and electrons of g-C3N4 can be efficiently separated from each other. Those separated electrons in the VB of

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Bi2Fe4O9 and those separated holes in the CB of g-C3N4 can carry out the reduction and oxidation reactions of RhB, which leads to the improved visible-light-driven photocatalytic activities compared with those of its single components.

Highlights 1. The novel Bi2Fe4O9/g-C3N4 composite photocatalysts have been designed and prepared. 2. Combination of Bi2Fe4O9 and g-C3N4 could enhance the visible-light-driven photocatalytic degradation for RhB.

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3. The effects of Bi2Fe4O9/g-C3N4 weight ratios on photocatalytic performance in the composite have been discussed.

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4. The Z-scheme photocatalytic mechanism was proposed and discussed in detail.

Abstract A series of Bi2Fe4O9/g-C3N4 composite photocatalysts are prepared via a

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facile mixing-calcination method. The crystal structures of composites are investigated

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by X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR). The

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Field Emission Scanning Electron Microscope (FE-SEM) in combination with Energy Dispersive X-ray Spectroscopy (EDS) mapping analysis and Transmission Electron

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Microcopy (TEM) images illustrate the uniform distribution of constituent elements and clear interface between Bi2Fe4O9 and g-C3N4 component. The visible-light-driven photocatalytic rates of Rhodamine B over Bi2Fe4O9/g-C3N4 composites increase and

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then decrease with the increase of g-C3N4 content. The effects of Bi2Fe4O9/g-C3N4 weight ratios on photocatalytic performance are thoroughly discussed. Meanwhile, the

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active species trapping experiments and the electrochemical impedance spectra as well as the electronic energy-band structure estimation analysis demonstrate that the photocatalytic mechanism for Bi2Fe4O9/g-C3N4 composites is ascribed to the Z-scheme.

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In addition, the recyclability and stability experiments of the composite are also studied.

Key words: Bi2Fe4O9; g-C3N4; Bi2Fe4O9/g-C3N4 composites; photocatalytic properties; visible-light irradiation 1. Introduction In recent years, typical semiconductor-based photocatalysts of TiO2 [1, 2], ZnO [3]

or ZnS [4] have attracted tremendous interest due to their potential utilization of solar energy for environmental purification and water splitting. However, their practical application is still greatly limited by its wide bandgap which responds only to ultraviolet light (5% of the total solar spectrum) [5]. Up till now, a lot of metal oxides, for example, Bi2WO6 [6], FeVO4 [7], CuInS2 [8], Ag3PO4 [9], BiFeO3 [10-12], Bi2Fe4O9 [13-16], have been characterized as novel visible-light-driven photocatalysts. Among them,

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Bismuth ferrites are perhaps the most studied environment-friendly multifunctional materials due to their well-known multiferroic, optical, catalytic, dielectric,

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electromagnetic and gas-sensoring properties [11, 17-22]. For example, Dutta et al. discover the coexistence of magnetic and ferroelectric properties in Sc3+ doped Bi2Fe4O9 nanoparticles, demonstrating the possibility of becoming the good

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multiferroic material [19]. Li et al. investigate the dielectric and microwave absorption

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properties of La/Nd doped BiFeO3 nanoparticles and observe that the relaxation moves when the temperature

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to lower frequencies and the imaginary permittivity increases

gets higher, providing a novel approach to explore absorber in microwave fields [17].

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Li and his cooperators further report an enhanced microwave attenuation performance

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in BiFeO3-based BiFeO3-BaFe7 (MnTi) 2.5O19 composite, which can be ascribed to the improved electromagnetic properties [20].By virtue of its narrow band gap, bismuth ferrites that have two main crystalline structures of mullite-type Bi2Fe4O9 (band gap

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between 1.8~2.2eV) and perovskite-type BiFeO3 (band gap of about 2.2eV) are becoming the current research focus on account of their visible-light sensitive

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photocatalytic, photovoltaic and photoluminescence activities [12, 18]. Despite its great potential, bismuth ferrites have typically shown insufficient photodegradation activities under visible-light irritation, which are mainly due to the fast recombination of photo-

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generated electrons/holes pairs in single-component photocatalyst [21]. For instance, although BiFeO3 submicrocubes show better photocatalytic activities than those of microspheres and microcubes samples, they still exhibit only 40% photocatalytic rate of Congo red under visible-light irradiation after 3h [10]. Zhang et al. synthesize Bi2Fe4O9 submicron particles using an EDTA-assisted sol-gel route which show 35% of photocatalytic efficiency for methyl orange in 3h under visible-light irradiation [22].

Therefore, more attention should be paid to strategies focusing on the improvement of the electron/hole separation to increase the photocatalytic efficiency of bismuth ferrites. Semiconductor coupling technology is considered to be an effective strategy to engineer surface and band gap position of a semiconductor, which can help develop highly-active semiconductor-based composite photocatalysts [23]. The strategy is based on an appropriate band alignment between semiconductors, which can dominate

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the transport direction, increase the separation distance and hinder the recombination

of photogenerated carriers, and thus resulting in the great improvement of composite

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photocatalysts [24, 25]. For instance, Liu et al. discover that the TiO2-based composite consisting of the CdSxSe1-x interlayer between the CdS and CdSe layer exhibits a

remarkably improved photocurrent of 14.78 mA cm-2 at -0.2 V vs SCE, which is much

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higher than that of pure TiO2 nanotubes [26]. Chen et al. fabricate ZnO-based core-

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shell nanowire arrays containing p-type Cu2-xSe layer and n-type CdSe layer, which can

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improve the photoelectrochemical performance and promote the separation of photogenerated carriers [27]. Important modification examples of composite bismuth ferrite

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with other semiconductor are as follows. Yang et.al report the improved visible-light-

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driven photocatalytic behaviors for methylene blue in BiFeO3/TiO2 p-n junction nanofibers [28]; Kong et.al observe 2.5 time higher of photocatalytic rate in BiFeO3(Bi/Fe)2O3 composite powders than that of pure BiFeO3 [29]; Zhang et.al discover 6.57

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time and 5.02 time higher of the photocatalytic rate in BiFeO3/Bi2Fe4O9 heterojunction nanofibers than those of BiFeO3 and Bi2Fe4O9 respectively [30]; H. Ramezanalizadeg

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et.al find out the outstanding photodegradating behavior vs. Methyl orange (MO) and RhB of BiFeO3/CuWO4 heterojunction compared with those of its single components [31]. On the other hand, graphitic carbon nitride (g-C3N4) is a metal-free semiconductor

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and can be easily fabricated by directly heating melamine at 500℃ to 580 ℃ [32], which makes it possess low cost, high chemical and thermal stability. It has been found that g-C3N4 semiconductor has a band gap of ~2.7eV, which demonstrates attractive visiblelight-driven photocatalytic activities for pollutant degradation, CO2 reduction and water photo-splitting [33]. Similar to bismuth ferrite, the visible-light-driven photocatalytic property of g-C3N4 is still restricted by the rapid recombination of photogenerated

electron-hole pairs [34]. Thus, various composite materials comprising of g-C3N4 and other semiconductor with different electronic energy-band structure have also been subsequently reported to promote photoactivites of g-C3N4, such as g-C3N4/BiFeO3 [35-37], g-C3N4/Bi2WO6 [38], g-C3N4/SmVO4 [39], g-C3N4/Ag3PO4 [40], gC3N4/V2O5 [41], Ag2CrO4/g-C3N4 [42] and CeVO4/g-C3N4 [43]. Due to the matching band edges between bismuth ferrite and g-C3N4, it is expected

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that the semiconductor composite containing bismuth ferrite and g-C3N4 may improve visible-light-driven photocatalytic abilities. Wang et.al reports improved visible-light-

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driven photocatalytic efficiency for MO in g-C3N4/BiFeO3 nanocomposites [36]. Later,

Fan et.al observes that g-C3N4/BiFeO3 composites with the content of g-C3N4 being 50wt% have the best photocatalytic activity [37]. Though researches begin focusing on

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g-C3N4/BiFeO3, relative works done on g-C3N4/Bi2Fe4O9 are very rare [44]. In this

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work, we have designed and synthesized semiconductor-based composite photocatalyst

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of Bi2Fe4O9/g-C3N4 based on the possible synergetic effect between Bi2Fe4O9 and gC3N4. As a function of Bi2Fe4O9 and g-C3N4 weight ratio, a series of Bi2Fe4O9/g-C3N4

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composites have been successfully fabricated via a facile mixing and heating method

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and their crystal structural, microstructural, and optical properties have also been thoroughly characterized and discussed. In terms of the effects of Bi2Fe4O9/g-C3N4 weight ratios on photocatalytic performance, the visible-light-driven photocatalytic

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activities of Bi2Fe4O9/g-C3N4 composites for RhB have been thoroughly studied. The Zscheme photocatalytic mechanism accounting for the enhanced photocatalytic

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efficiency of Bi2Fe4O9/g-C3N4 composites is also presented and discussed through radical trapping experiments, electrochemical impedance spectra analysis and electronic energy-band structure estimation.

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2. Experimental 2.1. Synthesis of Bi2Fe4O9/g-C3N4 composite photocatalysts The Bi2Fe4O9 micro-powders were synthesized via low-temperature solid state reaction method. The detailed synthesis procedures of the Bi2Fe4O9 fabrication were shown in our earlier work [18, 45]. The g-C3N4 powders were synthesized by heating the melamine (C3H6N6) directly [32]. Typically, a certain amount of melamine powders

were sintered up to 550℃ for 4h at a rate of 5℃/min in a muffle furnace. And the yellow g-C3N4 was finally collected and ground into the powder. The Bi2Fe4O9/g-C3N4 composites were prepared through a mixing-calcination process as follows. The aboveobtained Bi2Fe4O9 powders were mixed with the g-C3N4 powders according to the predetermined amount. Subsequently, the mixture was mechanically ground and heated at 300℃ for 1h. After the reactor cooled down, the final Bi2Fe4O9/g-C3N4 composites

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were obtained. To investigate the effect of weight ratio between Bi2Fe4O9 and g-C3N4

on their chemical and physical properties, a series of Bi2Fe4O9/g-C3N4 composites with

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different weight ratios of g-C3N4 to Bi2Fe4O9 at 0% (pure Bi2Fe4O9) , 30% , 50% , 70% ,

90% and 100% (pure g-C3N4) were prepared and correspondingly abbreviated as S1, S2, S3, S4, S5 and S6 respectively.

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2.2. Characterization

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The crystalline structures of all specimens were investigated by X-ray powder diffraction (XRD, Philips X’pert PW3373/10) employing Cu Kαradiation (λ=1.5406Å,

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2θ=10°-80°). Fourier transform-infrared (FT-IR) spectra of our samples were recorded

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on a FT-IR spectrophotometer (Nicolet 6700, American Thermo Electron Corporation)

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at room temperature. The microstructures and morphologies of samples were characterized using a Field Emission Scanning Electron Microscope (FE-SEM, HITACHI UHR FE-SEM SU8000) combined with an Energy Dispersive X-ray

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Spectroscopy (EDS) mapping analysis. Transmission Electron Microscope (TEM) and High-resolution TEM (HRTEM) were also applied to study the composite by using a

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JEOL JEM-2100 microscope. The UV-vis absorption spectra of all samples were performed by a UV–vis spectrophotometer (UV2550, Shimadzu, Japan). The electrochemical measurements were measured with CHI760 electrochemical

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workstation (Shanghai Chenhua Instrument Co. Ltd) using a conventional threeelectrode cell consisting of fluorine tin oxide (FTO) glass working electrode, Pt counter electrode and standard calomel reference electrode (SCE). The Na2SO4 solution was applied as electrolyte solution. 2.3 Measurement of photocatalytic activities The visible-light-driven photocatalytic activities of Bi2Fe4O9, g-C3N4 and

Bi2Fe4O9/g-C3N4 composites were evaluated by the photocatalytic decolorization for RhB through using a 500W Xe lamp with a cutoff filter (λ>420nm). In the experiment, 50 mg of the as-prepared powders were suspended in 100 mL of 8mg/L RhB aqueous solution. Prior to irradiation, the solution was continuously stirred in the dark for 1h to fulfill the adsorption–desorption equilibrium between RhB and the catalyst. During irradiation, 3ml of the suspension mixture was drawn out from the reaction cell at 10min

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intervals, centrifuged at 12,000 rpm min-1 for 15 min and then determined for RhB concentration. The RhB concentration was obtained via recording the absorbance at

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533 nm by using a UV-vis spectrophotometer. With the purpose of investigating the influences of superoxide radicals (•O2-), holes (h+) and hydroxyl radicals (•OH) on the photocatalytic performance in the composite, active species capturing experiments

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were also performed by adding 1mmol scavengers, including benzoquinone (BQ),

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the photocatalytic experiments, respectively.

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3. Results and discussion 3.1 XRD and FT-IR analysis

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disodium ethylenediaminetetraacetate (EDTA-2Na) and the 2-propanol (IPA) during

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Fig. 1 exhibits phase structures of all as-obtained Bi2Fe4O9, g-C3N4 and Bi2Fe4O9/g-C3N4 composites. All diffraction peaks appearing in the sample S1 can be well indexed to orthorhombic Bi2Fe4O9 with Pbam space group (JCPDS card No. 74-

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1098). The sharp intensities of diffraction peaks mean high crystallinity in the pure Bi2Fe4O9 powders. The sample S6 have two main peaks at 13.1° and 27.4° respectively,

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which can be indexed as (100) and (002) diffraction planes (JCPDS 87-1526) in correspondence with the graphitic materials [33]. In terms of Bi2Fe4O9/g-C3N4, weak diffraction peaks at 27.4° corresponding to g-C3N4 appears when the weight ratio of g-

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C3N4 beyond 50 wt%. With the concentration of g-C3N4 increasing from sample S3 to S5, the diffraction peaks of g-C3N4 become clearer and stronger, suggesting that Bi2Fe4O9 are successfully composited with g-C3N4 without destroying Bi2Fe4O9 crystal phase during the mixing and heating process. Moreover, only Bi2Fe4O9 diffraction peaks are observed when the g-C3N4 content is below 50 wt%, which results from the low g-C3N4 concentration within resolution limit of XRD and even dispersion of g-

C3N4 powders in the composites [37]. No secondary phases are found, demonstrating that Bi2Fe4O9/g-C3N4 composites with only two phases of Bi2Fe4O9 and g-C3N4 have been successfully formed. Similar phenomena have also been observed in the gC3N4/BiFeO3 and g-C3N4/Bi2WO6 hybrids [36, 38]. FT-IR spectrum is an ideal method to characterize chemical structures of carbon containing material. Fig. 2 presents the FT-IR spectra of Bi2Fe4O9, g-C3N4, and

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Bi2Fe4O9/g-C3N4 composites respectively. In terms of pure Bi2Fe4O9 and g-C3N4, their spectra are in good agreement with previous literatures [22, 40 and 46]. In the Bi2Fe4O9

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sample, the characteristic peaks located at 433cm-1 and 473cm-1 are attributed to the Fe

cations’ stretching vibration, being a characteristic of octahedral FeO6 groups in the mullite-type structure. The peaks which are at 523 cm-1 and 609 cm-1 are attributed to

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the bending vibration of O–Fe–O and Fe–O–Fe in the tetrahedral sites respectively.

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Another two strong peaks at 640 cm-1 and 813 cm-1 are associated with the Fe cations’

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stretching vibration in the FeO4 tetrahedral unit [22, 46 and 47]. As for g-C3N4 sample, all the absorption bands are distributed in the two main regions, that is, 810 cm-1 and

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1240-1638 cm-1, which is in line with the characteristic peaks of the typical stretching

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modes of g-C3N4 heterocycles [39]. Noticeably, the FT-IR spectra of all Bi2Fe4O9/gC3N4 composites mean the overlap of both FT-IR spectra of Bi2Fe4O9 and g-C3N4, suggesting the coexistence of Bi2Fe4O9 and g-C3N4. In addition, with the g-C3N4

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content increasing, the intensity of the peaks of g-C3N4 increases and Bi2Fe4O9 decreases. Those results further indicate the successful formation of Bi2Fe4O9/g-C3N4

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composites, as is also evidenced by our previous XRD analysis. 3.2 FE-SEM and TEM analysis Fig.3 shows the morphologies and microstructures of Bi2Fe4O9, g-C3N4 and

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Bi2Fe4O9/g-C3N4 composites. Pure Bi2Fe4O9 powders have regular cubic-like appearances with length and thickness of about 2μm (Fig.3a), whereas pure g-C3N4 powders show typically aggregated flake-like morphologies with a layered structure and a large particle size (Fig.3f). FE-SEM images of Bi2Fe4O9/g-C3N4 composites show that the surface of cubic-like Bi2Fe4O9 micro-powders is surrounded by the flake-like g-C3N4 particles (Fig.3b-e), which will be also confirmed by our TEM results. With the

increasing weight ratio of g-C3N4 surrounding the Bi2Fe4O9 surfaces, the morphology changes from cubic-like Bi2Fe4O9 shape (Fig.3b and c) to mixed morphology (Fig.3d) and then to flake-like g-C3N4 appearance (Fig.3e and f). Besides, the EDS elemental distribution mappings of one typical Bi2Fe4O9/g-C3N4 composite (the sample S4) are presented in Fig.4. The sample S4 contains O, C, Fe, Bi, N five elements and the different kinds of colors are uniform in the images, indicating that all the elements are

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distributed uniformly in the composites. Therefore, the XRD, FT-IR spectra and FESEM results confirm the successful preparation of Bi2Fe4O9/g-C3N4 composites, which

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only consist of Bi2Fe4O9 and g-C3N4 phases and the two phases are well distributed. To further reveal the microstructure features of Bi2Fe4O9/g-C3N4 composites (the sample S4), TEM analyses are carried out. As is presented in Fig. 5 (a), the TEM image shows

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the Bi2Fe4O9 particles anchor tightly on the g-C3N4 powders’ surfaces, leading to the

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occurrence of heterostructure. Fig. 5 (b) displays typical HRTEM images of the

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composite in correspondence with the square region of TEM image in Fig. 5 (a). In Fig. 5 (b), the d-spacing of Bi2Fe4O9 is determined to be 0.319nm corresponding to the

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lattice fringe of (121) planes, which display high crystallized Bi2Fe4O9 powders with

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orthorhombic. Meanwhile, the g-C3N4 particles present low crystallinity feature due to its polymer nature [42]. The strong and clear interface with well-defined boundary between Bi2Fe4O9 and g-C3N4 can be observed in Fig. 5 (b). The results mentioned

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above fully prove an intimate interface between Bi2Fe4O9 and g-C3N4, which is favorable for the movement of photogenerated carriers in the composite.

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3.3 Optical properties

Fig. 6 illustrates the UV-vis diffuse reflectance spectra of our samples. The optical

absorption features are related to the band gap of a semiconductor, which is one

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important factor to determine the visible-light-driven photocatalytic behavior of catalysts. As is shown in Fig.6, pure Bi2Fe4O9 and g-C3N4 possess photoabsorption from UV to the visible-light region until 675nm and 465nm, suggesting their potential visible-light-driven photocatalytic properties. Seen from the inset of Fig. 6, the optical band gaps of Bi2Fe4O9 and g-C3N4 are obtained to be about 1.84eV and 2.67 eV respectively. Those values are estimated from the plots of (F(R)) 1/2 versus photon

energy (hv) by extrapolating the linear portion of (F(R)) 1/2 to the energy (hv) axis at a =0 based on the according to Kubelka-Mumk (K-M) theory [31]. In addition, compared with the pure Bi2Fe4O9, the optical absorption edges of Bi2Fe4O9/g-C3N4 composites show a blue shift tendency in visible light and the absorption intensities decrease with the increase of g-C3N4 weight ratio. Similar to the g-C3N4/BiOCl heterostructured photocatalysts [48], the phenomenon mentioned above can be explained by the

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interaction between Bi2Fe4O9 and g-C3N4 in Bi2Fe4O9/g-C3N4 composites. 3.5 Photocatalytic degradation of RhB

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To evaluate the photocatalytic properties of Bi2Fe4O9, g-C3N4 and Bi2Fe4O9/g-

C3N4 composites under visible-light illumination, their photodegradation activities for RhB after physical adsorption in the dark are tested. Fig. 7 illustrates the

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photodegradation rate curves of RhB using Bi2Fe4O9, g-C3N4 and Bi2Fe4O9/g-C3N4

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composites as a function of time. We can see that RhB concentration decreases a litter

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bit with the pure Bi2Fe4O9 under visible-light illumination due to the prompt recombination of photogenerated electrons and holes [14]. Only 11.8% RhB is

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photodegraded after illumination for 50mins in pure Bi2Fe4O9. On the contrary, all the

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Bi2Fe4O9/g-C3N4 composites manifest obviously improved photocatalytic activities in comparison with those of Bi2Fe4O9. The removal efficiencies of RhB with 50mins reaction are 25.5%, 36.7%, 88.3% and 75.9% for the samples S2, S3, S4 and S5,

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respectively. The pure g-C3N4 powders exhibit 65.3% photodegradation rate under the same condition. The improvement could be attributed to the effective inhibition and

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transfer of photo-induced charges induced by the formation of heterojunction between Bi2Fe4O9 and g-C3N4, which will be discussed later. Radical trapping experiments of active species over Bi2Fe4O9/g-C3N4 composite (the sample S4) have been performed

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to clarify the photocatalytic mechanism during the photocatalytic reaction. The scavengers

used

are

benzoquinone

(BQ)

for

•O2-,

disodium

ethylenediaminetetraacetate (EDTA-2Na) for h+ and 2-propanol (IPA) for •OH. Fig.8 shows that the addition of EDTA-2Na and IPA leads to a slight decrease in photocatalytic degradation efficiency, while the introduction of BQ results in a much more severe reduce of efficiency. These observations suggest that •O2- are the main

active species for RhB during the photocatalytic process, while h+ or •OH radicals are the minor ones with a certain impact on the photodegradation. 3.5 Photocatalytic mechanism With an aim to study the enhanced photocatalytic mechanism, the valence band (VB) and conduction band (CB) potentials of Bi2Fe4O9 and g-C3N4 are calculated using

EVB=X-E0+0.5Eg

(1)

ECB=EVB-Eg

(2)

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following equations according to the classic Mulliken electronegativity theory [7]:

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In these equations, EVB and ECB are the valence band (VB) and conduction band (CB)

edge energy; X is electronegativity of a semiconductor (the geometric mean of the electronegativity of all constituent atoms, X for Bi2Fe4O9 and g-C3N4 are 6.00eV and

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4.72 eV [13, 33]); E0 is the energy of free electrons on the hydrogen scale (~4.5 eV) and

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Eg is the band gap energy of a semiconductor (Eg for Bi2Fe4O9 and g-C3N4 are 1.84eV

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and 2.67 eV). Their valence band (VB) and conduction band (CB) edge energy of gC3N4 are calculated to be 1.56 eV and -1.11 eV respectively, both of which are negative

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than those of Bi2Fe4O9 (2.42 eV and 0.58 eV). Fig.9 shows the energy band structure

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of g-C3N4 and Bi2Fe4O9. Obviously, the Bi2Fe4O9 (Eg=1.84 eV) and the g-C3N4 (Eg=2.67 eV) powders can generate excited electrons and holes by absorbing visible light. In terms of Bi2Fe4O9, the VB potential (2.42 eV) is more positive than the

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standard reduction potential •OH/H2O (2.27eV), so the photogenerated holes (h+) can reduce H2O to •OH and further react with RhB. However, the CB potential (0.58 eV) is

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not more negative than the standard redox potential O2/•O2- (-0.33 eV), so the photogenerated electrons (e-) cannot react with O2 to •O2- [44]. Nevertheless for pure g-C3N4, the photogenerated electrons (e-) in the CB of g-C3N4 can yield •O2- radicals

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by reacting with O2 because the position of CB (-1.11 eV) is more negative than that of O2/•O2- (-0.33eV), which is mainly responsible for photocatalytic reaction. Meanwhile, the left photogenerated holes (h+) in the VB of g-C3N4 have low oxidation capacity to oxidize H2O to generate •OH because the position of CB (1.56 eV) is not more positive than that of •OH/H2O (2.27eV) [41, 43]. Therefore, the Bi2Fe4O9 and g-C3N4 have the photocatalytic abilities of dye degradation under visible-light degradation. The CB/VB

edge potentials, the matching band edges (a stagger band gap structure) and closed interfaces between Bi2Fe4O9 and g-C3N4 lead to the improved photocatalytic activities in the Bi2Fe4O9/g-C3N4 composites. If the photogenerated charge carriers in Bi2Fe4O9/g-C3N4 interface follow a traditional heterojunction-type mechanism, the photoinduced electrons in the CB of g-C3N4 and the holes in the VB of Bi2Fe4O9 will move to the CB of Bi2Fe4O9 and the VB of g-C3N4, respectively. In this case, although

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the charge carriers’ separation between Bi2Fe4O9 and g-C3N4 is probably promoted,

the electrons accumulated in the CB of Bi2Fe4O9 and the holes gathered in the VB of g-

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C3N4 have lower redox potentials than those of g-C3N4 and Bi2Fe4O9 respectively,

thereby reducing the redox capacity of composite [48, 49]. Most importantly, the accumulated electrons in the CB of Bi2Fe4O9 cannot react with O2 to generate •O2-,

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which is not in agreement with our radical trapping experiments results. Therefore, the

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traditional heterojunction-type mechanism may not dominate the photocatalytic

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process.

On the basis of the band edge position, the radical trapping experiments and the

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photocatalytic performances, a Z-Scheme photocatalytic mechanism of photogenerated

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electronic-hole pair transfer at the interface of Bi2Fe4O9/g-C3N4 composites is presented in Fig. 9. Both Bi2Fe4O9 and g-C3N4 semiconductors are immediately excited with the photogenerated electrons and holes produced under visible-light illumination. The

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photoinduced electrons in the CB of Bi2Fe4O9 will move to the VB of g-C3N4 and recombine with those holes in its VB due to the electrostatic attraction between e- and

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h+ [40, 41 and 49]. That means that with the efficient separation of the holes of Bi2Fe4O9 and the electrons of g-C3N4, the lifetime of the photogenerated carriers will increase. To verify the above assumption, the electrochemical impedance spectra (EIS) are used

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to evaluate the electron/hole recombination in our samples. Fig. 10 illustrates the EIS changes of pure g-C3N4, Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 (the sample S4), respectively. Generally speaking, the smaller semicircles of the Nyquist plots mean a smaller chargetransfer resistance on the electrode surface, giving rise to a higher electron/hole transfer and separation [42]. The data in Fig.10 present that Bi2Fe4O9/g-C3N4 composite has a smaller relative arc sizes, which imply the occurrence of a quicker

charge transfer in the composite than that in the pure g-C3N4 and Bi2Fe4O9. Therefore, the EIS results prove that the Bi2Fe4O9/g-C3N4 composite structure will lead to the reduction of electron/hole. As is discussed before, those electrons separated in the CB of g-C3N4 as well as holes in the VB of Bi2Fe4O9 can simultaneously react with O2 to generate •O2- and with H2O to form •OH, both of which can further carry out the redox reactions of RhB. Meanwhile, the more positive VB potential of Bi2Fe4O9 than that of

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g-C3N4 indicate the stronger oxidation activity of photoinduced holes, which can also

directly decompose RhB. These results indicate that •O2-, •OH and h+ all have effects

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on the photocatalytic degradation of RhB and •O2- is easier to be generated followed

by the Z-scheme photocatalytic mechanism in our composite, which match well with our radical trapping experiments results. In general, the redox capacity of the

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photoinduced electrons and holes will be enhanced by the Z-Scheme carriers transfer

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process in Bi2Fe4O9/g-C3N4, thereby resulting in the enhancement of photocatalytic

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efficiency. Recently, the Z-scheme photocatalytic mechanism has also been found in other composites photocatalysts such as Ag3PO4/g-C3N4 and WO3/g-C3N4, in which the

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energy band structures between g-C3N4 and Ag3PO4 or WO3 are similar to those

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between Bi2Fe4O9 and g-C3N4 [40, 50].

According to the above discussion, the probable photocatalytic reactions over Bi2Fe4O9/g-C3N4 composites are described as follows:

e- + O2

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Bi2Fe4O9/g-C3N4 + hv

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h++ H2O

•O2-+ RhB

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•OH + RhB h++ RhB

Bi2Fe4O9 (e-+h+) + g-C3N4 (e-+h+)

•O2-

H+ + •OH Degradation products Degradation products Degradation products

(3) (4) (5) (6) (7) (8)

As we know, proportions of every component in composites also have great effects

on photocatalytic or photoelectrochemical performance [27, 50-53]. For instance, Chen et al. report that the photocurrent densities of ZnO-based core–shell nanowire arrays are improved significantly as a result of the synergistic effects of the CdSe and Cu2-xSe layers [27]. It is notable that in comparison with that of pure Bi2Fe4O9 powders,

those Bi2Fe4O9/g-C3N4 composites present remarkably enhanced photocatalytic rates, whose performances are similar to those of g-C3N4, implying the key role of the introduction of g-C3N4 and the heterostructure in promoting the photocatalytic activity [54]. The obvious difference in the optimal weight ratio of Bi2Fe4O9 and g-C3N4 could be attributed to the content of g-C3N4. With a litter g-C3N4 dispersing on the surface of Bi2Fe4O9, a few Bi2Fe4O9/g-C3N4 heterojunction are generated. Compared with pure

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Bi2Fe4O9, this kind of structure is beneficial for promoting the separation of electronsholes pairs, thus improving the photocatalytic efficiency. However, the small total area

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of the solid-solid contact interface cannot ensure electrons movement from Bi2Fe4O9 to g-C3N4 effectively and thus will inhibit the utilization of solar light [49]. When the weight ratio of g-C3N4 and Bi2Fe4O9 increases to 70%, the broadened interface between

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g-C3N4 and Bi2Fe4O9 provides enough channels for the transfer of electrons, the

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separation of electron-hole pairs and the absorption of visible-light, all of which lead

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to the improved photocatalytic efficiency [30, 31 and 38]. Flake-like g-C3N4 morphology appears due to the excessive g-C3N4 powders covering the surface of

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Bi2Fe4O9 particles, which inhibits the interaction between Bi2Fe4O9 and g-C3N4,

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reduces the active sites on the heterojunction surfaces and shields the visible light absorption. Moreover, the distance between Bi2Fe4O9 and g-C3N4 is extended owing to the excessive g-C3N4, which inhibits the electrons transfer and aggravates the electron-

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hole recombination. As a consequence, the photocatalytic activity is damaged. 3.6 Recyclability and stability

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From the view of application, the recyclability and stability of photocatalysts is

important to evaluate the quality of catalysts. Therefore, the additional recyclability and stability experiments of the best-performing sample S4 are carried out to

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photodegrade RhB under the same condition for three times. As is shown in Fig. 11, the slight decrease of photocatalytic efficiency of Bi2Fe4O9/g-C3N4 composites implies the satisfactory recyclability and stability of the composite during the photocatalytic process. The photocatalytic degradation of 79.5% and 69.5% for RhB after 2nd and 3rd cycles could be mainly ascribed to the weight loss of the samples during the recovery and cleaning process [51]. All of these characteristics indicate that the Bi2Fe4O9/g-

C3N4 composite can be regarded as a stable photocatalyst with the long-term practical use. 4. Conclusions Different weight ratios of Bi2Fe4O9/g-C3N4 composites are successfully fabricated using a simple mixing–calcination process combined with a low-temperature solid state reaction method. Results of XRD and FT-IR confirm the successful formation of

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Bi2Fe4O9/g-C3N4 composites. The introduction of g-C3N4 leads to the variation of Bi2Fe4O9 morphology from cubes to flakes and the clear interfaces are observed in the

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composites, both of which have been proven by the FE-SEM, TEM and the EDS mapping techniques. The Bi2Fe4O9/g-C3N4 composites show strong absorption in the

range of visible light. Compared with Bi2Fe4O9 and g-C3N4, Bi2Fe4O9/g-C3N4

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composites exhibit improved photodegradation efficiency for RhB. The highest

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photocatalytic performance for RhB of 88.3% for 50mins illumination is achieved in

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the samples S4. The mechanisms of Bi2Fe4O9/g-C3N4 weight ratios on photocatalytic performance have been revealed. According to the active species trapping experiments

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results, electrochemical impedance spectra and electronic energy-band structure

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estimation analysis, the enhanced photocatalytic activities of Bi2Fe4O9/g-C3N4 composites can be ascribed to the synergetic effect of hybridization and close interfacial connection through the Z-scheme photocatalytic mechanism. The recyclability and

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stability measurements show that the best-performing sample S4 has satisfactory photocatalytic stability. These findings provide a new example of the construction of

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the Z-scheme photocatalytic system and meanwhile indicate that Bi2Fe4O9/g-C3N4 composites are a promising candidate of visible-light photocatalyst in terms of removing environmental pollutants.

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Acknowledgments This work was sponsored by the National Natural Science Foundation of China

(11704288). References

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Figure Captions

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Fig. 1: XRD patterns of g-C3N4, Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 composites.

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Fig. 2: FT-IR spectra of g-C3N4, Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 composites.

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Fig. 3: FE-SEM micrographs of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5 and (f) S6.

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Fig. 4: The distribution of constituent elements of O, C, Fe, Bi and N by the EDS

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mapping analysis in the sample S4.

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Fig. 5: TEM image (a) and the corresponding HRTEM images (b) of the sample S4.

Fig. 6: The UV-vis absorption spectra of g-C3N4, Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 composites and the corresponding (F(R)) 1/2~hv curves in the inset.

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Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 composites.

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Fig. 7: Photodegradation activities of RhB under visible-light irradiation over g-C3N4,

Fig. 8: Degradation of RhB over the sample S4 with different scavengers.

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Fig.9: Energy band structures of g-C3N4 and Bi2Fe4O9 and a possible Z-scheme

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photocatalytic mechanism of electron-hole separation, and transportation in the

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Bi2Fe4O9/g-C3N4 composite under visible-light irradiation.

Fig. 10: EIS measurement of g-C3N4, Bi2Fe4O9 and Bi2Fe4O9/g-C3N4 composite.

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Fig.11: Recyclability and stability of photocatalytic degradation of RhB over the

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sample S4.