The p-n-type Bi5O7I-modified porous C3N4 nano-heterojunction for enhanced visible light photocatalysis

Accepted Manuscript The p-n-type Bi5O7I-modified porous C3N4 nano-heterojunction for enhanced visible light photocatalysis Zongjun Dong, Jiaqi Pan, Beibei Wang, Ziyuan Jiang, Chuang Zhao, Jingjing Wang, Changsheng Song, Yingying Zheng, Can Cui, Chaorong Li PII:

S0925-8388(18)30980-0

DOI:

10.1016/j.jallcom.2018.03.112

Reference:

JALCOM 45342

To appear in:

Journal of Alloys and Compounds

Received Date: 8 January 2018 Revised Date:

8 March 2018

Accepted Date: 9 March 2018

Please cite this article as: Z. Dong, J. Pan, B. Wang, Z. Jiang, C. Zhao, J. Wang, C. Song, Y. Zheng, C. Cui, C. Li, The p-n-type Bi5O7I-modified porous C3N4 nano-heterojunction for enhanced visible light photocatalysis, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.112. 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.

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ACCEPTED MANUSCRIPT The p-n-type Bi5O7I-modified porous C3N4 nano-heterojunction for enhanced visible light photocatalysis Zongjun Donga, Jiaqi Pana, Beibei Wanga, Ziyuan Jianga, Chuang Zhaoa, Jingjing

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Wanga, Changsheng Songa, Yingying Zhenga, Can Cuia, Chaorong Lia, * Department of Physics, and Key Laboratory of ATMMT ministry of Education,

Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China

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E-mail address: [email protected]

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Abstract: A p-n-type Bi5O7I-modified porous g-C3N4 nano-heterojunction is synthesized via a simple preparation technique using the template method and annealing-hydrothermal co-deposition method. The results of SEM, EDS, elemental mapping, XRD, TEM, XPS and FT-IR imply that the Bi5O7I nanoparticles were

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successfully deposited on the surface of the g-C3N4 nanosheets. The photocatalytic activity of the as-prepared p-n-type nano-heterojunction remarkably exhibits an enhancement of 30 times that of the unmodified samples. Furthermore, the

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photocatalytic degradation enhancement mechanism has been studied, which could be

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ascribed to the p-n-type nano-heterojunction and lamellar-porous structure of the g-C3N4.

Key words: p-n-type, photocatalytic degradation, nano-heterojunction, cycling 1. Introduction

With increasing environment pollution, caused by industrial wastewater, household garbage or chemical dyes, low cost and recyclable semiconductor photocatalysts, such as ZnO, TiO2, and SnO2[1-3], in recent years, have attracted wide attention for 1

ACCEPTED MANUSCRIPT environment purification and have been utilized in many studies. Currently, graphite-like carbon nitride (g-C3N4)[4-7], which is metal-free, exhibits high thermal-chemical stability and can be used in visible light (band gap of 2.6 eV), has

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been one of the most active research topics[8-9]. Dong et al. reported a g-C3N4 nanohybrid with SPR-enhanced visible-light photocatalytic performance[10], and Jiang’s group obtained yolk–shell g-C3N4 spheres for water pollution treatment and

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hydrogen production[11]. Although g-C3N4 exhibits a remarkable visible light response,

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the lifetime of the photon-generated carriers and the specific surface area would seriously restrict improvements to the photocatalytic efficiency. Thus, many researchers have attempted to solve those issues, and many inspiring results have been reported, such as noble metal deposition (Au, Ag)[12-13], surface modification, and

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doping with metal (K, Fe)[14-15] or nonmetal elements (P, S)[16-17]. Surface heterojunction modification characterized by high carrier separation demonstrates a remarkable carrier lifetime for efficiently enhancing photocatalytic

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performance[18-19]. Furthermore, compared with other traditional methods, the facile

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preparation could allow many materials to match the band structure of g-C3N4[20]. He et al. prepared ZnO/g-C3N4 photocatalysts to convert CO2 to fuel[21]. Hao’s group reported TiO2/g-C3N4 heterojunction photocatalysts with remarkable visible light photocatalytic performance[22], and Yang et al. prepared Bi2SiO5/g-C3N4 with a high photocatalytic

performance[23][24].

In

the

above

studies,

the

Bi-O-I-based

semiconductor with a visible light response and an intrinsic p-type semiconductor character has attracted great attention[25-28]. Compared with traditional modification 2

ACCEPTED MANUSCRIPT methods, modification with the intrinsic p-type Bi-O-I-based semiconductor generated nanocomposites with typical p-n junctions possessing more efficient photon-generated carrier separation than the traditional heterojunctions[29-31]. Chen’s

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group prepared a series of BiOxIy/g-C3N4 nanocomposites with enhanced visible light photocatalytic activity[32], Song et al. synthesized the 3D nanoplate-built BiOI/CdWO4 heterostructures with enhanced photocatalytic performance under light

irradiation[33],

Reddy’s

group

fabricated

a

p-BiOI/n-ZnTiO3

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visible

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heterojunction with excellent visible light degradation[34], and Jiang et al. prepared BiOpBrq/BiOmIn heterojunctions with high visible light photocatalytic performance[35]. Furthermore, Bi5O7I[36], with a suitable band gap (2.9 eV), higher chemical stability and transfer efficiency, has been reported an efficient modification material by Cheng

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et al., who obtained a novel visible light-driven photocatalyst of Bi5O7I/Bi2O3 heterojunction[37], Cui et al., who fabricated a Z-scheme AgI/Bi5O7I photocatalyst with excellent photocatalytic activity[38], Huang’s group, who reported the

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g-C3N4/Bi5O7I heterojunction

for enhancing

the visible-light

photocatalytic

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performance[39], Liu’s group, who fabricated the PbBiO2Cl/BiOCl nanocomposite with enhanced visible light photocatalytic activity[40], Lin et al., who reported a visible light photocatalyst of PbBiO2Br/BiOBr heterojunction[41]. These studies provide a number of choices for heterojunction modification. Furthermore, the specific surface area is another important factor in photocatalytic performance, such that a large specific surface area could provide sufficient reactive sites for the photocatalytic process, and many studies have attempted for address this 3

ACCEPTED MANUSCRIPT issue. Han reported seaweed-shaped g-C3N4 for enhanced hydrogen evolution[42], and Zheng obtained helical g-C3N4 with improved photocatalytic and optical performance[43]. Notably, in the pores nanosheets, the porous morphology and

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lamellar structure could effectively increase the specific surface area [44]; for instance, Yang et al. prepared porous g-C3N4 nanosheet-based heterostructures to enhance the visible light photocatalytic activity[45], Liu et al. prepared hollow, mesoporous,

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ultrathin g-C3N4 with spatial anisotropic charge separation for superior photocatalytic

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H2 evolution[46], Guo’s group reported ultrathin g-C3N4 nanosheets with excellent visible light photocatalytic activity[47], Wan et al. reported multifunctional holey C3N4 nanosheets with a high-efficient photocatalytic performance[48], and Tian et al. obtained 3D mesoporous g-C3N4 for superior hydrogen evolution[49]. All of the above

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indicate that multiple modification methods would efficiently increase the specific surface area.

In this work, we prepared a novel p-n-type Bi5O7I-modified porous g-C3N4

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nano-heterojunction via a simple template method and hydrothermal-chemical

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co-deposition method. Compared with unmodified g-C3N4 nanosheets, the p-n-type Bi5O7I/g-C3N4 nano-heterojunction exhibits an obvious enhancement in the visible light photocatalytic degradation of phenol. Furthermore, the stability over repeated cycles and the mechanism of the photocatalytic activity enhancement were studied. 2. Experimental 2.1 Preparation of the SiO2 nanosphere: A classical Stöber method was used to prepare the SiO2 nanosphere template[50]. Here, 4 ml of aqueous ammonia (32 wt.%, 4

ACCEPTED MANUSCRIPT Sigma-Aldrich), 10 ml of deionized water and 98 ml of ethanol were mixed together at 30 °C with 30 min of uniform stirring. Then, 5.6 ml of tetraethoxysilane (TEOS, Sigma-Aldrich) was added, and the solution was vigorously stirred for 15 min. Next,

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to form uniform nonporous silica spheres, the solution was left to sit for 1 h. To gain a more disperse SiO2, doping was performed using 2 ml of n-octadecyltrimethoxysilane (C18TMOS, Sigma-Aldrich) and 2 ml of tetraethoxysilane while stirring, and the

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solution was left to sit without stirring for 3 h at room temperature. Monodisperse

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SiO2 was obtained by centrifuging and drying at 70 °C, followed by annealing at 550 °C for 6 h in air at a rate of 1.5 °C/min.

2.2 Preparation of the porous g-C3N4: The above monodisperse silica nanoparticles were used as the template to prepare porous g-C3N4. After mixing with

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0.2 g of SiO2 and 1 g of a cyanamide solution (50% in H2O, Macklin), the turbid liquid after 2 h sonication was heated at 70 °C to achieve a well-dissolved solution, and it was subsequently stirred for 10 h. Then, the g-C3N4/SiO2 was annealed at

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550 °C for 4 h. After removing the silica template with 4 M NH4HF2 for 12 h, the

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obtained porous g-C3N4 was rinsed by centrifuging, washing and drying. 2.3 Preparation of the p-n-type Bi5O7I-modified g-C3N4 nano-heterojunction:

The nano-heterojunction was synthesized by a hydrothermal method. Typically, 100 mg of p-type g-C3N4 was dispersed in 38 ml of ethylene glycol with stirring and ultrasound. Subsequently, the Bi(NO3)3·5H2O powder was poured into ethylene glycol that included g-C3N4 and was accompanied by stirring and ultrasound for 10 min. A KI solution (KI dissolved in 2 ml of deionized water) was added dropwise to 5

ACCEPTED MANUSCRIPT the mixed solution. After forceful stirring at room temperature for 0.5 h, the mixture was transferred to a 50 ml Teflon-lined autoclave and maintained at 150 °C for 6 h. Finally, the obtained samples were collected by centrifugation via washing two times

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with deionized water and ethanol, and they were dried at 80 °C for 12 h. Then, samples with different amounts (0.0, 25.3, 51.7, and 79.2 mg) of Bi(NO3)3·5H2O and (0.0, 1.0, 2.0, and 3.0 mg) KI were denoted by g-C3N4, C-Bi-1, C-Bi-2, C-Bi-3,

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respectively. Furthermore, pure Bi5O7I was prepared by the same method.

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2.4 Photocatalytic activity: In this experiment, 50 mg of the photocatalysts were added into 50 ml (10 mg/L) of the phenol solutions and stirred for 30 min in the dark to reach the adsorption–desorption equilibrium. Then, the activity was studied under visible light (CEL-HXF300/CEL-HXUV300, λ≥400 nm). During the process, with

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the given time intervals (10 min), 3 ml of solution was taken for photocatalytic examination by a UV–Vis spectrophotometer (Hitachi U3900). The specific surface area was measured via the Brunauer–Emmett–Teller method (BET, 3H-2000PS1).

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Moreover, the carrier trapping was also performed under the same conditions by

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adding isopropyl alcohol (IPA, •OH trapping agents), ethylenediamine tetraacetic acid (EDTA, h+ trapping agents) and benzoquinone (BQ, O2- trapping agents). 2.5 Characterization:

The micromorphology, including elemental mapping and EDS, were characterized

by scanning electron microscopy (FESEM, Hitachi S-4800). The crystal structure and phase composition were determined by transmission electron microscopy (TEM, JEM-2100) and X-ray diffraction (XRD, Bruker D8 Discover). The specific surface 6

ACCEPTED MANUSCRIPT area was measured by an F-sorb 3400 micropore analysis system (3H-2000PS1). The X-ray photoelectron spectroscopy (XPS) spectra were obtained by a Kratos Axis Ultra system with a monochromatic Al K X-ray source. The UV–Vis diffuse reflectance

(Hitachi-U3900).

The

PL spectra

were

recorded

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spectra and photocatalytic degradation were obtained by a UV–Vis spectrophotometer by

a

Hitachi

F-7000

spectrofluorimeter. Fourier transform infrared spectroscopy (FT-IR) spectra were

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measured by a Thermo Nicolet Nexus FTIR model 670 spectrometer.

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3. Results and discussion

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Fig. 1 The XRD patterns of the C3N4 nanosheets modified with different ratios of Bi5O7I

Fig. 1 shows the XRD patterns of the as-prepared Bi5O7I-modified g-C3N4

nano-heterojunctions. As revealed, the obvious diffraction peak at 27.4° could be ascribed to the (002) plane of the g-C3N4 (JCPDS-87-1526)[51]. With the deposition of the Bi5O7I nanoparticles, new diffraction peaks that appeared in C-Bi-1, C-Bi-2 and C-Bi-3 obviously increased with the ratio of the modification. As displayed, the new 7

ACCEPTED MANUSCRIPT peaks located at 28.0, 31.0, 32.0, 33.4, 46.0, 47.6, 53.4, 55.9, 56.4 and 57.9° are assigned to the (312), (004), (204), (020), (024), (224), (316), (912), (1000) and (624) planes of the p-type Bi5O7I (PDF-40-0548), which corresponds to pure Bi5O7I[28, 42].

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Interestingly, the peak of the (002) plane of g-C3N4 partially overlaps with the (312) plane of the Bi5O7I. As revealed, with increasing Bi5O7I, the intensity of the peaks at 28.0° (312 plane) increases gradually and it can be distinguished clearly in C-Bi-3.

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samples are mainly composed of Bi5O7I and g-C3N4.

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There are no other obvious peaks that could be observed, which indicate that the

Fig. 2 The SEM of the as-prepared samples at different states, (a) HRSEM of the

SiO2 nanospheres template, (b) HRSEM of the SiO2 nanosphere/g-C3N4 nanosheets composites, (c) HRSEM of the g-C3N4, (d) HRSEM of the C-Bi-1, (e) HRSEM of the C-Bi-2, (f) HRSEM of the C-Bi-3 The morphology of the samples at different states is displayed in Fig. 2. Fig. 2a is the high-resolution SEM (HRSEM) of the as-prepared SiO2 nanosphere template. As 8

ACCEPTED MANUSCRIPT revealed, the SiO2 nanospheres with a diameter of 150 nm are uniform. Fig. 2b is the HRSEM of the SiO2 nanosphere/g-C3N4 nanosheets composites. After annealing, the SiO2 nanospheres are evenly dispersed in the g-C3N4 nanosheets. Subsequently, the

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SiO2 nanospheres are removed, and the porous g-C3N4 nanosheets are displayed in Fig. 3c. As shown, the structure of the porous nanosheets provides abundant loading sites for the deposition of the Bi5O7I. Fig. 2d, 2e and 2f are the HRSEM of the C-Bi-1,

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C-Bi-2 and C-Bi-3, respectively. As shown in Fig. 2d, with the deposition of Bi5O7I,

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nanoparticles appear on the surface of the C3N4 nanosheets. Compared with Fig. 2e and 2f, as the ratio of the Bi5O7I increases further, the surface of the g-C3N4 nanosheets becomes increasingly rough. Due to the high density of the Bi5O7I nanoparticles and the resolution of the SEM, the dense Bi5O7I nanoparticles could not

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be easily distinguished, requiring characterization by TEM. Further, the elemental mapping (ESI Fig. S1) of the Bi5O7I-modified C3N4 nanosheets (C-Bi-2) indicates that the elements Bi, O, I, C and N are uniformly distributed on the surface. Then, the

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EDS are shown in Fig. S2 (in ESI); as revealed, the atomic ratio of the Bi to I is

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approximately 5.04 (2.27/0.45, the stoichiometric ratio of the Bi5O7I is 5/1), which corresponds to the XRD results, indicating that Bi5O7I nanoparticles were introduced into the system and uniformly distributed on the surface of the g-C3N4 nanosheets. Additionally, as displayed in Fig. S3a (in ESI), the N2 adsorption–desorption

isotherm measurement indicates that the p-n-type Bi5O7I-modified porous g-C3N4 nano-heterojunction has a large Brunauer–Emmett–Teller (BET) specific surface area of 21.01 m2/g, which is typical of a porous lamellar material. As calculated by the 9

ACCEPTED MANUSCRIPT Barrett–Joyner–Halenda (BJH) equation, the average size of the pores is approximately 13.57 nm (in ESI Fig. S3b), indicating that this porous lamellar

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structure would be an ideal choice as a photocatalyst.

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Fig. 3 The TEM micrographs of the Bi5O7I-modified g-C3N4 nano-heterojunction, (a) the TEM of the as-prepared Bi5O7I/g-C3N4 nano-heterojunction, (b) the HRTEM of the Bi5O7I, and (c) the HRTEM of the g-C3N4

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Fig. 3 shows the TEM data for the as-prepared Bi5O7I-modified g-C3N4

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nano-heterojunction. As shown in Fig. 3a, the nano-heterojunction is laminar and porous, and the Bi5O7I nanoparticles are uniformly distributed in the surface of the g-C3N4 nanosheets, which corresponds to the SEM results. Furthermore, the data of the HRTEM characterization of Bi5O7I and g-C3N4 are shown in Fig. 3b and 3c, the lattice spacing of 0.318 nm and 0.328 nm are ascribed to the (312) plane of Bi5O7I and the (002) plane of C3N4[39, 51-52].

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Fig. 4 The XPS spectra of the as-prepared Bi5O7I/g-C3N4, (a) full survey, (b) Bi 4f spectra, (c) I 3d spectra, (d) O 1s spectra, (e) C 1s spectra, (f) N 1s spectra the

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Furthermore,

components

of

the

as-prepared

Bi5O7I/g-C3N4

nano-heterojunction were characterized by XPS. Fig. 4a presents the full survey data that indicates the presence of Bi, I, O, C and N in the as-prepared sample. Fig. 4b shows the high-resolution spectrum of Bi 4f. As shown, two peaks, Bi 4f7/2 and Bi 4f5/2, could be observed at 159.4 and 164.7 eV, respectively, which could be assigned to Bi3+ in the Bi5O7I[32, 53]. Fig. 4c is the high-resolution spectrum of the I 3d. As revealed, the peaks of the I 3d are at 619.1 and 630.4 eV, which are attributed to the I 11

ACCEPTED MANUSCRIPT 3d5/2 and I 3d3/2[32]. To obtain more insight into the present chemical bonding between the oxygen, carbon and nitrogen in the nano-heterojunction, the high-resolution XPS spectra of O 1s, C 1s and N 1s are fitted by the Gaussian analysis method. As

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displayed in Fig. 4d, the O 1s spectrum could be divided into three peaks located at 529.3, 531.0 and 532.4 eV. The peak at 529.3 is ascribed to the Bi-O bond of the lattice oxygen atoms in the Bi-O-I system[32, 53]. Two other peaks could be assigned to

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the absorbed –OH and H2O molecule group at the surface[32, 53], respectively. Fig. 4e

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is the spectrum of C 1s. As can be seen, two distinct peaks at 284.8 and 288.4 eV and a lower peak at 286.2 eV are observed. The two distinct peaks could be attributed to the sp2 C-C bonds (284.2 eV) and carbon in N-C=N (288.4 eV)[53] while the lower peak at approximately 286.2 eV could be assigned to the C−NH2 species of the

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g-C3N4[28, 32]. Four peaks are deconvoluted in the N 1s spectrum in Fig. 4f. The peak located at 398.5 could be assigned to sp2 hybridized nitrogen in the triazine rings (C-N=C), and the peak at 399.7 could be ascribed to tertiary nitrogen N-(C)3 groups[28, together with N-C=N (288.4 eV), which construct the basic g-C3N4 unit.

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53]

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Furthermore, the peaks at 400.8 and 404.2 eV could be assigned to the free amino groups (C-N-H) and π-excitations, respectively[32, 53].

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Fig. 5 The FT-IR spectra of g-C3N4 nanosheets modified with different amount of Bi5O7I

Furthermore, the FT-IR spectra of the samples modified with different ratios of Bi5O7I are displayed in Fig. 5. By comparing with the reference data, the peak at 1628

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cm-1 could be attributed to the C=N stretching vibration, and the peaks located at 1240 and 1320 cm-1 could be ascribed to the aromatic C-N stretching vibration[39]. In

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addition, the peak at 808 cm-1 is related to the triazine units[39]. It is obvious that, with the modification of Bi5O7I, the characteristic peaks of g-C3N4 show minimal shifts,

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which indicates that there is no other surface group and the photocatalysis enhancement is caused by the nano-heterojunction. Therefore, based on the above results, the p-n-type Bi5O7I/g-C3N4 nano-

heterojunction was successfully prepared.

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Fig. 6 The photocatalytic degradation of phenol, (a) effect of difference in the

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modification ratios of the Bi5O7I, (b) the histograms of the samples

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The photocatalytic phenol degradation activities of the different Bi5O7I/g-C3N4 nano-heterojunctions are displayed in Fig. 6, including the control group (pure Bi5O7I). As revealed, the pure as-prepared g-C3N4 nanosheets exhibit a weak photocatalytic activity of phenol degradation under visible light, which could be

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ascribed to the intrinsic photocatalytic properties of the g-C3N4 nanosheets. It is obvious that the photocatalytic properties improve with the introduction of Bi5O7I nanoparticles and achieve an optimal value at the C-Bi-2 (approximately 96%) and

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then decrease. Furthermore, the corresponding kinetics curves are fitted as[54] -ln(C/C0)

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= Kt, where C0 and C correspond to the concentration of phenol at irradiation time t=0 and t=tcurrent, respectively. The K values of the corresponding samples are calculated as 0.0018 (g-C3N4), 0.0198 (C-Bi-1), 0.0552 (C-Bi-2), 0.0320 (C-Bi-3) and 0.0062 (pure Bi5O7I) min-1. Compared with the unmodified pure g-C3N4 nanosheets, C-Bi-2 achieved a photocatalytic performance that was enhanced by approximately 30 times, outperforming the other studied materials. All the results indicate that the p-n-type Bi5O7I/g-C3N4 nano-heterojunction could effectively enhance the photocatalytic 14

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activity.

Fig. 7 The repeated cycling performance of the p-n-type Bi5O7I/g-C3N4 nano-heterojunction under visible light As a remarkable photocatalyst, the cycling stability of C-Bi-2 was studied out

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under visible light. As shown in Fig. 7, after 6 consecutive cycles, the photocatalysis shows a stable performance. Compared with the initial value, the decrease of 8% could be regarded as a decent result. Furthermore, the XRD of C-Bi-2 after cycling

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(in ESI Fig. S4) shows similar pattern to the previous sample, which indicates that the

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structure of the nano-heterojunction is stable in the photocatalytic process. The above results indicate that the nano-heterojunction exhibits a remarkable cycling stability during the photocatalytic process. Thus, it is important to explore the photocatalytic mechanism of the p-n-type

Bi5O7I/g-C3N4 nano-heterojunction under visible light.

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Fig. 8 The UV–Vis absorption spectra of nano-heterojunction containing different amounts of Bi5O7I Fig.

8

shows

the

UV–Vis

absorption

spectra

of

the

Bi5O7I/g-C3N4

nano-heterojunction with different ratios. As shown, the absorption peak for the

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as-prepared g-C3N4 nanosheets is located at approximately 470 nm, which is ascribed to the intrinsic band gap of g-C3N4[8, 10]. The absorption characteristics of pure Bi5O7I

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are also displayed, which indicate that the band gap of Bi5O7I is wider than that of g-C3N4. Therefore, with the introduction of Bi5O7I nanoparticles, the absorption of the

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different samples does not notably change in visible light, in agreement with the results of the earlier published studies[36, 38]. The similar absorption values indicate that the p-n-type nano-heterojunction plays an important role in the photocatalytic process.

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Fig. 9 The different carrier trapping in the Bi5O7I/g-C3N4 nano-heterojunction, IPA (isopropyl alcohol, •OH trapping agents), EDTA (ethylenediamine tetraacetic acid, h+ trapping agents), BQ (benzoquinone, ·O2- trapping agents) Furthermore, the carrier type is another important factor for this process, and

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carrier trapping was carried out. Fig. 9 shows the carrier trapping of the Bi5O7I/g-C3N4 nano-heterojunction in visible light photocatalysis. As revealed, the samples mixed with IPA (isopropyl alcohol, •OH trapping agents) and EDTA

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(ethylenediamine tetraacetic acid, h+ trapping agents) show a similar degradation

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activity to that of the blank sample, indicating that presence of •OH and h+ is not the main factor in this system[54]. Interestingly, the sample with BQ (benzoquinone, ·O2- trapping agents) exhibits an obvious decrease in photocatalytic performance, which indicates that the probability of forming ·O2- is much higher[4, 23, 29, 35], and ·O2- plays an important role in the degradation process, as demonstrated in the previous literature[55-59].

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visible light

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Fig. 10 The mechanism of the p-n-type Bi5O7I/g-C3N4 nano-heterojunction under

Based on the above results from XRD, TEM, EDS, elemental mapping, XPS, FT-IR, UV–Vis and carrier trapping, the mechanism of the photocatalytic degradation enhancement under visible light is proposed and shown in Fig. 10, and it could be

structure.

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mainly ascribed to the unique p-n-type nano-heterojunction and remarkable laminar

Compared with the normal heterojunctions, Bi5O7I-modified g-C3N4 could form a

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unique p-n junction structure by using deposition. Under visible light illumination, the

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internal electric field of the p-n junction could drive the photon-generated carrier transfer, promoting the photon-generated electron (CB of p-type Bi5O7I) transfer to the g-C3N4 (CB of n-type g-C3N4) and the photon-generated holes (VB of g-C3N4) transfer to the Bi5O7I (VB of Bi5O7I), thus allowing a more efficient charge carrier separation than that in a common heterojunction and increasing the lifetime of the photon-generated electrons forming ·O2- (also including the •OH and h+) to improve the photocatalytic degradation[54-57]. Since the probability of forming ·O2- is much 18

ACCEPTED MANUSCRIPT higher than that for •OH and h+, the main process could be described as follows[4, 35, 60]

: -

(1),

-

(2),

·O2 + 2H+ = H2 O2

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·O2 + Phenol→ Intermediates →Degraded Products

which corresponds to previous Bi-based works[4, 20, 23, 29, 41, 55-61]. This consequence could be proved by the absorption and fluorescence (PL) of the samples. As revealed

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in Fig. 8, with the introduction of Bi5O7I, the absorption of the sample does not

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notably change, which indicates that the enhancement in photocatalysis is derived from the structure of the nano-heterojunction, instead of the increasing absorption. Furthermore, as shown in Fig. S5 (in ESI), the PL of the C-Bi-2 is obvious weaker than that of other composites, which means that the recombination has decreased, and

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the separation of the photon-generated carriers have increased[38]. Together with the above results, the p-n-type heterojunction is considered as the main reason for the enhanced photocatalytic degradation. Interestingly, C-Bi-3 exhibits a decrease in

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photocatalytic degradation despite exhibiting a similar visible light absorption, which

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could be ascribed to the higher recombination of photon-generated carriers compared to C-Bi-2, which could be proved by Fig. S5 (in ESI)[28]. The presence of lamellar and porous structure of the Bi5O7I-modified g-C3N4 is also

a prominent factor that could provide a larger surface area (Fig. S3a, 21.01 m2/g) than that of common g-C3N4 (approximately 10 m2/g) for the deposition of the Bi5O7I nanoparticles and the photocatalytic degradation. The p-n-type Bi5O7I-modified porous g-C3N4 nano-heterojunction exhibits 19

ACCEPTED MANUSCRIPT excellent photocatalytic degradation under visible light. 4. Conclusions We have successfully prepared a p-n-type Bi5O7I-modified porous g-C3N4

as-prepared

by

p-n-type

an

annealing-hydrothermal

nano-heterojunction

exhibits

deposition

method.

The

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nano-heterojunction

remarkable

photocatalytic

enhancement under visible light towards the degradation of phenol, with a catalytic

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activity nearly 30 times higher that of unmodified g-C3N4. The main reason is the

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unique p-n-type heterojunction in which the internal electric field of the p-n junction could drive the transfer of photon-generated carriers to promote the photon-generated carrier separation and increase the lifetime of the photon-generated electrons. In addition, the lamellar and porous structure of g-C3N4 could provide an abundant

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specific surface area for the deposition of the Bi5O7I and for photocatalysis. Additionally, the p-n-type photocatalyst exhibits remarkable photocatalytic stability under a visible light.

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Thus, the novel p-n-type Bi5O7I-modified porous g-C3N4 nano-heterojunction with

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remarkable photocatalysis activity is considered a potential material in the field of the environmental protection. Acknowledgments

This work was supported by the Natural Science Foundation of China (Nos.

51672249 and 51603187) and the Zhejiang Provincial Natural Science Foundation of China (No. LQ17F040004). References 20

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ACCEPTED MANUSCRIPT Highlights >The p-n type Bi5O7I/C3N4 nano-heterojunction could promote the separation of the photon-generated carriers,

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>The samples exhibit remarkable photocatalytic degradation of phenol and stability, >The lamellar and porous structure of the C3N4 could provide large surface area for

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EP

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SC

photocatalysis.