g-C3N4 with high visible light activity

Accepted Manuscript Title: Design of a direct Z-scheme photocatalyst: Preparation and characterization of Bi2 O3 /g-C3 N4 with high visible light activity Author: Jinfeng Zhang Yingfei Hu Xiaoliang Jiang Shifu Chen Sugang Meng Xianliang Fu PII: DOI: Reference:

S0304-3894(14)00718-3 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.08.055 HAZMAT 16229

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

29-4-2014 12-7-2014 10-8-2014

Please cite this article as: J. Zhang, Y. Hu, X. Jiang, S. Chen, S. Meng, X. Fu, Design of a direct Z-scheme photocatalyst: preparation and characterization of Bi2 O3 /g-C3 N4 with high visible light activity, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.08.055 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.

Design of a direct Z-scheme photocatalyst: preparation and characterization of Bi2O3/g-C3N4 with high visible light activity Yingfei Hub

Xiaoliang Jiangb

Sugang Mengb

Xianliang Fu b

Department of Chemistry, Anhui Science and Technology University, Anhui

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a

Shifu Chena,b*

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Jinfeng Zhanga,b

b

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Fengyang, 233100, People’s Republic of China

Department of Chemistry, Huaibei Normal University, Anhui Huaibei, 235000,

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People’s Republic of China

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Abstract: A direct Z-scheme photocatalyst Bi2O3/g-C3N4 was prepared by ball milling and heat treatment methods. The photocatalyst was characterized by X-ray

electron

microscopy

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powder diffraction (XRD), UV–Vis diffuse reflection spectroscopy (DRS), scanning (SEM),

transmission

electron

microscopy

(TEM),

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Brunauer–Emmett–Teller (BET) surface areas, photoluminescence technique (PL), and electron spin resonance (ESR) technology. The photocatalytic activity was evaluated by degradation of methylene blue (MB) and rhodamine B (RhB). The results showed that Bi2O3/g-C3N4 exhibited a much higher photocatalytic activity than pure g-C3N4 under visible light illumination. The rate constants of MB and RhB degradation for Bi2O3(1.0 wt.%)/g-C3N4 are about 3.4 and 5 times that of pure g-C3N4, respectively. The migration of photogenerated carriers adopts a Z-scheme mechanism. The photoexcited electrons in the CB of Bi2O3 and photogenerated holes in the VB of g-C3N4 are quickly combined, so the photoexcited electrons in the CB of g-C3N4 and

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holes in the VB of Bi2O3 participate in reduction and oxidation reactions, respectively. •O2−, •OH and h+ are the major reactive species for the Bi2O3/g-C3N4 photocatalytic

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system.

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Keywords: Z-scheme photocatalyst; Bi2O3/g-C3N4; Preparation; Characterization;

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Mechanism

*

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To whom correspondence should be addressed. Tel: +86-550-6732001, Fax:

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+86-550-6732001. E-mail: [email protected]

1. Introduction

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Photocatalysis has been attracting much attention for scientists of many fields in recent years because it can be applied to wastewater treatment, environmental

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remediation, producing hydrogen from water splitting and synthetic organics [1-5]. A variety of materials such as oxides, sulfides, nitrides and solid solutions etc. have been exploited as photocatalysts[6-10]. However, rapid recombination of the photoexcited electron-hole pairs is likely to occur for the single-phase photocatalysts, which will restrict the photocatalytic activity. Thus, developing composite photocatalysts were considered as an effective way for enhancement of photocatalytic activity[11-15]. It is proposed that the increased photocatalytic activity of the composite may be attributed to the effective separation of the photoexcited carriers which are transferred into valence band (VB) and conduction band (CB) of opposite semiconductor respectively

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due to their potential difference of VB and CB [12-14]. However, the oxidation and reduction ability of the transferred photoexcited carriers is lower than that of the original due to the difference of band positions. Recently, the Z-scheme principle of

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photocatalyst has become a research hotspot owing to its stronger oxidation and

far,

various

Z-scheme

H2WO4H2O/Ag/AgCl,

photocatalysts

CaFe2O4/Ag(or

have

been

reported,

such

as

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So

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reduction capability and higher photocatalytic performance than the single component.

ITO)/WO3,

AgBr-Ag-Bi2WO6,

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Ru/(SrTiO3:Rh)-(BiVO3)-(Fe3+/Fe2+), IrO2/TaON and so on [16-19]. However, the transfer of photogenerated electrons and holes in these Z-scheme systems was

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achieved only by means of noble metal (Ag, Au, Ru) or IO3-/I-, Fe3+/Fe2+ ion pair. Furthermore, the cost and the stability of these photocatalysts are still far from what is

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expected. So it is an inevitable choice to construct a highly efficient and stable Z-scheme photocatalytic system within two different semiconductors. However, this

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kind of Z-scheme photocatalyst has scarcely been reported up to now[20, 21]. Although the direct Z-scheme photocatalysts such as g-C3N4–TiO2, Bi2O3/NaNbO3,

BiOCl–g-C3N4 and WO3/g-C3N4 have been reported recently [22-25], the transfer mechanisms of the photoexcited carriers have not been investigated in detail. In recent years, graphite C3N4 (g-C3N4) material has attracted much attention in

the field of photocatalysis due to its non-toxic, inexpensive and easy to prepare by heating of melamine, dicyandiamide and urea [24-31]. It is known that the band gap of g-C3N4 is about 2.7 eV, which can absorb visible light up to 460 nm. However, it exhibits low photocatalytic activity because of the high recombination rate of

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photogenerated electron-hole pairs. In order to improve the photocatalytic activity, many strategies such as doping and coupling with other semiconductor, etc. were used to modify g-C3N4 [32-36]. Bismuth oxide (Bi2O3) which can be excited by visible

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light is an important metal-oxide semiconductor with a direct band gap of 2.8

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eV[37-42]. However, the photocatalytic activity of Bi2O3 is limited by the

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photocorrosion and fast recombination of photogenerated charge carriers. In order to increase the photocatalytic activity of Bi2O3, the heterojunction photocatlysts have

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been prepared, such as Bi2O3/BaTiO3, Bi2O3/TiO2 and Bi2O3/Bi2WO6 etc.[40-44]. It is known that the VB position of g-C3N4 is about 1.57 eV, and the CB position

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is about -1.13 eV [26, 27]. The ECB of Bi2O3 is about 0.33 eV, and the EVB is 3.13 eV [37, 39]. When Bi2O3 is combined with g-C3N4, a Z-scheme system photocatalyst may

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be formed due to the short distance between the VB of g-C3N4 and the CB of Bi2O3. If so, the holes generated on the VB of g-C3N4 are easily combined with the electrons

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generated on the CB of Bi2O3. Consequently, the photogenerated electrons on the CB of g-C3N4 exhibit strong reduction ability, and the photogenerated holes on the VB of

Bi2O3 show excellent oxidation ability. However, to the best of our knowledge, there

has been no report on the investigation of the mechanism for the Bi2O3/g-C3N4 photocatalyst.

In this paper, a direct Z-scheme photocatalyst Bi2O3/g-C3N4 exhibiting excellent photocatalytic activity was synthesized by ball milling and heat treatment. The photocatalytic activity of Bi2O3/g-C3N4 was investigated using Methylene blue (MB) and Rhodamine B (RhB) as probe molecules of organic pollutants. The novelty of the

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work compared to the reported papers is that the composite Bi2O3/g-C3N4 as a typical Z-scheme photocatalyst was proved by physical and chemical methods (such as ESR, PL-TA and PL etc). It is suggested that the enhancement of the photocatalytic activity

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may be attributed to the formation of the Z-scheme system between Bi2O3 and g-C3N4,

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which could result in the photoexcited electrons of g-C3N4 with a high reducibility

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and photoexcited holes of Bi2O3 with a high oxidizability participating in oxidation

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and reduction reactions, respectively.

2. Experimental

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2.1 Materials

Melamine powder (99%) used in the experiments was supplied by Aladdin

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Chemistry Co. Ltd. Bismuth (Ⅲ) nitrate pentahydrate was supplied by Sinopharm Chemical Reagent Co., Ltd. MB, RhB and other chemicals used in the experiments

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were purchased from Shanghai and other China chemical reagent Ltd. They are of analytically pure grade and used without further purification. Deionized water was used throughout this study.

2.2 Preparation of samples

g-C3N4 powder was synthesized via heating melamine in a tube furnace. A certain

amount of melamine was put into an alumina crucible which was first heated at 520 ºC for 2 h and was further heated at 540 ºC for 2 h with a temperature rise rate of 10 ºC/min. After the reaction, the alumina crucible was cooled naturally to room temperature. Pure Bi2O3 was prepared using the same heat treatment method, which

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used Bismuth (Ⅲ) nitrate pentahydrate as a precursor. The typical preparation of Bi2O3/g-C3N4 photocatalyst was as follows: firstly, the appropriate amounts of melamine and Bi(NO3)3·5H2O were added into a Zirconia

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tank, respectively. Two different sizes of Zirconia balls were mixed in a Zirconia tank

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and water was used as a dispersant. The precursor samples were milled for 1 h at a

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speed of 400 rpm, and then the wet powder ball milled was dried at 60 °C in air. The obtained powders were heated at the temperatures of 520°C and 540°C for 2 h,

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respectively. In this way, different Bi2O3(wt.%)/g-C3N4 photocatalysts (wt.= 0, 0.1, 0.5, 1.0, 3.0, 5.0) were obtained, respectively.

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2.3 Photoreaction apparatus and procedure

Experiments were carried out in a photoreaction apparatus[45,46]. The

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photoreaction apparatus consists of two parts. The first part is an annular quartz tube. A 500 W Xenon lamp (Institute of Electric Light Source, Beijing) with a maximum

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emission at about 470 nm was used as visible light source. The wavelength of the visible light was controlled through a 400 nm cutoff filter (λ>400nm, Instrument

Company of Nantong, China). The lamp is laid in the empty chamber of the annular tube, and running water passes through an inner thimble of the annular tube. Owing to continuous cooling, the temperature of the reaction solution is maintained at approximately 30C. The second part is an unsealed beaker of a diameter 12 cm. At

the start of the experiment, the reaction solution (volume, 300 ml) containing reactants and photocatalyst was put in the unsealed beakers, and a magnetic stirring device was used to stir the reaction solution. The distance between the light source

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and the surface of the reaction solution is 11 cm. In the experiments, the initial pH of the reaction solution was about 7.0, and the amount of the photocatalyst used was 1.0 g/L. The initial concentrations of MB and RhB were 1.1×10-5 and 1.0×10-5mol/L,

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respectively. In order to disperse the photocatalyst powder, the suspensions were

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ultrasonically vibrated for 20 min prior to irradiation. After illumination, the samples

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(volume of each was 5 ml) taken from the reaction suspension were centrifuged at 7000 rpm for 20 min and filtered through a 0.2-µm millipore filter to remove the

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particles. The filtrate was then analyzed. In order to determine the reproducibility of the results, at least duplicated runs were carried out for each condition for averaging

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the results, and the experimental error was found to be within ± 4%. 2.4 Characterization

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X-ray diffraction measurement was carried out at room temperature using a Bruker D8 advance X-ray powder diffractometer with Cu Kα radiation and a scanning

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speed of 3º/min. The accelerating voltage and emission current were 40 kV and 40 mA, respectively. The microcrystalline structure and surface characteristics of the photocatalysts were also investigated by using (JEOL JSM-6610LV) scanning electron microscope. Transmission electron microscopy and high-resolution transmission electron microscopy (HR-TEM) images were performed with a JEOL-2010 transmission electron microscope, using an accelerating voltage of 200 kV. UV-vis diffuse reflectance spectroscopy measurements were carried out using a Hitachi UV-365 spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 350 to 550 nm, and BaSO4 was used as a reflectance

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standard. The Brunauer–Emmett–Teller (BET) surface areas were measured using a Micromeritics ASAP 2020 N2–physisorption method at 77 K. Photoluminescence emission spectra were recorded on a JASCO FP-6500 type fluorescence

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spectrophotometer over a wavelength range of 360-500nm. ESR signals of

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spin-trapped paramagnetic species with 5,5-dimethyl-l-pyrroline N-oxide (DMPO)

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were recorded with a Bruker A300E spectrometer. 2.5 Analysis

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The maximum absorption wavelengths of MB and RhB are 664 and 554 nm, respectively. The maximum absorbance of MB and RhB were respectively selected to

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determine the degradation process. The concentration of MB and RhB in solution was determined by spectrophotometer. The photoxidation efficiency of MB and RhB was

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calculated from the following expression:

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η = [( C0−Ct )/C0]×100 %

(1)

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where η is the photocatalytic efficiency; C0 is the concentration of reactant before illumination; Ct is the concentration of reactant after illumination time t. In order to confirm the extent of mineralization, total organic carbon analyses

(TOC) of the samples were carried out by a Shimadzu TOC-VCPH total organic carbon

analyzer.

3. Results and discussion 3.1 Characterization of Bi2O3 (wt.%)/C3N4 photocatalysts 3.1.1 XRD analysis

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Fig. 1 shows the XRD patterns of g-C3N4, Bi2O3 and Bi2O3/g-C3N4 with different amounts of Bi2O3. It is obvious that the diffraction peaks at 2θ=27.8° and 13.3° can be ascribed to (002) and (100) planes of g-C3N4 (JCPDF 87-1526). The former, which

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corresponds to the interlayer distance of 0.325nm, is attributed to the long-range

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interplanar stacking of aromatic systems; the latter with a much weaker intensity,

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which corresponds to a distance d=0.676nm, is related to an in-plane structural packing motif. The two diffraction peaks are in good agreement with the reported

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results of g-C3N4[26, 27, 30, 32]. The diffraction peaks of Bi2O3 are identical with the reported data of a monoclinic phase in the JCPDS (65-2366). From Fig 1, it can be

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seen that, when the amount of Bi2O3 is lower than 1.0 wt.%, the peaks corresponding to monoclinic Bi2O3 is not found. It demonstrates that the Bi2O3 powders are evenly

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dispersed on the surface of g-C3N4 particles. And with the content of Bi2O3 increasing from 1.0 to 5.0 wt.%, the diffraction peaks of Bi2O3 increase remarkably and the

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diffraction peaks of g-C3N4 decrease gradually. Furthermore, no other new crystal phases in the ball milling process are found.

Fig. 1 XRD patterns of g-C3N4, Bi2O3 and Bi2O3(wt.%)/g-C3N4 photocatalysts

3.1.2 UV-vis analysis It is known that the optical absorption of a semiconductor is closely related to its electronic structure. The UV-vis diffuse reflectance spectra of g-C3N4, Bi2O3 and Bi2O3(wt.%)/g-C3N4 samples are shown in Fig.2(a). It can be seen that the steep

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absorption edge of Bi2O3 is about 442 nm, and the pure g-C3N4 shows absorption wavelengths from the UV to the visible range up to 460 nm, which can be assigned to the intrinsic band gap of g-C3N4 (2.7 eV) [26, 28, 31]. Compared with pure g-C3N4,

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the absorption wavelength regions of the Bi2O3(wt.%)/g-C3N4 samples exhibit

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significant blue shift. It may be attributed to the interaction between the g-C3N4 and

2(a)

UV–vis

diffuse

reflectance

spectra

of

Bi2O3,

g-C3N4

and

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Fig.

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Bi2O3 and smaller particle size of Bi2O3 in the composite samples [43, 46].

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Bi2O3(wt.%)/C3N4 samples

It is known that the band gap energy of the photocatalysts can be calculated by

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the following equation [45, 47]:

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αhv = A(hv - Eg)n/2

(2)

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In this equation, α, h, v, A, and Eg are absorption coefficient, Planck constant,

light frequency, proportionality and band gap energy, respectively; n keys the properties of the transition in a semiconductor (n = 1 for direct transition and n =4 for indirect transition). The values of n for g-C3N4 and Bi2O3 are 4 and 1, respectively. By

applying this equation, the band gap of g-C3N4 and Bi2O3 is 2.7 eV and 2.8 eV, respectively, which agrees with the previous reports [26, 28, 37, 40]. The result is shown in Fig.2 (b).

Fig. 2(b) Band gap energies of g-C3N4 and Bi2O3

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The band positions of the photocatalyst can be calculated by the following empirical formulas [45, 47]: (3)

ECB = EVB – Eg

(4)

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EVB = X + 0.5Eg – Ee

cr

where EVB is the valence band potential; ECB is the conduction band potential; X is the absolute electronegativity of the semiconductor; Ee is the energy of free electrons on

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the hydrogen scale (Ee = 4.5 eV). The X values for g-C3N4 and Bi2O3 are 4.73 and

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6.23 eV, respectively. From the calculation, it is known that the ECB of g-C3N4 and Bi2O3 are about -1.12 and 0.33 eV, respectively. The EVB of g-C3N4 and Bi2O3 are

3.1.3 SEM analysis

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reported results [26,30, 37, 39].

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estimated to be 1.58 and 3.13 eV, respectively. The result is in accordance with the

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SEM was used to investigate the morphology of the photocatalysts. Fig.3 shows

the SEM photographs of pure Bi2O3, g-C3N4 and Bi2O3(1.0wt.%)/C3N4, respectively.

It can be seen that the appearance of the Bi2O3 in Fig.3a is a regular spheroidal structure with mean size of about 0.5-2 µm. And from Fig.3b, the pure g-C3N4 displays aggregated morphologies, which are comprised of block-based flakiness and particles. For the Bi2O3 (1.0wt.%)/g-C3N4 sample, the morphology shows many aggregated flakiness. It indicates that Bi2O3 particles have been deposited on the surface of g-C3N4, and formed the composite structures. In addition, the selected energy dispersive spectrum (EDS) of the composite is shown in Fig. 3(d). It is

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obvious that the sample is composed of C, N, Bi and O elements.

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Fig. 3 SEM images of Bi2O3 (a), g-C3N4 (b), Bi2O3(1.0wt.%)/C3N4 (c) samples and

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EDS spectrum (d)

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3.1.4 TEM analysis

In order to investigate the interface of the sample, the Bi2O3(1.0wt.%)/g-C3N4

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photocatalyst that performed the best photocatalytic activity was chosen for TEM and HRTEM characterization. Fig.4 gives an overview of the typical TEM image of the

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Bi2O3(1.0wt.%)/g-C3N4 photocatalyst. It can be seen that Bi2O3 particles were uniform deposited on the surface of g-C3N4. The particle size is about 20-50 nm. HR-TEM

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shows the existence of the heterojunction between g-C3N4 and Bi2O3. It is obvious that g-C3N4 and Bi2O3 display the different orientations and lattice spacing. Two

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different lattice fringes were clearly observed. One fringe with d=0.326 nm matches the (120) crystallographic plane of Bi2O3, and the other with d= 0.325nm is attributed

to the (002) crystallographic plane of g-C3N4 (JCPDS 87-1526). In a word, the obvious interface between g-C3N4 and Bi2O3 was formed, which is favorable for the transport of photoexcited carriers.

Fig. 4 TEM and HR-TEM images of Bi2O3(1.0wt.%)/C3N4 sample

3.1.5 BET surface area analysis

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The

nitrogen

adsorption-desorption

isotherms

of

g-C3N4

and

Bi2O3(5.0wt.%)/g-C3N4 samples are presented in Fig. 5. Obviously, the samples exhibit type IV isotherms with type H3 hysteresis loop according to the IUPAC

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classification, indicating that the samples have a mesoporous structure [18, 44, 47].

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The corresponding pore size distributions of the samples are shown in the inset of Fig.

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5, which further indicates the presence of mesoporous. The pore size of mesoporous is not uniform ranging from 5 to 60 nm. The BET surface areas of the g-C3N4 and

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Bi2O3(5.0wt.%)/g-C3N4 are 10.12 and 7.77 m2/g, respectively. It is clear that the BET surface areas of the samples are slightly decreased with the increase in the amount of

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Bi2O3. It may be attributed to different pore size distributions of the samples.

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Fig.5 Nitrogen adsorption-desorption isotherms and corresponding pore size

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distribution curves (inset) of g-C3N4 and Bi2O3(5.0wt.%)/g-C3N4samples

3.2 Evaluation of photocatalytic activity In order to investigate the photocatalytic activity of g-C3N4 and Bi2O3

(wt.%)/C3N4 samples, MB and RhB were selected as probe molecules of organic pollutants, because it is a common contaminant in industrial wastewater. Fixed illumination time is 60 min. Fig. 6 shows the photocatalytic activity of the samples under visible light illumination. It is clear that the photocatalytic efficiencies of g-C3N4 for MB and RhB are 35.6% and 11.1%, respectively. The photocatalytic

activity of Bi2O3(wt.%)/C3N4 increases with the increase in the content of Bi2O3 up to

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1.0 %. When the content of Bi2O3 is greater than 1.0 %, the photocatalytic activity of the sample is decreased gradually. It is obvious that the amount of Bi2O3 has an important effect on the photocatalytic activity of the samples. Although the addition

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of Bi2O3 may enhance the photocatalytic activity, the extents of increased

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photocatalytic activity are different. The reason may be that when the amount of

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Bi2O3 is higher than 1%, the surface of g-C3N4 is covered by Bi2O3 powders, and thus hinders light absorption of g-C3N4 and decreases the generated rate of the

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photoexcited charge carriers. Under the experimental conditions, the optimum amount

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of Bi2O3 in the Bi2O3(wt.%)/C3N4 sample is 1.0 %.

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Fig. 6 Effect of the amount of Bi2O3 on the photocatalytic activity of the samples

Fig.7a and b show the relationship between the concentrations of MB and RhB

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and the illumination time. It is clear that the photocatalytic activity decreases as follows: Bi2O3(1.0wt.%)/C3N4 > Bi2O3(3.0wt.%)/C3N4 > Bi2O3(5.0wt.%)/C3N4 >

Bi2O3(0.5wt.%)/C3N4 > Bi2O3(0.1wt.%)/C3N4

> pure g-C3N4. The degradation

process of MB and Rh follows first-order kinetics equation. Fig. 8a and b show the first-order kinetics of MB and Rh degradation with the different samples. According to the first-order kinetics, the rate constants of Bi2O3 (1.0wt.%)/C3N4 and g-C3N4 are

calculated to be 0.0253 min-1 and 0.0074 min-1 for MB, respectively. And for RhB, the rate constants of Bi2O3 (1.0wt.%)/C3N4 and g-C3N4 are 0.0101 min-1 and 0.002 min-1. It is clear that the rate constants of Bi2O3(1.0wt.%)/C3N4 for MB and RhB are

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about 3.42 times and 5.05 times that of pure g-C3N4.

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Fig.7(a) Concentration change of MB as the function of the illumination time

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Fig.7(b) Concentration change of RhB as the function of the illumination time

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Fig. 8(a) The first-order kinetics of MB photocatalytic degradation

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Fig. 8(b) The first-order kinetics of RhB photocatalytic degradation

UV-Vis spectra and TOC values of the samples are shown in Fig. 9. It can be seen

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that compared with the UV-Vis spectra of MB and RhB before irradiation, the absorption intensities of the samples are decreased slightly after 60 min irradiation

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without catalyst. However, the absorption intensities are decreased rapidly after 60 min irradiation with Bi2O3(1.0wt.%)/C3N4 catalyst. The photoxidation efficiency of

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MB and RhB are 78.1% and 45.6%, respectively. That is, the effective degradation of MB and RhB can be achieved only in the coexisting conditions of illumination and catalyst. From the TOC values of the samples, it is obvious that the conclusion is consistent with that of the photoxidation efficiency. The TOC of MB and RhB decreased by about 63.8 % and 35.9 % respectively with Bi2O3(1.0wt.%)/C3N4 catalyst after irradiation for 60 min. It is clear that the reduction rate of TOC is slower than that of MB and RhB degradation. That is, the mineralization rates of MB and RhB are lower than that of decoloration.

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Fig. 9 UV-Vis spectra and TOC values of samples with the different conditions a, d : UV-Vis spectra and corresponding TOC values of MB and RhB before irradiation.

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b, e : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min irradiation without photocatalyst.

c, f : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min

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irradiation with photocatalyst.

From the BET surface areas of the g-C3N4 and Bi2O3(5.0wt.%)/g-C3N4, it is

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obvious that the increased photocatalytic activity of Bi2O3(wt.%)/g-C3N4 sample

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cannot be attributed to the BET surface area. Therefore, the enhanced photocatalytic activity of Bi2O3(wt.%)/g-C3N4 sample can only ascribed to the presence of Bi2O3.

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The catalyst’s lifetime is an important parameter of the photocatalytic process, so

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it is essential to evaluate the stability of the catalyst for practical application. The

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Bi2O3(1.0wt.%)/C3N4 photocatalyst was selected as a sample to ascertain the stability. In the cyclic experiment, the recovered photocatalyst was centrifuged and dried at 80 ºC for 2 h. Then the photocatalyst was weighed again and the lost portion (fresh samples less than 5.0 wt.%) was added so as to be used for the next run. The repetition tests reveal that there is no obvious decrease in the photocatalytic efficiency of RhB after 5 times experiments, which indicates that Bi2O3 (1.0wt.%)/C3N4 has a

good

stability

in

the

photocatalytic

reaction

process.

The

stability

of

Bi2O3(1.0wt.%)/C3N4 was also investigated by XRD patterns of the fresh and used samples. As shown in Fig. 10, the XRD patterns of the fresh and used samples have no obvious change. 16

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Fig.10 XRD patterns of fresh and used 5 times samples (a: fresh, b: used 5 times)

3.3. Proposed photocatalytic mechanism

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3.3.1 Role of the reactive species

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It has been reported that the •O2−, h+ and •OH are the major reactive species for the photocatalytic oxidation. In order to distinguish the roles of the reactive species,

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the scavenger investigation was performed. By means of adding different scavengers to remove the corresponding reactive species, the roles of reactive species in the

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photocatalytic process were ascertained respectively according to the changes of photocatalytic efficiency. In the paper, ammonium oxalate (AO), isopropanol (IPA)

respectively [48-50].

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and benzoquinone (BQ) were selected as the quenchers of h+, •OH and •O2− ,

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Fixed illumination time is 60 min. Fig. 11a shows the photocatalytic

efficiencies of MB and RhB with pure g-C3N4 in the conditions of adding various scavengers. It is known that the photocatalytic efficiencies of MB and RhB are 35.6% and 11.1% without scavengers, respectively. When the AO was added into reaction solution, the photocatalytic efficiency is almost invariable. Therefore, hole is not the major reactive specie. However, when the IPA was added into reaction solution, the degradation efficiencies of MB and RhB decrease to 22.1% and 5.3%, respectively. When adding BQ into reaction solution, the photocatalytic efficiencies of MB and RhB are whittled down into 17.9% and 6.2%, respectively. Based on the results, it is

17

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clear that •O2− and •OH are the major reactive species in the pure g-C3N4 reaction system. For the Bi2O3(1.0wt.%)/g-C3N4 photocatalyst, the effects of various scavengers

ip t

on the photocatalytic efficiencies of MB and RhB are shown in Fig. 11b. It can be

cr

seen that the degradation efficiencies of MB and RhB are 78.1% and 45.6%

us

respectively when no scavenger was added. When the AO was added into reaction solution, the degradation efficiencies of MB and RhB are decreased to 46.4% and

an

31.4%, respectively. When adding IPA into the reaction solutions, the degradation efficiencies of MB and RhB decrease to 36.4% and 22.5%, respectively. When adding

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BQ into the reactions, the degradation efficiencies of MB and RhB are whittled down into 42.6% and 28.5%, respectively. It is clear that h+, •OH and •O2− are the major

te

d

reactive species for the Bi2O3 (1.0wt.%)/g-C3N4 photocatalyst. It is obvious in Fig. 11c that the photocatalytic efficiencies of MB and RhB for

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pure Bi2O3 are 25.7% and 9.8% without scavengers, respectively. When the BQ was added into reaction solution, the photocatalytic efficiency is not obviously changed. When AO was added into reaction solution, the photocatalytic efficiencies of MB and RhB decrease to 10.5% and 3.3%. When adding IPA into the reaction solution, the degradation efficiencies of MB and RhB are decreased to 12.9% and 4.1%, respectively. Obviously, the major reactive species for the Bi2O3 are h+ and •OH. Fig.11. Effects of a series of scavengers on the degradation efficiency (a) g-C3N4, (b) Bi2O3(1.0wt.%)/g-C3N4, (c) Bi2O3 (The dosage of scavengers = 0.1mmol/L, Illumination time t = 60min)

18

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It is known that the photocatalytic oxidation and reduction abilities of semiconductors are determined by the potentials of their valence and conduction

ip t

bands, respectively. In theory, the photoexcited electrons in the CB of Bi2O3 can not

cr

reduce O2 to give •O2− and •HO2 because the CB edge potential of Bi2O3 (0.33 eV vs.

NHE) is more positive than the standard redox potentials of EΘ(O2/•O2−) (−0.33 eV vs.

us

NHE) and EΘ(O2/•HO2) (−0.05 eV vs. NHE) [45, 48, 49]. However, the photoexcited

an

holes in the VB of Bi2O3 can oxidize OH to give •OH due to the VB potential of Bi2O3 is more positive than the standard redox potential of OH/•OH (Eo(OH/•OH) =

M

2.4eV) [42]. For the g-C3N4 photocatalyst, because the CB edge potential of g-C3N4 (-1.12 eV vs. NHE) is more negative than the standard redox potentials of

te

d

EΘ(O2/•O2−), the photoexcited electrons can reduce O2 to give •O2−. Meanwhile, owing to the VB potential of g-C3N4 is lower than the standard redox potential of

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OH/•OH, the photoexcited holes in the VB of g-C3N4 can not oxidize OH to give •OH. However, the result shows that the •OH can be produced in the reaction system of g-C3N4 photocatalyst. It is suggested that the •OH may be produced from the •O2−,

which is another source of •OH[45, 48, 49]. e- + O2 → •O2−

(5)

•O2− + e- + 2H+ → H2O2

(6)

•O2− + H2O2 → •OH +OH- + O2

(7)

H2O2 → 2 •OH

(8)

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3.3.2 Transport process of photoexcited carriers It is known that when Bi2O3 and g-C3N4 are combined with each other, the heterojunction photocatalyst Bi2O3/g-C3N4 will be formed. According to the band gap

ip t

structures of Bi2O3 and g-C3N4, the separation processes of photoexcited electron-hole

cr

could be expressed in Fig. 12(a) and Fig. 12(b), respectively. If the charge carriers of

us

Bi2O3/g-C3N4 transfer according to Fig.12(a), which is the common electron-hole separation process for a great number of composite photocatalysts, then the electrons

an

in the CB of g-C3N4 will migrate to the CB of Bi2O3, and holes in the VB of Bi2O3 will migrate to the VB of g-C3N4. As a result, these accumulated electrons in the CB

M

of Bi2O3 can not reduce O2 to yield •O2− and •HO2, and the holes in the VB of g-C3N4 can not oxidize OH to give •OH. From the above analysis, if the charge carriers

te

d

transfer in accordance with the traditional model, which is not favorable for the formation of active species. However, under the experimental conditions, the results

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show that •O2−, •OH and h+ are the major reactive species in the Bi2O3/g-C3N4 photocatalytic system which has high photocatalytic activity, so the separation process of the photoexcited electron-hole pairs should not follow Fig.12(a), but Fig.12(b). Namely, the fast combination between the photoexcited electrons in the CB of Bi2O3 and photoexcited holes in the VB of g-C3N4 was achieved. At the same time,

accumulation of electrons in the CB of g-C3N4 have more negative potential to reduce the molecular oxygen to yield •O2−, and holes in the VB of Bi2O3 have more positive potential to generate abundant active •OH radicals. Therefore, the photocatalytic activity of the Bi2O3/g-C3N4 system was significantly increased. Based on the results,

20

Page 20 of 52

it is proposed that the transport process of photoexcited carriers follows Fig.12(b).

ip t

And the Bi2O3/g-C3N4 is a typical Z-scheme photocatalyst.

cr

Fig.12. Schematic diagram of photoexcited electron-hole separation process

us

3.3.3 Evidence of the mechanism 3.3.3.1 Experiments of PL-TA and ESR

an

Hydroxyl radicals (•OH) generated on the photocatalyst surface can be detected by the fluorescence spectrometer [45, 48, 49]. Terephthalic acid (TA) readily reacts

M

with •OH to produce a highly fluorescent product, i.e. 2-hydroxyterephthalic acid, whose PL peak intensity is in proportion to the amount of •OH radicals formed in

te

d

water. The experimental procedures were reported in the earlier reports[45, 46]. The fixed illumination time is 60 min. Fig. 13 shows the PL spectra of the photocatalysts.

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From Fig. 13, it can be seen that the PL intensity of the Bi2O3(1.0wt.%)/g-C3N4

photocatalyst exhibits the strongest fluorescence intensity, suggesting the formation rate of •OH radical on the surface of Bi2O3(1.0wt.%)/g-C3N4 is the highest. However,

the generation rate of •OH radicals on the g-C3N4 surface is the lowest. The intensity decreases as follows: Bi2O3(1.0wt.%)/g-C3N4＞Bi2O3＞g-C3N4.

Fig.13 The PL intensity of samples in TA solution under visible light irradiation (a) Bi2O3(1.0wt.%)/g-C3N4, (b) Bi2O3, (c) g-C3N4, (d) blank

21

Page 21 of 52

The production of •O2− radicals in the reaction system can be detected by the ESR technique [45, 48]. The result is shown in Fig.14. It is clear that the four characteristic peaks of DMPO-•O2− adducts for g-C3N4 and Bi2O3(1.0wt.%)/g-C3N4

ip t

samples are observed. It is obvious that •O2− radicals were generated on the surface of

cr

two samples after irradiation. Furthermore, the intensity of the peaks of

us

Bi2O3(1.0wt.%)/g-C3N4 is stronger than that of g-C3N4. So the amount of •O2− radicals generated on the surface of Bi2O3(1.0wt.%)/g-C3N4 is more than that of g-C3N4.

an

However, for the Bi2O3, there is no characteristic peak of DMPO-•O2−. It means that

M

few •O2− radicals were generated on the sample.

dispersion

d

Fig. 14 ESR signals of the DMPO-•O2−with irradiation for 20 s in methanol

te

In summary, •O2− and •OH radicals were produced on pure g-C3N4, while only

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•OH radicals were generated on pure Bi2O3. When g-C3N4 is combined with Bi2O3,

•O2− and •OH radicals are produced on the Bi2O3/g-C3N4 surface. Meanwhile, the amount of •O2− and •OH radicals produced on the surface of Bi2O3/g-C3N4 is the

highest under the experimental conditions. The results further prove that the transfer process of the photogenerated electron-hole pairs for the Bi2O3/g-C3N4 photocatalyst

follows Fig.12(b).

3.3.3.2 Photoluminescence emission spectra The PL emission spectra were employed to investigate the combination and

22

Page 22 of 52

separation of the photoinduced carriers [45, 48, 49]. The intensity of PL emission spectra may indicate the recombination rate of photoexcited electron-hole pairs. The stronger the PL intensity is, the faster the photoexcited electron-hole combine.

ip t

Generally, a higher PL intensity indicates a higher recombination rate and a lower PL

cr

intensity expresses a lower recombination rate.

us

Fig. 15 shows the PL spectra of the Bi2O3(wt.%)/g-C3N4 with wt.% of 0.1 , 0.5, 1.0, 3.0, 5.0 and g-C3N4, respectively. It is clear that the PL spectra have a strong

an

emission peak at around 450 nm, which could be related to the recombination of the photoexcited electron-hole of g-C3N4. From Fig.15, it can be seen that the PL intensity

M

of the Bi2O3(1.0wt.%)/g-C3N4 photocatalyst exhibits the strongest emission, suggesting that the recombination of the photoexcited electron-hole on the

te

d

photocatalyst surface is the highest. The g-C3N4 shows the weakest emission, suggesting that the recombination of the photoexcited electron-hole is the lowest. The

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PL intensity of the samples are decreased as follows: Bi2O3(1.0wt.%)/g-C3N4 ＞ Bi2O3(0.5wt.%)/g-C3N4 ＞ Bi2O3(3.0wt.%)/g-C3N4 ＞ Bi2O3(5.0wt.%)/g-C3N4 ＞ Bi2O3(0.1wt.%)/g-C3N4 ＞ g-C3N4 . It is obvious that the above results are also

consistent with the results of photocatalytic activity of the samples. It means that a higher PL intensity indicates a higher photocatalytic activity under the experimental conditions. In theory, if the photoexcited electrons and holes are transferred as Fig.12a, the PL intensities of the Bi2O3/g-C3N4 photocatalysts must be lower than that of pure g-C3N4. However, the results show that with the increasing amount of Bi2O3 from 0.1

23

Page 23 of 52

to 5.0 %, the PL intensity of the Bi2O3/g-C3N4 is much higher than that of pure g-C3N4. It means that the transfer of photoexcited electrons and holes should not follow Fig.12(a) but Fig.12(b). In Fig.12(b), the higher PL intensity of the samples are

ip t

attributed to the higher recombination rate between the photoexcited electrons in the

cr

CB of Bi2O3 and the photoexcited holes in the VB of g-C3N4. The accumulated

us

photoexcited electrons in the CB of C3N4 can reduce O2 to yield •O2− and photoexcited holes in the VB of Bi2O3 can oxidize OH to give •OH. Based on the

an

analysis, it is concluded that the Bi2O3/g-C3N4 system is a typical Z-scheme photocatalyst. The PL spectra further demonstrate that the transport of photoexcited

M

carriers of the Bi2O3/g-C3N4 photocatalyst follows the Fig.12(b).

Ac ce p

samples

te

d

Fig.15. Photoluminescence emission spectra of g-C3N4 and Bi2O3 (wt.%)/g-C3N4

4. Conclusions

A direct Z-scheme photocatalyst Bi2O3 (wt.%)/g-C3N4 was fabricated via ball

milling and heat treatment methods. The Bi2O3/g-C3N4 exhibited a much higher

photocatalytic activity than single-phase C3N4 under visible light illumination. The rate constants of MB and RhB degradation for Bi2O3(1.0wt.%)/C3N4 are 3.4 times and 5 times that of pure g-C3N4. The Bi2O3/g-C3N4 is a direct Z-scheme photocatalyst,

which was testified by PL technique, ESR technology and scavengers method. The increased photocatalytic activity is attributed to the quick combination between the

24

Page 24 of 52

photoexcited holes of g-C3N4 and photoexcited electrons of Bi2O3. The accumulated photoexcited electrons in the CB of g-C3N4 with high reduction ability and photoexcited holes in the VB of Bi2O3 with high oxidation ability participate in redox

ip t

reaction, respectively. •O2−, •OH and h+ are the major active species in the

us

cr

Bi2O3/g-C3N4 photocatalytic reaction system.

Acknowledgements

an

This study was supported by the Natural Science Foundation of China (NSFC,

M

grant No. 20973071, 51172086, 51272081 and 21103060).

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d

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Fig. 1 XRD patterns of g-C3N4, Bi2O3 and Bi2O3(wt.%)/g-C3N4 photocatalysts

Fig.

2(a)

UV–vis

diffuse

reflectance

spectra

of

Bi2O3,

g-C3N4

and

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Bi2O3(wt.%)/C3N4 samples

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Fig. 2(b) Band gap energies of g-C3N4 and Bi2O3

Fig. 3 SEM images of Bi2O3 (a), g-C3N4 (b), Bi2O3(1.0wt.%)/C3N4 (c) samples and

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EDS spectrum (d)

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Fig. 4 TEM and HR-TEM images of Bi2O3(1.0wt.%)/C3N4 sample

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Fig. 5 Nitrogen adsorption-desorption isotherms and corresponding pore size

d

distribution curves (inset) of g-C3N4 and Bi2O3(5.0wt.%)/g-C3N4samples

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Fig. 6 Effect of the amount of Bi2O3 on the photocatalytic activity of the samples

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Fig.7(a) Concentration change of MB as the function of the illumination time

Fig.7(b) Concentration change of RhB as the function of the illumination time

Fig. 8(a) The first-order kinetics of MB photocatalytic degradation

Fig. 8(b) The first-order kinetics of RhB photocatalytic degradation

Fig. 9 UV-Vis spectra and TOC values of samples with the different conditions a, d : UV-Vis spectra and corresponding TOC values of MB and RhB before irradiation. b, e : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min 33

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irradiation without photocatalyst. c, f : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min irradiation with photocatalyst.

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Fig.10 XRD patterns of fresh and used 5 times samples (a: fresh, b: used 5 times)

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Fig.11 Effects of a series of scavengers on the degradation efficiency (a) g-C3N4, (b) Bi2O3(1.0wt.%)/g-C3N4, (c) Bi2O3

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(The dosage of scavengers = 0.1mmol/L, Illumination time t = 60min)

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Fig.12 Schematic diagram of photoexcited electron-hole separation process

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Fig.13 The PL intensity of samples in TA solution under visible light irradiation (a) Bi2O3(1.0wt.%)/g-C3N4, (b) Bi2O3, (c) g-C3N4, (d) blank

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Ac ce p

dispersion

d

Fig.14 ESR signals of the DMPO-•O2−with irradiation for 20 s in methanol

Fig.15 Photoluminescence emission spectra of g-C3N4 and Bi2O3 (wt.%)/g-C3N4 samples

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d

Fig. 1 XRD patterns of g-C3N4, Bi2O3 and Bi2O3(wt.%)/g-C3N4 photocatalysts

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UV–vis

diffuse

reflectance

spectra

of

Bi2O3,

g-C3N4

and

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Bi2O3(wt.%)/C3N4 samples

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Fig.

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Ac ce p

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d

Fig. 2(b) Band gap energies of g-C3N4 and Bi2O3

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Fig. 3 SEM images of Bi2O3 (a), g-C3N4 (b), Bi2O3(1.0wt.%)/C3N4 (c) samples and

EDS spectrum (d)

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TEM and HR-TEM images of Bi2O3(1.0wt.%)/C3N4 sample

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d

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Fig. 4

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ip t cr us an M d te Ac ce p Fig. 5 Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves (inset) of g-C3N4 and Bi2O3(5.0wt.%)/g-C3N4samples

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Ac ce p

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d

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Fig. 6 Effect of the amount of Bi2O3 on the photocatalytic activity of the samples

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Ac ce p

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d

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Fig.7(a) Concentration change of MB as the function of the illumination time

Fig.7(b) Concentration change of RhB as the function of the illumination time

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ip t cr us an

Ac ce p

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d

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Fig. 8(a) The first-order kinetics of MB photocatalytic degradation

Fig. 8(b) The first-order kinetics of RhB photocatalytic degradation

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Fig. 9 UV-Vis spectra and TOC values of samples with the different conditions

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irradiation.

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a, d : UV-Vis spectra and corresponding TOC values of MB and RhB before

b, e : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min irradiation without photocatalyst. c, f : UV-Vis spectra and corresponding TOC values of MB and RhB after 60 min irradiation with photocatalyst.

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Ac ce p

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times)

d

Fig. 10 XRD patterns of fresh and used 5 times samples (a: fresh, b: used 5

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Fig.11 Effects of a series of scavengers on the degradation efficiency

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(a) g-C3N4, (b) Bi2O3(1.0wt.%)/g-C3N4, (c) Bi2O3

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d

(The dosage of scavengers = 0.1mmol/L, Illumination time t = 60min)

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Ac ce p

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d

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Fig.12 Schematic diagram of photoexcited electron-hole separation process

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Fig.13 The PL intensity of samples in TA solution under visible light irradiation

Ac ce p

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d

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(a) Bi2O3(1.0wt.%)/g-C3N4, (b) Bi2O3, (c) g-C3N4, (d) blank

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Fig. 14 ESR signals of the DMPO-•O2−with irradiation for 20 s in methanol

Ac ce p

te

d

dispersion

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Fig.15 Photoluminescence emission spectra of g-C3N4 and Bi2O3 (wt.%)/g-C3N4

Ac ce p

te

d

samples

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Graphical abstract Design of a direct Z-scheme photocatalyst: preparation and characterization of Bi2O3/g-C3N4 with high visible light activity Yingfei Hub

Xiaoliang Jiangb

Department of Chemistry, Anhui Science and Technology University, Anhui

Fengyang, 233100, People’s Republic of China

Department of Chemistry, Huaibei Normal University, Anhui Huaibei, 235000,

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b

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a

Xianliang Fu b

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Sugang Mengb

Shifu Chena,b*

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Jinfeng Zhanga,b

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People’s Republic of China

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◆A direct Z-scheme photocatalyst Bi2O3/g-C3N4 was prepared.

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Highlights

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◆The photocatalyst exhibited a much higher activity than pure g-C3N4. ◆Bi2O3/g-C3N4 as a typical Z-scheme photocatalyst was proved.

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◆e- in the CB of Bi2O3 and h+ in the VB of g-C3N4 are quickly combined.

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d

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◆e- in the CB of g-C3N4 and h+ in the VB of Bi2O3 participate in redox reaction.

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