α-Bi2O3 with enhanced photocatalytic performance and adsorptive ability

Accepted Manuscript Title: Simple thermal decomposition of bismuth citrate to Bi/C/␣-Bi2 O3 with enhanced photocatalytic performance and adsorptive ability Authors: Yuanyuan Ma, Qiaofeng Han, Te-Wei Chiu, Xin Wang, Junwu Zhu PII: DOI: Reference:

S0920-5861(18)31387-7 https://doi.org/10.1016/j.cattod.2018.10.005 CATTOD 11668

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

8-8-2018 30-9-2018 3-10-2018

Please cite this article as: Ma Y, Han Q, Chiu T-Wei, Wang X, Zhu J, Simple thermal decomposition of bismuth citrate to Bi/C/␣-Bi2 O3 with enhanced photocatalytic performance and adsorptive ability, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.10.005 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.

Simple thermal decomposition of bismuth citrate to Bi/C/α-Bi2O3 with enhanced photocatalytic performance and adsorptive ability Yuanyuan Ma,1 Qiaofeng Han,1,* Te-Wei Chiu,2,* Xin Wang,1 Junwu

Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education,

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Zhu1

Nanjing University of Science and Technology, Nanjing 210094, China. 2

Department of Materials and Mineral Resources Engineering, National Taipei

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University of Technology, 1, Sec.3, Zhongxiao E. Rd., Taipei, 106 Taiwan.

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*Corresponding authors. E-mail: [email protected] (Q. Han); [email protected] (T. Qiu).

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Graphical abstract:

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C/Bi/α-Bi2O3 was obtained by calcining bismuth citrate in N2, which exhibited

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superior visible light photocatalytic activity and adsorption ability with help of trace

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amount of EDTA-2Na.

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Highlights：

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 C/Bi/α-Bi2O3 was easily synthesized by calcining bismuth citrate in N2.

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 The remaining carbon and metal Bi could enlarge light response.  The pyrophorization of carbon in air induce low-temperature

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information of α-Bi2O3.

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 C/Bi/α-Bi2O3 exhibited better photocatalytic activity than α-Bi2O3

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and β-Bi2O3.

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 The as-prepared Bi2O3 exhibited enhanced adsorption for MG with

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help of EDTA-2Na.

Abstract: Exploring a simple method to prepare efficient photocatalyst is very

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important technology for practical application. Herein, α-Bi2O3, β-Bi2O3 and C/Bi/α-Bi2O3 were easily obtained by calcining bismuth citrate under different conditions. Especially, C/Bi/α-Bi2O3 obtained at 450 °C in nitrogen atmosphere exhibited superior photocatalytic activity for methyl orange (MO) and malachite 2

green (MG) degradation under visible light irradiation, higher than β-Bi2O3 and α-Bi2O3

calcined in air

at 350 °C and above 450 °C, respectively. The presence of

non-burned carbon in N2 atmosphere benefits to complete transformation of β-Bi2O3 into α-Bi2O3 at a annealing temperature low to 300 °C, as well as partial reduction of remaining carbon and metal Bi

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Bi2O3 into Bi nanoparticles. More importantly, the

could enhance visible light absorption of α-Bi2O3, generate hot charge carriers and

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boost electron migration efficiency, which thus dramatically improve photocatalytic activity of α-Bi2O3. Furthermore, these bismuth oxides exhibit exceptionally strong

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adsorption ability for cationic dyes like MG and methyl violet (MV) with help of trace

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amount of ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) and the

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maximum adsorption capacity for MG on α-Bi2O3 reached 312 mg/g. The as-prepared

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C/Bi/α-Bi2O3 also possesses good stability and recyclibility in photocatalysis and

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adsorption.

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Keywords: Bismuth citrate; Thermal decomposition; C/Bi/Bi2O3 composite; Visible light photocatalysis; Adsorption

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1. Introduction

Among the widely investigated bismuth-based photocatalysts, Bi2O3 attracts more

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attention due to simple composition, nontoxicity, thermal stability and earth abundance. Bi2O3 has four main polymorphs: α-monoclinic, β-tetragonal, γ-body centered cubic and δ-face centered cubic, each with different physicochemical properties [1]. For example, α-Bi2O3, with a band gap energy (Eg) of 2.7-2.9 eV, is thermodynamically most stable, and while, β-Bi2O3 is defined as most active due to 3

low Eg (~2.5 eV) and good visible light response. Bi2O3 has been proved to be a photocatalyst for water splitting and pollutant decomposing under solar light irradiation. Nevertheless, the photocatalytic activity of

pure Bi2O3 is far from

practical application due to low quantum yields. Many strategies have been developed

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to modify their photocatalytic performance. On the one hand, since the photocatalytic activity of a semiconducting material is greatly dependent on crystallinity,

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morphology, dimensions, exposed facets and so on, a great deal of effort has been

paid on controllable synthesis of Bi2O3 with various microstructures. For example,

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four polymorphs of Bi2O3 in various morphologies, including needles, nanowires,

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nanoflowers, nanotubes and so on, have been prepared by various methods and the

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structure-dependent photocatalytic activity have been investigated [1-19]. On the

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other hand, it is an effective strategy to improve light absorption, tune band structures

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and thus enhance the photocatalytic activity of a semiconductor by constructing a composite or heterojunction and doping metal or nonmetal elements. For example,

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α/β-Bi2O3 and Bi2O4-x/Bi2O3 [20-23], Bi2S3/Bi2O3 and Bi2S3/Bi2O3/MoS2 [24,25],

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Bi2O3/g-C3N4 heterojunctions [26,27], and Bi, Cu or rare earth metal deposited or doped Bi2O3 [28-31] exhibited enhanced photocatalytic activity for NO removal and organic pollutants degradation. Recently, noble metals like Ag and Au have been

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reported to be able to improve photocatalytic activity of

Bi2O3 due to surface

plasmon resonance (SPR) effect [32,33]. Besides these noble metals, semimetal bismuth (Bi), a low price and nontoxic metal, was found to show a similar SPR effect to noble metals when the particle size was decreased to smaller than tens of 4

nanometers [34-36]. Moreover, as compared to the deposition method used for the preparation of noble metal coupled photocatalysts, Bi nanoparticles could be generated from Bi-based compounds matrix via in situ reduction, which has more advantages such as no Bi nanoparticles agglomeration, lattice matching and intimate

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contact at the interface between Bi and Bi-based semiconductors, thus benefiting the transfer of the charge carriers and enhancement of the photocatalytic activity.

solid

state

reaction

route

is

considered

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Nevertheless, most of the reduction reaction was carried out in solution. Relatively, time-saving,

and

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environmental-friendly [37].

cost-effective

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Furthermore, the promising carbonaceous materials have demonstrated good light

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absorption and electrical conductivity, which may facilitate the generation and transformation of charge carriers and

hamper the recombination of electron-hole

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pairs . Recently, Zhu et al. reported one-pot synthesis of C/Bi/Bi2O3 composite by

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calcining the precursor in air with high photocatalytic activity for 2,4-dichlorophenol degradation [38]. However, the precursor was synthesized via a complicated route

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from the reaction between ethylene diamine teraacetic acid (EDTA) and Bi(NO3)3·5H2O in nitric acid and ammonium hydroxide solution. Jaroniec et al.

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reported superior photocatalytic activity of Bi/δ-Bi2O3@C synthesized by mixing resorcinol, F127 and formaldehyde with Bi(NO3)3·5H2O to form a gel and then subjecting a two-stage thermal treatment in nitrogen [39]. The improvement of photocatalytic activity as compared to bare Bi2O3 could be attributed to the synergistic effects of porous carbon and metal Bi. Motivated by these, herein, we developed a 5

very simple method to synthesize C/Bi/α-Bi2O3 composite by the thermal decomposition of commercially purchased bismuth citrate in N2 atmosphere, and no preparation of the precursor was in need. Most importantly, the obtained C/Bi/α-Bi2O3 composite exhibited much better photoreactivity for methyl orange (MO) and

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malachite green (MG) degradation than pure α-Bi2O3 and even β-Bi2O3 photocatalysts. Especially, we found that all of the-as prepared Bi2O3 possessed strong adsorption

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ability for MG and methyl violet (MV) with help of a trace amount of ethylene diamine tetraacetic acid disodium salt (EDTA-2Na). To the best of our knowledge,

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EDTA-modified adsorption ability for Bi2O3 has not been reported yet. The

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photocatalytic and adsorptive mechanism was also discussed in detail.

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2.1. Materials and preparation

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2. Experimental section

All of the reagents were of analytical grade and used as received without further

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purification. Typically, C/Bi/α-Bi2O3 was prepared as follows. A certain amount of

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bismuth citrate was ground for about 10 min, and then put into aluminum crucible for calcination. The calcination was conducted in nitrogen atmosphere with a heating rate of 2 °C/min and annealed at certain temperature ranging from 300 °C to 500 °C for 4

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h (denoted as C/Bi/α-Bi2O3-T). After the calcination, the oven was allowed to cool down to room temperature naturally. It is worth noting that all products were pyrophoric for about two minutes when taken out from the oven. The samples were then collected by washing with deionized water and absolute ethanol for several times

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and drying in air. All samples obtained in N2 atmosphere appeared black, indicating the presence of residual carbon due to incomplete calcination process. Pure β-Bi2O3 (orange) and α-Bi2O3 (light yellow) were prepared by calcining in air atmosphere with the annealing temperature of 350°C and 450 °C, respectively, while keeping other

calcined in air. The samples were collected in a similar way.

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2.2. Characterization

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conditions constant. No pyrophoric phenomenon was observed for the products

X-ray diffraction test (XRD) was conducted on a Bruker D8 Advance X-ray

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diffractometer (Cu Kα radiation, λ = 1.542 Å). The morphology and microstructures

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were checked via transmission electron microscope (TEM) (JEM-2100; JEOL). The

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chemical states of the elements were analyzed by X-ray photoelectron spectroscopy

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(XPS) with monochromatized Al Kα as the exciting source (Quantera II SXM; PHI),

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and the binding energies of all elements were calibrated by using the C1s peak of adventitious carbon at 284.80 eV. Nitrogen adsorption-desorption isotherm was

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measured on a Micromeritics ASAP 2010 analyzer. The UV-vis diffuse reflectance

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spectrum (DRS) was obtained on a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere, using BaSO4 as a reference. Total organic carbon (TOC)

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was measured by using Multi N/C 2100 analyzer (Jena). 2.3. Photocatalytic activity, electrochemical and photoelectrochemical property test The photocatalytic performance of samples was evaluated by using MO or MG as a probe under visible light irradiation. Typically, 20 mg of catalyst was dispersed in 50 7

mL of MO (10 mg/L) or MG (10 mg/L) aqueous solution. Before the irradiation, dye adsorption on the catalyst was carried out in dark under continuous stirring to ensure an adsorption-desorption equilibrium. Then, the suspension was illuminated by a xenon lamp of 500 W with a 420 nm cutoff filter under stirring. At a given time

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intervals, about 3 ml of the suspension was sampled, and centrifugated at 8000 rpm for about 10 min to separate the catalyst powders and solution. Dye concentration was

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determined using the characteristic absorption bands of the dyes by using a UV-vis

spectrophotometer. The residual powders in centrifuge tube and aliquot dye solution

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were returned to the photocatalytic reactor in order to preserve the same amount of

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catalyst and solution. The photodegradation efficiency was calculated by (C0-C)/C0 ×

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100%, where C0 and C represent dye concentration at the irradiation time of t = 0 and

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t, respectively.

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Mott-Schottky plot, photocurrent response and electrochemical impedance spectra

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(EIS) were conducted on an electrochemistry workstation (CHI 760E, Shanghai Chenhua Instruments, China) by using a platinum plate and saturated calomel

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electrode (SCE) as the counter electrode and reference electrode, respectively, and Na2SO4 (0.5 M) aqueous solution as the electrolyte. The working electrode was made

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by mixing 20 mg of sample with 2 mL mixture of distilled water/ethanol/Nafion (volume ratio: 20:40:1) and then coating onto a 1 cm × 1 cm F-doped tin oxide (FTO) glass electrode. 2.4. Adsorption test

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The adsorption property of the as-prepared samples with or without EDTA was investigated. Typically, 1 mL EDTA (50 mM) was mixed with 20 mg of α-Bi2O3. Then, 50 mL of MG solution with known various concentrations was separately added to the above mixture. The solutions were stirred at a constant speed of 700 rpm.

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At a given contact time intervals, about 5 ml of the suspension was taken out and then centrifuged to remove the adsorbent particles and MG concentrations in the filtrate

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were determined on a UV-vis spectrophotometer.

3. Results and discussion

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3.1. Structure and composition

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The phase and structure of the as-prepared samples were firstly characterized by

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XRD. As shown in Fig. 1, XRD patterns of the samples prepared by calcining in air at

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450 °C and 350 °C could be perfectly indexed as monoclinic α-Bi2O3 (JCPDS No. 41-1449) (Fig. 1a) and tetragonal β-Bi2O3 (JCPDS No. 27-0050) (Fig. 1c),

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respectively. No impurity peaks were detected, indicating that the as-prepared

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samples are in high purity. The sharp and strong diffraction peaks imply that the samples were well crystalline. Obviously, XRD patterns of the samples calcinied in nitrogen at a temperature ranging from 300 °C to 500 °C indicate that the coexistence

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of Bi (JCPDS No. 44-1246) and α-Bi2O3 (JCPDS No. 41-1449) (Fig. 1b; Fig. S1, Supporting information). No peaks assigned to carbon could be detected although the samples appeared black gray (Fig. S2, Supporting information), revealing that carbon was amorphous. The formation of Bi metal should result from the reduction of Bi2O3

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by residual carbon [38,39]. Given that the composites only consisted of two phases, the weight ratio of Bi to α-Bi2O3 could be calculated by the Reference Intensity Ratio (RIR) method [39,40]. Herein, since the most intense diffraction peaks of metal Bi and α-Bi2O3 overlap at around 27°, the intensity of the second most intense peaks were

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used. According to the RIR values of Bi and α-Bi2O3 phases read from the PDF detabase, which are 2.57 and 1.4, respectively, the calculated molar percentage of Bi

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in the composites obtained at 300 °C, 350 °C, 450 °C and 500 °C is 9.1%, 29.7%, 19.1% and 25.0%, respectively. It seems that no correlation between Bi content and

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annealing temperature. In addition, different from air atmosphere, no β-Bi2O3 was

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obtained in nitrogen even when the calcination temperature was reduced to 300 °C.

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An important experimental phenomenon, that the products were pyrophoric for about

released heat by pyrophorization could induce rapid transformation

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suggests that the

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two minutes after cooled to room temperature and taken out from tubular furnace,

of β-Bi2O3 to α-Bi2O3. Similar morphology between C/Bi/α-Bi2O3-T and β-Bi2O3

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obtained in air at 350 °C

supports this hypothesis (shown below). Usually, the

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transformation of β-Bi2O3 to α-Bi2O3 by calcining Bi(NO3)3·5H2O was completed at 650 °C [21]. The more detailed formation mechanism of α-Bi2O3 at so low temperature is not very clear and further investigation is underway. For comparison,

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the calcination of Bi(NO3)3·5H2O instead of bismuth citrate was conducted. Analogous to the report [21], the temperature of complete decomposition of Bi(NO3)3·5H2O into pure β-Bi2O3 (500 °C) and α-Bi2O3 (650 °C) in air is obviously higher than that of bismuth citrate. Therefore, a large amount of organic carbon in 10

bismuth citrate benefits to lower its decomposition temperature due to burning heat of organic carbon. Song et al also reported that bismuth citrate began to decompose at 285 °C and completely decomposed into Bi2O3 at 413 °C [41].

10

20

 

b

(121)

30

40

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   

a Bi 44-1246 -Bi2O3 41-1449

50

60

70

80

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2 Theta / degree

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  

(041)

-Bi2O3 Bi

(104) (110)

 

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c



(111) (120) (012) (012)

Intensity / a.u.

-Bi2O3 27-0050

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Fig. 1. XRD patterns of the products prepared under different calcining conditions: (a)

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450 °C in air, (b) 450 °C in nitrogen, (c) 350 °C in air.

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The morphology and microstructures of samples are important factors to influence the photocatalytic performance. TEM observation indicates that pure α-Bi2O3 obtained by

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calcining in air mainly consisted of irregular worm-like particles with a diameter of Pure β-Bi2O3 was composed of a quasi-sphere-like structure

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about 500 nm (Fig. 2a).

with a size of 30 nm (Fig. 2b). Analogous to β-Bi2O3, all of the composites

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Bi/C/α-Bi2O3 obtained in nitrogen at a temperature ranging from 300 to 500 °C appeared quasi-sphere-like morphology with a size of 30 nm (Fig. 2c; Fig. S3a and b, Supporting information), which implies that Bi/C/α-Bi2O3 may come from the transformation of β-Bi2O3 driven by the heat from the self-ignition of organic carbon in air, while keeping morphology unchanged. The measured lattice spacing of α-Bi2O3 11

is 0.271 nm, corresponding to the (200) plane of α-Bi2O3 (Fig. S3c, Supporting information). HRTEM image of β-Bi2O3 indicates two sets of lattice spacings of 0.194 and 0.125 nm, corresponding to (222) and (224) planes of β-Bi2O3, respectively (Fig. S3d, Supporting information). HRTEM images of C/Bi/α-Bi2O3-450 reveals two

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sets of fringes with d-spacings of 0.271 and 0.188 nm, which could be ascribed to (200) and (202) planes of α-Bi2O3 and Bi, respectively (inset in Fig. 2c and Fig. 2d).

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Since there co-existed α-Bi2O3 and Bi in a small area of about several nanometers, it

could be supposed that Bi metal particles intimately attached to the surface of Bi2O3

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particles, which would be beneficial to the realization of the SPR effect. No fringes

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corresponding to carbon could be identified due to its amorphous structure.

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Fig. 2.

TEM (a-c) and HRTEM (inset in c, d) images of (a) α-Bi2O3, (b) β-Bi2O3 and (c, d)

C/Bi/α-Bi2O3-450.

The composition and elemental chemical states of α-Bi2O3 and C/Bi/α-Bi2O3-450 were checked by XPS. The survey spectra of two samples show that all of the

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detected elements are in accord with their compositions (Fig. 3a). For

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C/Bi/α-Bi2O3-450, the high resolution XPS spectroscopy of Bi 4f shows two doublets peaks (Fig. 3bA). One doublet at 164.3 and 159.0 eV could be assigned to the binding

energy (BE) of Bi 4f5/2 and Bi 4f7/2 of Bi3+, respectively. The other with lower BE at

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162.3 and 157.0 eV corresponds to Bi 4f5/2 and Bi 4f7/2 of Bi metal, respectively [35],

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which demonstrates the presence of metal Bi. The ratio of peak areas of Bi 0 to Bi3+ is

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0.26:1, i.e. the molar ratio of Bi to Bi2O3 is about 0.13, which is a little lower than the calculated value by XRD. The spectra of C 1s could be deconvoluted into three peaks

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at 284.8, 286.1 and 288.6 eV (Fig. 3cA), which could be assigned to

the

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amorphous carbon as well as adventitious carbon, C-O and C=O of non-burned organic carbon or adsorbed CO2, respectively. The O 1s XPS spectra located at 529.9,

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530.9 and 532.7 eV (Fig. 3dA) could respectively be ascribed to oxygen atoms in Bi-O, C-O or C=O and surface adsorbed H2O or hydroxyl group. In the case of

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α-Bi2O3, Bi 4f XPS spectra show that there exists a very small amount of metal Bi with a molar ratio of Bi to Bi2O3 at about 0.005 (Fig. 3bB), and also, C 1s spectra could be fitted into three peaks ascribed to adventitious carbon as well as amorphous carbon, C-O and C=O of non-burned organic carbon (Fig. 3cB), which suggests that a small

amount of organic carbon and metal Bi were still present on the surface of 13

α-Bi2O3 although the color of the sample is light yellow. As compared to C/Bi/α-Bi2O3-450, the BE of Bi 4f ascribed to Bi3+ in α-Bi2O3 shifted to 164.1 and 158.7 eV, and the other doublet ascribed to metal Bi shifted to 162.0 and 156.9 eV,

Bi 4f 7/2 Bi 4f 5/2

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B 1000

800

600

400

200

0

168

166

Binding energy / eV

160

158

156

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O 1s

Intensity / a.u.

A

N

d

C 1s

A

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A

B 292

162

Binding energy / eV

290

288

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Intensity / a.u.

c

164

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B

Intensity / a.u.

Bi4f

b Bi5d O2s

C1s

O1s Bi4d3 Bi4d5

Bi4p3

A

O KLL

Intensity / a.u.

a

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implying that the chemical environment around these atoms had slightly changed.

286

284

B 536

282

534

532

530

528

Binding energy / eV

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Binding energy / eV

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Fig. 3. XPS spectra of (A) C/Bi/α-Bi2O3-450 and (B) α-Bi2O3 : (a) survey, (b) Bi 4f, (c) C 1s and (d) O1s.

BET surface areas and pore size distribution of α-Bi2O3 and C/Bi/α-Bi2O3-450 were

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measured by N2 adsorption experiments. The isotherm curve of C/Bi/α-Bi2O3-450 shows typical type IV with H3 hysteresis loop at p/p0 > 0.5 (Fig. 4a), which were characteristics of mesoporous materials. The pore size distribution indicates bimodal characteristics with one peak at 10 nm and the other at 45 nm. The former most likely 14

arise from intra-sphere spacings, and while the latter should be due to inter-sphere pores. The mean pore diameter calculated from desorption branch of the nitrogen isotherm by the BJH (Barrett-Joyner-Halenda) method is 13.48 nm. The calculated BET surface area of C/Bi/α-Bi2O3-450 is 14.03 m2/g, which is about 8 times higher

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than that (1.76 m2/g) of α-Bi2O3 (Fig. 4b; Table S1, Supporting information). The higher surface area and larger pore size of C/Bi/α-Bi2O3-450 should be induced by

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smaller particle size and the presence of more organic carbon, which play a key role

0.00 0

20

40

60

80

100 120

Pore Diameter (nm)

5

Adsorption Desorption

0 0.2

0.4

0.6

0.8

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0.0

2.5 2.0

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0.005 0.004 0.003

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3 -1

0.01

3.0

Pore Volume (cm3g-1nm-1)

10

0.02

3.5

0.002 0.001

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15

0.03

4.0

M

20

a

0.04

Volume adsorbed (cm g STP)

25

Pore Volume (cm3g-1nm-1)

3 -1

Volume adsorbed (cm g STP)

in the enhancement of its photocatalytic activity due to more exposed active sites.

1.0

Relative Pressure (P/Po)

1.5

0.000

1.0

0

10

20

30

40

50

Pore Diameter (nm)

0.5

Adsorption Desorption

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

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Fig. 4 Nitrogen adsorption-desorption isotherms of (a) C/Bi/α-Bi2O3-450 and (b) α-Bi2O3, the

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insets showing pore size distribution.

3.2. Light absorption property

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The light absorption property of the as-prepared samples was investigated by DRS

(Fig. 5a). Three samples possess almost the same absorption onsets at about 420 nm. Nevertheless, their absorption edge positions are different from each other. For pure α-Bi2O3, the absorption edge situated around 450 nm, and while for C/Bi/α-Bi2O3-350 and C/Bi/α-Bi2O3-450, the absorption edges red shifted to 568 and 485 nm, 15

respectively, which should mainly be induced by the difference in carbon or Bi contents and particles size. . In addition, . as compared to α-Bi2O3 obtained in air, there is a bump around 585 nm for C/Bi/α-Bi2O3 samples, which should be induced by SPR effect of Bi metal [35]. This absorption peak is slight, which may be relevant

visible-light absorption of C/Bi/α-Bi2O3 benefits for

generating more charge

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carriers.

enhanced

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to low spatial density of Bi on the surface of α-Bi2O3 [42]. The

It is reported that the band gap energy (Eg) could be calculated according to 1240/λ,

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where λ is wavelength at the absorption onset, or by Tauc’s method. Herein, the band

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gap energy (Eg) were calculated according to classic Tauc’s method αhν = A(hν -

Furthermore, herein, better linear relationship of (αhν)2 versus hν than

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of n is 1.

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Eg)n/2. Since α-Bi2O3 is known to be a direct transition semiconductor [27], the value

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(αhν)1/2 versus hν also confirms that the as-prepared materials belong to direct transition semiconductors. From the intercept of the tangent

the estimated Eg values of α-Bi2O3,

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versus photon energy (hν) to α = 0,

of the plot of (αhν)2

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C/Bi/α-Bi2O3-350 and C/Bi/α-Bi2O3-450 are 2.82, 2.33 and 2.63 eV, respectively. 1.2

a



450 C  350 C

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Absorbance

(h)2(eV)2

1.0

A

12

-Bi2O3

0.8 0.6 0.4

0.2 200

b

8

-Bi2O3

6

450 C  350 C



4 2

300

400

500

600

700

800

900

1000

Wavelength / nm

16

0 1.8

2.4

3.0

h (eV)

3.6

Fig. 5.

(a) DRS spectra and (b) plots of (αhν)2 vs hν of the as-prepared α-Bi2O3,

C/Bi/α-Bi2O3-350 and C/Bi/α-Bi2O3-450.

3.3. Photocatalytic performance

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The photocatalytic activity of the as-prepared samples was firstly evaluated by the degradation of azo dye MO in neutral medium under visible light irradiation. As

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displayed in Fig. 6a, the photolysis of MO could be negligible within 180 min in the absence of a catalyst. As compared to pure α-Bi2O3 with a degradation efficiency of about 20%, all C/Bi/α-Bi2O3 composites exhibited enhanced photocatalytic activity.

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Especially, C/Bi/α-Bi2O3-450 possessed the highest photocatalytic activity and about

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93% of MO was degraded within 180 min of irradiation. The average MO degradation

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curves with error bars were determined from these samples under the same preparation conditions, and the maximum error in the degradation efficiency on

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C/Bi/α-Bi2O3-450 is 6.5% after 60 min, which testify the repeatability of the synthetic

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process. Although β-Bi2O3 was previously reported as most photoactive among several polymorphs of Bi2O3 due to good visible light response [1], its photocatalytic

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activity is still lower than that of C/Bi/α-Bi2O3-450. The enhancement of the photocatalytic activity of C/Bi/α-Bi2O3-450 could be attributed to SPR effect of metal

A

Bi and enlarged visible light absorption due to the presence of organic carbon [38]. Since all C/Bi/α-Bi2O3 samples possess similar morphology, particle size and crystallinity, higher photocatalytic activity of C/Bi/α-Bi2O3-450 may be relevant to its suitable Bi and carbon

contents . Although Bi content in C/Bi/α-Bi2O3-450 (19.1%)

is lower than those of C/Bi/α-Bi2O3-350 °C (29.7%) and C/Bi/α-Bi2O3-500 °C (25.0%) 17

according to XRD calculation, XPS quantitative analysis indicates that Bi content on the surface of C/Bi/α-Bi2O3-450 (11.3%) is higher than those of C/Bi/α-Bi2O3-350 °C (1.5%) and C/Bi/α-Bi2O3-500 °C (6.9%) (Fig. S4, Supporting information). Because photocatalysis belongs to heterogeneous reaction, Bi and carbon contents on the

IP T

surface of a catalyst play a key role in the photocatalytic activity. Unfortunately, it is inaccurate to calculate the carbon content by XPS because of the presence of

a

1.0

samples exposed to air.

SC R

adventitious carbon on the surface of the

1.0

0.2

-Bi2O3

-30

-Bi2O3

450 oC

0.0

0

30

60

90

t / min

M

-60

350 oC 500 oC

0.2

450 oC

0.0

0.4

N

350 oC 500 oC

blank -Bi2O3

A

0.4

0.6

C / C0

blank -Bi2O3

C / C0

0.6

U

0.8

0.8

b

120 150 180

-60

-30

0

30

60

90

120 150 180

t / min

ED

Fig. 6. Photodegradation efficiency of (a) MO and (b) MG over the as-prepared catalysts under

PT

visible light irradiation, and error bars represent one standard deviation from triplicate experiments.

CC E

Temporal UV-Vis spectra of MO degradation shows that the characteristic absorbance peak of MO located at 464 nm dramatically dropped (Fig. S5a, Supporting information), implying that MO could be quickly decolorized. In order to further

A

investigate mineralization nature of MO on

C/Bi/α-Bi2O3-450, the TOC

disappearance was examined after 180 min of the visible light irradiation.

The result

showed that the TOC value decreased from 10.00 mg/L to 3.69 mg/L, and the

18

mineralization rate reached approximately 63%. Therefore, the mineralization of MO was predominant. Furthermore, the as-prepared C/Bi/α-Bi2O3-450 also exhibited good photocatalytic activity for MG degradation with an efficiency of 82% within 3 hours of irradiation,

SC R

C/Bi/α-Bi2O3-500 (49.6%) (Fig. 6b; Fig. S5b, Supporting information).

IP T

higher than α-Bi2O3 (19.9%), β-Bi2O3 (60.6%), C/Bi/α-Bi2O3-350 (48.9%) and

The pseudo-first-order kinetic model equations were used to understand the degradation kinetics of MO and MG: -ln(C/C0) = kt. The obvious linear relationship

N

U

between -ln(C/C0) and irradiation time t for all samples indicated that the

A

time-dependent photodegradation of MO and MG follows the pseudo first-order

M

kinetics (Fig. S6, Supporting information). The apparent rate constant k of C/Bi/α-Bi2O3-450 obtained from the tangent value of the line are 0.013 min-1 and

ED

0.007 min-1 for MO and MG, respectively, which are higher than those of the other

PT

samples, indicating faster degradation rate of MO and MG on C/Bi/α-Bi2O3-450.

CC E

The durability of the C/Bi/α-Bi2O3-450 was evaluated by recycling experiments. The decrease in the photocatalytic activity for MO and MG degradation is less than 10% after four cycles (Fig. S7a, Supporting information), indicating that the

A

as-prepared C/Bi/α-Bi2O3-450 possessed favorable recycling characteristics under visible light illumination. XRD patterns of C/Bi/α-Bi2O3-450 after four cycles indicates that all the diffraction peaks are in basically agreement with fresh catalyst

19

(Fig. S7b, Supporting information), suggesting that the catalyst was structural stable throughout the photocatalytic process. In order to exclude photosensitization effect of dye degradation, the degradation of colorless salicylic acid (SA) on C/Bi/α-Bi2O3-450 was conducted under identical

IP T

conditions with MO degradation. The results show that about 50% of SA was

SC R

decomposed within 5 h of visible light irradiation (Fig. S8, Supporting information), implying that the photocatalytic degradation of MO or MG is mainly caused by direct semiconductor photocatalysis.

N

U

3.4. Photocatalytic mechanism

A

To investigate the photocatalytic mechanism of C/Bi/α-Bi2O3-450, the role of

M

possible reactive species in the photodegradation of MO and MG were evaluated via

ED

the active species trapping experiments by separately adding 1,4-benzoquinone (BQ, a quencher of •O2−), ammonia oxalate (AO, a quencher of h+) and isopropanol (IPA, a

PT

quencher of •OH) into the photoreaction system under otherwise identical experiment

CC E

conditions without any scavengers [43]. As displayed in Fig. 7, the change of degradation efficiency of MO or MG indicates that the photogenerated holes h+ were main active species and •O2− radicals were miner active species. The addition of IPA

A

had almost no inhibition to photocatalytic activity, indicating that •OH could not be formed in this photocatalytic system.

20

a

0.6

0.6

C / Co

0.8

C / Co

0.8

0.4

0.4

AO EDTA BQ IPA no scavenger

0.2 0.0 -60

-30

0

b

1.0

AO EDTA BQ IPA no scavenger

0.2

30

60

t / min

90

0.0

120 150 180

-60

-30

0

30

60

90

t / min

IP T

1.0

120 150 180

SC R

Fig. 7. Trapping experiment in the presence of various quenchers (1 mM) on C/Bi/α-Bi2O3-450 for the photodegradation of (a) MO and (b) MG.

U

The band potentials of a semiconductor are important factors to photocatalysis,

N

which could be evaluated by the Mott-Schottky method. As shown in Fig. S9a, the

A

negative slope of the C-2-E plot indicates that C/Bi/α-Bi2O3 belongs to n-type

M

semiconductors, similar to previous report [25]. From the intercept of tangent line to x

ED

axis, the flat band potential (Vfb) of C/Bi/α-Bi2O3 was -0.33 V vs. SCE, i.e. 0.30 V vs. standard hydrogen electrode (SHE), which was calculated by the formula VSHE = VSCE

PT

+ 0.059pH + V0SCE, where, V0SCE = 0.245 V at pH = 6.8, VSCE = -0.33 V (i.e.

CC E

experimental potential measured against the Hg/Hg2Cl2/saturated KCl reference electrode), and pH = 6.47 for Na2SO4 (0.5 M) solution [25]. For n-type semiconductors, the CB position is more negative by 0.1 V than the flat band potential.

A

Therefore, the ECB value of C/Bi/α-Bi2O3 is 0.20 V. Similarly, the ECB value of α-Bi2O3 was calculated to be -0.03 V (Fig. S9b). According to the Eg values assessed from the DRS analysis, the valence band potentials (EVB) of C/Bi/α-Bi2O3 and

21

α-Bi2O3 are 2.83 and 2.79 V, respectively, calculated by using the equation ECB = EVB - Eg. In view of active species trapping experiment results and band structures of the catalysts, a schematic illustration of possible charge transfer process in the

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C/Bi/α-Bi2O3-450 is proposed and shown in Scheme 1. Under visible light irradiation, the electrons could be excited from the valence band (VB) to the conduction band

the surface of α-Bi2O3 and the Fermi level of

metal Bi is -0.17 eV, which is

more

ECB of α-Bi2O3, the photogenerated electrons could transfer from

U

negative than the

SC R

(CB) of α-Bi2O3, leaving the holes in the VB. Because metal bismuth was present on

be trapped by O2 in solution to form •O2−

A

electrons in the CB of α-Bi2O3 could

Whether the photogenerated

N

the Fermi level of bismuth to the CB of α-Bi2O3.

M

depend on the standard redox potential of O2/•O2− (E0). It is reported that the E

ED

(O2/•O2−) value is -0.046 V or -0.28 V vs. NHS [44,45]. Because the redox potentials of O2 into •O2− depend on the acidity of the medium [46], E (O2/•O2−) could be

PT

calculated by using an equation: E = E0 - 0.05915 pH, where E0 is redox potential

CC E

with pH = 0 and it is -0.046 V [46]. After BQ as a scavenger was added to C/Bi/α-Bi2O3-450 suspension of MO and MG , the solution pH is 4.15 and 4.27 and are more

A

the calculated E (O2/•O2−) is -0.29 and -0.30 V, respectively, which

negative than the ECB values of C/Bi/α-Bi2O3 or α-Bi2O3 (0.20 V for C/Bi/α-Bi2O3 and -0.03 V for α-Bi2O3) . Therefore, the photogenerated electrons in the CB of α-Bi2O3 could not reduce O2 into •O2−. Since •O2− was demonstrated reactive species in this photocatalytic system, it is supposed that hot electrons generated from non-radiative 22

decay due to SPR effect of Bi could reach more negative band position and thus reduced O2

into •O2− [47] . though the Fermi level of metal Bi is -0.17 eV. Once the

electrons were released, Bi would shift to more positive potentials [48], and then, Bi would accept electrons from VB of α-Bi2O3 and then return to its original state.

IP T

Therefore, the photogenerated electrons and holes in α-Bi2O3 could be effectively separated. In addition, the remaining carbon could accept and transfer electrons from

SC R

CB of α-Bi2O3 and thus prohibit the recombination of electron-hole pairs. The holes h+ in the VB of α-Bi2O3 could directly oxidize MO or MG into smaller molecules or

Scheme 1.

PT

ED

M

A

N

U

CO2 and H2O.

Schematic diagram of the proposed mechanism for the degradation of dyes over the

CC E

C/Bi/α-Bi2O3 composite under visible light irradiation.

The enhancement of the photocatalytic activity of C/Bi/α-Bi2O3-450 could be

A

attributed to the synergistic effects of several factors. First of all, the remaining non-burned carbon could absorb more visible light and increase the specific surface areas of α-Bi2O3 as well. Moreover, the remaining carbon could also reduce resistance for carriers migration. Thirdly, Bi nanoparticles present on the surface of α-Bi2O3

23

could enhance surface electron excitation and interfacial electron transfer due to its SPR effect. All these benefit for generating more charge carriers and inhibiting the recombination rate of the carriers, thus enhancing photocatalytic performance.

IP T

3.5. Photocurrent and electrochemical impedance spectra The superior separation, migration and transferring ability of the photogenerated

SC R

electrons and holes on the C/Bi/α-Bi2O3-450 was further verified by the photocurrent experiment. As shown in Fig. 8a, the transient photocurrent responses of all samples displayed the switched on/off behaviors with the pulsed visible light illumination.

N

U

C/Bi/α-Bi2O3-450 possessed the highest current density among all of the samples,

A

which means that it has the strongest ability to transfer photogenerated electrons due

M

to the involvement of suitable Bi and C in α-Bi2O3. The order of photocurrent intensity of all samples is consistent with their photocatalytic activity. The

ED

transformation behavior of the carriers on these photocatalysts were further

PT

investigated by using electrochemical impedance spectroscopy (EIS) (Fig. 8b). All of the composites exhibited a smaller arc radius on the EIS Nyquist plots than α-Bi2O3,

CC E

which means smaller interfacial resistance for carrier migration. Compared to the other samples, C/Bi/α-Bi2O3-450 has the lowest arc radius, implying that

A

C/Bi/α-Bi2O3-450 possesses smallest interface layer resistance and highest charge transferring efficiency.

24

2

0.20

0.12

C/Bi/-Bi2O3-450

Light on Light off

C/Bi/-Bi2O3-500

-Bi2O3

C/Bi/-Bi2O3-500

0.16

a

C/Bi/-Bi2O3-450

C/Bi/-Bi2O3-350

b

C/Bi/-Bi2O3-350 -Bi2O3

-Z"/ohm

-Bi2O3

0.08

-Bi2O3

0.04 0.00 10

20

30

40

50

60

0

2000

4000

8000

10000

(a) Transient photocurrent responses under visible light irradiation and (b) EIS plots of

SC R

Fig. 8.

6000

Z'/ohm

Time /s

IP T

photocurrent density (A/cm )

0.24

the as-prepared samples.

U

3.6. Adsorption property

N

Analogous to AO, EDTA-2Na is commonly used as a scavenger for h+ [49].

M

A

However, different from MO photoreaction system (Fig. 7b), the solution was swiftly decolorized within 30 min when a trace amount of EDTA-2Na (1 mM) was added

ED

into MG photoreaction system (Fig. 7b). For MV, similar phenomenon was also

PT

observed except that complete decolorization time was extended to 100 min possibly due to its larger molecular diameter than MG (Fig. S10, Supporting information).

CC E

Further investigation indicates that the decolorization similarly occurred in dark. Furthermore, the as-prepared

α-Bi2O3 and β-Bi2O3

also

exhibited strong

A

decolorization ability for MG with help of a little amount of EDTA-2Na (above 0.5 mM) in dark (Fig. 9a). Therefore, we propose that the addition of EDTA-2Na improved the adsorption capacity of these samples. In addition, found that

further investigation

α-Bi2O3 microrods (Bi2O3-mr) prepared in our previous work [6] and

commercial α-Bi2O3 (Bi2O3-com) also exhibit enhanced adsorption ability in the 25

presence of EDTA-2Na (Fig. 9a). Nevertheless, their adsorption capacity is lower than that of the as-prepared α-Bi2O3, which should be due to lower BET surface areas (1.24 m2/g for Bi2O3-mr; <1 m2/g for Bi2O3-com) [6,50]. Because the decolorization enhancing phenomenon did not exist for anionic dye methyl orange (MO), we

IP T

suppose that EDTA-2Na may change surface electrical property of these samples,

 -Bi2O3

0.6

-Bi2O3-com

C / C0

-Bi2O3-mr

0.4 0.2

20

30

40

50

60

-10 -20 -30

Fig. 9.

2

4

6 pH

8

10

M

t / min

Bi2O3+EDTA

0

A

10

Bi2O3

10

-40

0.0 0

b

20

U

0.8

a

N

-Bi2O3

Zeta potential (mV)

C/Bi/-Bi2O3

1.0

SC R

resulting in strong adsorption for cationic dyes.

(a) Decolorization efficiency of MG on various samples with the help of EDTA-2Na; (b)

ED

Zeta potentials of α-Bi2O3 before and after adding EDTA-2Na.

PT

As representation, the zeta potentials of the as-prepared α-Bi2O3 were measured.

CC E

As displayed in Fig. 9b, α-Bi2O3 with the addition of EDTA-2Na possessed higher surface electric negativity with ζ = -34.0 mV, which is two times of that without EDTA-2Na (ζ = -17.5 mV). This may well explain enhanced adsorption capacity for

A

cationic dyes in the presence of EDTA-2Na. In addition, the amount of EDTA-2Na also influence adsorption capacity. If the concentration of EDTA-2Na was reduced to 0.3 mM, it took a longer time (1.5 h) for MG to decolorize completely. We propose that enhanced surface electric negativity of α-Bi2O3 by EDTA-2Na may be induced by

26

the strong chelation of EDTA-2Na with Bi3+, leading to more O2- exposure on the surface. Finally, the adsorption isotherm experiments for MG with various initial concentrations on the as-prepared α-Bi2O3 were conducted. As displayed in Fig. S11,

IP T

the kinetic curves show a very rapid initial uptake within first 20 min for MG at

SC R

various initial concentrations, and the time to reach equilibrium prolonged as MG increasing. The adsorption capacity qt (mg/g) at any time t and removal efficiency r

(%) after the equilibrium were calculated using the equations (1) and (2) [51],

r = 100% (C0 - Ce)/C0

(2)

A

(1)

M

qt = (C0 - Ct)V/W

N

U

respectively.

ED

where C0 is the original concentration, Ct is the concentration at reaction time t (mg/L), V is the volume of solution (L) and W is the weight of the adsorbent (g). The

PT

adsorption capacity and pollutant concentration at the equilibrium are marked as qe

CC E

(mg/g) and Ce (mg/L), respectively. As shown in Table 1, the equilibrium adsorption capacity is 97 mg/g with a

A

removal rate of 97% at a MG initial concentration of 40 mg/L. The investigation on the adsorption kinetics of MG on α-Bi2O3-EDTA indicates that the adsorption data agree well to the pseudo second order model with a correlation coefficient (R2) of 0.9989 (Table 1 and Fig. S12, Supporting information). These suggest that the adsorption is not simply controlled by mass transfer. As revealed in Table 1 and Fig. 27

S13, the adsorption isotherm experiments on α-Bi2O3-EDTA with various initial MG concentrations show that the experimental data fit better for Langmuir model (R2 ≈ 0.96) than Freundlich isotherm model (R2 ≈ 0.77), implying that the adsorbent surface was uniform and the adsorption was close to a monolayer mechanism. The maximum

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MG adsorption capacity calculated from Langmuir isotherms is 312 mg/g, which is much higher than those of the reported materials and the as-prepared α-Bi2O3 without

Table 1.

SC R

EDTA-2Na (qmax = 40 mg/g) (Table S2, Supporting information).

Equilibrium adsorption capacity (qe), removal rate (r), and kinetic and isotherm

120

160

200

97

185

228

263

273

97

93

76

66

55

0.8443

Pseudo-second-order (R2)

0.9989

Langmuir (R2)

0.9644

qmax（mg/g）

312

Freundlich (R2)

0.7677

A

CC E

ED

Pseudo-first-order (R2)

PT

r（%）

80

A

qe（mg/g）

40

M

Initial concentration（mg/L）

N

MG

U

constants for MG adsorption on α-Bi2O3 in the presence of EDTA.

4. Conclusions In summary, by using bismuth citrate as a precursor, the composites of non-burned carbon, metal bismuth and bismuth oxide (marked as C/Bi/α-Bi2O3) could 28

be easily prepared via a calcination technique in nitrogen atmosphere. The surface plasmon resonance effect of metallic bismuth together with good visible light absorption and electrical conductivity of the residual carbon endow α-Bi2O3 with good photocatalytic activity for dyes degradation under visible light irradiation, better than

IP T

active β-Bi2O3. Furthermore, the forming temperature of α-Bi2O3 from the decomposition of bismuth citrate is much lower than from Bi(NO3)3·5H2O due to a

SC R

large amount of carbon. Finally, the surface negative electricity of the as-prepared α-Bi2O3, β-Bi2O3 and C/Bi/α-Bi2O3 could be increased in the presence of trace

U

amount of EDTA-2Na due to its strong chelation effect, resulting in high adsorption

N

capacity for cationic dyes like MG and MV. The result provides new insights into the

A

design of nontoxic bismuth oxides as photocatalysts and adsorbents for environmental

ED

M

remedies.

PT

Conflicts of interest

There are no conflicts of interest to declare.

CC E

Acknowledgments

The work is financially supported by the National Natural Science Foundation of

A

China (51772155, 51472122).

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Fig. captions

ED

Fig. 1. XRD patterns of the products prepared under different calcining conditions: (a) 450 °C in air, (b) 450 °C in nitrogen, (c) 350 °C in air. TEM (a-c) and HRTEM (inset in c, d) images of (a) α-Bi2O3, (b) β-Bi2O3 and (c, d)

PT

Fig. 2.

CC E

C/Bi/α-Bi2O3-450.

Fig. 3.

XPS spectra of (A) C/Bi/α-Bi2O3-450 and (B) α-Bi2O3: (a) survey, (b) Bi 4f, (c) C 1s

A

and (d) O1s.

Fig. 4.

Nitrogen adsorption-desorption isotherms of (a) C/Bi/α-Bi2O3-450 and (b) α-Bi2O3, the

insets showing pore size distribution.

37

(a) DRS spectra and (b) plots of (αhν)2 vs hν of the as-prepared α-Bi2O3,

Fig. 5.

C/Bi/α-Bi2O3-350 and C/Bi/α-Bi2O3-450.

Fig. 6. Photodegradation efficiency of (a) MO and (b) MG over the as-prepared catalysts under

Fig. 7.

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visible light irradiation, and error bars represent one standard deviation from triplicate experiments.

Trapping experiment in the presence of various quenchers (1 mM) on C/Bi/α-Bi2O3-450

Scheme 1.

SC R

for the photodegradation of (a) MO and (b) MG.

Schematic diagram of the proposed mechanism for the degradation of dyes over the

(a) Transient photocurrent responses under visible light irradiation and (b) EIS plots of

N

Fig. 8.

U

C/Bi/α-Bi2O3 composite under visible light irradiation.

M

Fig. 9.

A

the as-prepared samples.

(a) Decolorization efficiency of MG on various samples; (b) Zeta potentials of α-Bi2O3

Equilibrium adsorption capacity (qe), removal rate (R), and kinetic and isotherm

PT

Table 1.

ED

before and after adding EDTA-2Na.

A

CC E

constants for MG adsorption on α-Bi2O3 in the presence of EDTA.

38