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Uniform Fe2O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity
Accepted Manuscript Title: Uniform Fe2 O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity Author: Na Li Yujian Jin Xia Hua Kai Wang Jingjing Xu Mindong Chen Fei Teng PII: DOI: Reference:
S1381-1169(14)00402-6 http://dx.doi.org/doi:10.1016/j.molcata.2014.08.045 MOLCAA 9260
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
19-6-2014 28-8-2014 28-8-2014
Please cite this article as: N. Li, Y. Jin, X. Hua, K. Wang, J. Xu, M. Chen, F. Teng, Uniform Fe2 O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.08.045 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.
Uniform Fe2O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity
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Na Li, Yujian Jin, Xia Hua, Kai Wang, Jingjing Xu, Mindong Chen, Fei Teng
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Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and
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Pollution Control, School of Environmental Sciences and Engineering, Nanjing
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University of Information Sciences and Engineering, Nanjing 210044, China
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Abstract
The xFe2O3/yBiOCl (x/y, molar ratio) composites are prepared via a facile
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hydrothermal process, in which the uniform Fe2O3 nanocubes are well deposited on
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the BiOCl nanosheets. The xFe2O3/yBiOCl shows much higher activities than BiOCl or Fe2O3 for the degradation of rhodamine B (RhB) and methyl orange (MO).
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Specifically, the degradation rate of the 5Fe2O3/100BiOCl catalyst is 7.2 and 3.7 times as high as those of the bare BiOCl for MO and RhB, respectively. This has been mainly attributed to the formation of p/n Fe2O3/BiOCl heterojunctions.
Keywords: Uniform Fe2O3 nanocubes; BiOCl nanosheets; p/n heterojunction; Photocatalysis
Corresponding author. Email: [email protected] (F. Teng); Phone/Fax: 0086-25-58731090 1
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1.
Introduction
BiOCl, as an efficient, inexpensive photocatalyst, has been widely studied in the degradation of organic pollutants [1-5]. Its layered structure favors for the separation
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of the photo-generated carriers, thus leading to a high activity [6]. So far, semiconductor coupling is usually used to further improve its performances, for
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example, BiOCl/Bi2O3 [7,8], PANI/BiOCl [9], BiOI/BiOCl [10], WO3/BiOI [11],
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NaBiO3/BiOCl [12], BiOCl/BiOBr [13], Bi2S3/BiOCl [14,15], Ag/AgCl/BiOCl [16], Ag/AgX/BiOX [17], BiOCl/Ag3PO4 [18], etc. Among the coupled materials,
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nevertheless, Ag, AgX, Ag3PO4 and BiOI are fairly expensive, which limits their practical applications. Thus, it is desirable to explore the low-cost coupling materials.
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Furthermore, some of the preparation methods are cumbersome, for example, using toxic solvents [9] or expensive surfactant [11,16,17], or finely controlling pH values
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highly needed.
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[7,8,12,14]. Hence, a simple, economical and environmentally friendly method is
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Herein, Fe2O3/BiOCl is prepared by a facile hydrothermal method. Fe2O3 is selected as a coupling component with the BiOCl on base of its following advantages: First, Fe2O3 is an abundant, low-cost and environmentally friendly chemical in nature, which is widely applied in magnetism, sensors and biologics and photocatalysis [19-25]. Secondly, its energy bands can well match those of the BiOCl, which favor for the transfers of photogenerated charges from one material to another. Thirdly, a stable heterojunction is easy to form between the n-typed Fe2O3 and the p-typed BiOCl semiconductors, which favors for the separation of photogenerated electrons and holes thus refrain effectively their recombination. Furthermore, more than one kind of pollutants (e.g., different organic compounds, heavy metal ions, alkaline or acidic substances, etc.) is generally contained in 2
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practical industrial wastewaters. It is needed to develop an efficient and inexpensive photocatalyst that can effectively degrade industrial wastewaters containing multiple organic dyes.
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Herein, the Fe2O3/BiOCl heterojunctions are, for the first time, prepared by a simple hydrothermal method. We have investigated the activities of the catalysts at
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different molar ratios of the Fe2O3 to BiOCl. Furthermore, our main attention is paid to their photodegradation performances for the simulated the RhB-MO mixture
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wastewater. Here RhB is considered to be a cationic dye while MO is an anionic dye.
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The influences of different dye degradation have been briefly discussed. This contribution is aimed to extend photocatalysis technology to the practical
Experimental
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2.
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applications.
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2.1. Synthesis of xFe2O3/yBiOCl sample
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The BiOCl employed was prepared according to previously reported method [26]. The preparation method of Fe2O3/BiOCl is as follows: 0.4 mmol of the FeCl3·6H2O was added into 50 mL of distilled water. After the FeCl3·6H2O was dissolved fully, 1 mmol of the as-prepared BiOCl powder was then added to the solution and stirred for 30-min by a magnetic stirrer. After that, the mixture was transferred into a Teflon-lined stainless steel autoclave and maintained at 160 °C for 12 h. The as-obtained sample was then washed with distilled water for several times and dried at 60 °C for 5 h. In order to achieve the optimum photocatalysts，the molar ratio of the Fe2O3 to BiOCl varies from 5/100, 10/100, 20/100, 30/100 to 40/100 in our experiment. Furthermore, the phase-pure Fe2O3 sample was also prepared under the
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same conditions without adding the BiOCl. Herein, the as-prepared pure Fe2O3, xFe2O3/yBiOCl and the phase-pure BiOCl samples were labeled as 0Fe/100Bi, 5Fe/100Bi, 10Fe/100Bi, 20Fe/100Bi, 30Fe/100Bi, 40Fe/100Bi and 100Fe/0Bi,
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respectively.
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2.2. Characterization
The crystal structures of the samples were determined by X-ray powder diffractometer
(Rigaku
D/max-2550VB),
using
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polycrystalline
graphite
monochromatized CuK radiation (λ= 0.154 nm), operating at 40 kV and 50 mA. The
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XRD patterns were scanned in the range of 20-80o (2θ) at a scanning rate of 5o min-1.
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The samples were characterized on a scanning electron microscope (SEM, Hitachi SU-1510) with an acceleration voltage of 15 keV. The samples were coated with
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5-nm-thick gold layer before observations. The fine surface structures of the samples
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were determined by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) equipped with an electron diffraction (ED) attachment with an
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acceleration voltage of 200 kV. The texture properties of the samples were measured by nitrogen sorption isotherms. The surface areas and pore size distribution of the samples
were
calculated
by
the
Brunauer-Emmett-Teller
(BET)
and
Barret-Joyner-Halender (BJH) methods, respectively. UV-vis diffused reflectance spectra (UV-DRS) of the samples were obtained using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).
2.3. Photocatalytic degradation reactions Photocatalytic activities of the samples were evaluated by photocatalytic decomposition of rhodamine B (RhB) and methyl orange (MO), respectively. Typically, 0.05 g of powder was respectively added into the solution (200 mL, 10 4
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mg/L), which was irradiated with a 300 W Xe arc lamp equipped with ultraviolet light (λ ≤ 420 nm). The suspension was stirred for 30 min to reach an adsorption–desorption equilibrium of dye molecules on the surface of photocatalyst.
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During photoreaction, 4 mL of suspension was collected at a given interval time and
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centrifuged to remove the particles. The concentration of dye remained in the solution
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was determined by using UV–vis spectrophotometer. For the RhB-MO mixture wastewater, 200 mL of 10 mg/L RhB and 10 mg/L MO are employed. Results and discussion
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3.
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3.1. Characterization of the samples
Fig. 1(a,b) shows the representative SEM image of the typical BiOCl sample.
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Moreover, these microspheres are well separated one another, although they have
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discrepant diameters. It is interesting that the microspheres are assembled by
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numerous nanosheets. Besides, the phase-pure BiOCl is obtained (Fig. 2). After being coupled with the Fe2O3, some of microspheres have broken into pieces, the others
microspheres have become loose, as shown in Fig. 1(c,d). It is perhaps on this account that the surface area of xFe/yBi samples is smaller than that of the pure BiOCl (Table 1). In Fig. 2, the diffraction peaks of the Fe2O3 can not clearly be observed for
10Fe/100Bi sample, which may be due to some diffraction peaks of Fe2O3 overlap with those of the BiOCl (e. g., the diffraction peaks at 33.5° and 54°).
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(b)
(c)
(d)
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(a)
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Fig. 1. Scanning electron microscopy (SEM) images of the xFe/yBi samples (x/y, the
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molar ratio of Fe2O3 to BiOCl) samples: (a,b) x/y = 0/100; (c,d) x/y = 10/100
Table 1 Texture properties of the xFe/yBi samples (x/y, the molar ratio of Fe2O3 to
BiOCl )
Sample
0Fe/100Bi 5Fe/100Bi 10Fe/100Bi 20Fe/100Bi 30Fe/100Bi 40Fe/100Bi
[a]
BET surface area (m2/g) 15.9 1.6 1.8 1.7 1.1 2.1
[b]
Pore volume (cm3/g) 0.056 0.009 0.009 0.015 0.012 0.023
[b]
Pore size (nm) 10.7 36.8 35.7 34.3 39.5 40.7
Notes: [a], calculated by the Brunauer-Emmett-Teller (BET) method; [b], calculated by the Barret-Joyner-Halender (BJH) method
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Fig. 2 shows the XRD patterns of the samples. At higher x/y values than 10/100 (molar ratios), besides the diffraction peaks of the BiOCl, the diffraction peaks of the Fe2O3 are observed. Moreover, the (110) peak intensity of the Fe2O3 increases
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gradually with increasing the amount of the Fe2O3 added. Furthermore, Fig. 3 displays the HRTEM images and SAED pattern of the 10Fe/100Bi sample. It is clearly
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observed from Fig. 3a that the cubic Fe2O3 nanoparticles uniformly distribute on the BiOCl nanosheets. It is clear that the sides lengths of the Fe2O3 cubes are about 30~60
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nm. Fig. 3b shows SAED pattern of the sample. The clear diffraction spots result from
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the (200) and (110) Bragg reflections of the BiOCl and the (110) Bragg reflections of the Fe2O3, respectively. Moreover, the interface between the Fe2O3 and the BiOCl is
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observed by HRTEM (Fig. 3c). The lattice spacing of 0.248 nm is well indexed to the (110) crystal plane of the Fe2O3, while that of 0.273 nm matches the (110) crystal
d
plane of the BiOCl. The HRTEM results firmly confirm the formation of xFe/yBi
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heterojunctions.
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(110)
Intensity (a.u.)
Standard Fe2O3
100Fe/0Bi
40Fe/100Bi
*
30Fe/100Bi
*
20Fe/100Bi
*
10Fe/100Bi 5Fe/100Bi
0Fe/100Bi Standard BiOCl
10
20
30
40
50
60
70
80
2 Theta (degree)
Fig. 2. XRD patterns of the xFe/yBi samples, standard Fe2O3 and BiOCl 7
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(b)
(a)
BiOCl (200)
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BiOCl (100)
Fe2O3 (100)
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5 1/nm
0.248nm Fe2O3 (110)
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(c)
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0.274nm BiOCl (110)
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d
5 nm
Fig. 3. High-resolution electron emission microscopy (HRTEM) images and selected
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area electron diffraction pattern (SAED) of the 10Fe/100Bi sample: (a) TEM; (b) SAED; (c) Lattice fringe images
0.25
0Fe/100Bi 5Fe/100Bi 30Fe/100Bi 100Fe/0Bi
Absorbance(a.u.)
0.20
0.15
0.10
0.05
0.00 200
300
400 500 600 Wavelength(nm)
700
800
Fig. 4. UV–vis diffuse reflectance spectra (UV-DRS) of the typical xFe/yBi samples
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Besides, Fig. 4 shows the UV–DRS spectra of the typical xFe/yBi samples. When the Fe2O3 were loaded on the surface of the BiOCl sample, the absorption ability of light was enhanced in the wavelength range of 340 – 540 nm. It was also found that
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the absorption of xFe/yBi samples in the ultraviolet light range apparently increased
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and a red shift appeared upon the addition of the Fe2O3. The absorption ability of light
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of the 30Fe/100Bi was greater than that of the 5Fe/100Bi, but the degradation rates of dyes were lower than that of the 5Fe/100Bi. The possible reason may be that too
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much amount of Fe2O3 would agglomerate or grow into large particles, resulting in the decease of active sites. The UV–DRS spectra demonstrate that the light absorption
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ability of the xFe/yBi samples has also been improved than BiOCl or Fe2O3, also
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d
favoring for the improvement of activity.
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3.2. Photodegradation activities of RhB and MO Under visible light irradiation, the xFe/yBi heterojunctions almost have no catalytic
activity (Fig. S1 of SI (Supporting information)), which may be closely relative to the visible light responsive Fe2O3. It has been reported that the photogenerated electron–hole pairs can not be separated easily but easily recombine again, due to the short diffusion length of photogenerated holes in the Fe2O3 [23,27,28]. Thus, all the photocatalytic activities of the samples were evaluated under ultraviolet light irradiation.
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1.1
(a)
1.0
(c)
1.0 0.9 0.8
0.8
0.3 0.2 0.1 0.0
0
20 Time/min
30
2
0
40 4
(b)
A B C D E F G
3
ln(C0/C)
10
A B C D E F G
3
1
5
2
10
15 20 Time/min
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0.0
0.4
25
30
cr
0.2
A B C D E F G
0.5
(d)
an
0.4
0.6
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C/C0
A B C D E F G
ln(C0/C)
C/C0
0.7
0.6
1
0
0 5
10 Time/min
15
20
0
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0
5
10 Time/min
15
20
d
Fig. 5. Degradation curves and kinetic curves of RhB and MO dyes over the xFe/yBi
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samples under ultraviolet light irradiation (≤ 420 nm): (a,b) RhB (200 mL, 10 mg/L); (c,d) methylene orange (MO) (200 mL, 10 mg/L); A: 0Fe/100Bi; B: 5Fe/100Bi; C:
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10Fe/100Bi; D: 20Fe/100Bi;
E: 30Fe/100Bi; F: 40Fe/100Bi; G: 100Fe/0Bi
Observed from Fig. 5(a,b) that the xFe/yBi heterojunctions have the improved
photocatalytic activities. Specifically, the 10Fe/100Bi sample exhibits the highest photocatalytic activity for the degradation of RhB among the samples. With increasing the amount of Fe2O3, the activity of the xFe/yBi catalyst decreases. It may
be that at a too high x/y ratio, more Fe2O3 nanoparticles have agglomerated and even separated from the surface of BiOCl (Fig. S3d). Furthermore, it has been reported [23,27,28] that the photogenerated electron–hole pairs of Fe2O3 are easy to recombine again. So the photocatalytic degradation efficiencies of the xFe/yBi samples decrease
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Page 10 of 17
at too high Fe2O3 amounts. Further, the photodegradation of anionic mehylene orange dye (MO) is also investigated. In Fig. 5(c,d), the xFe/yBi heterojunctions also show the improved activity for the degradation of MO, compared with pure BiOCl.
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Moreover, the optimum molar ratio of Fe2O3 to the BiOCl is 5/100. Fig. 5b and 5d present the degradation reaction kinetic curves of RhB and MO, respectively. The
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xFe/yBi heterojunctions show the high reaction rates than pure BiOCl or Fe2O3. Table S1 summarize the calculated apparent rate constants (ka). For the decomposition of
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RhB, 10Fe/100Bi sample has the highest reaction rate among them, whose ka value is
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4 and 16 times higher than those of pure BiOCl and Fe2O3, respectively. For the degradation of MO, the ka value of the 5Fe/100Bi sample is the largest among them,
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which is 7 and 19 times higher than those of pure Fe2O3 and BiOCl, respectively. In order to determine whether the xFe/yBi heterojunctions can mineralization of
d
dyes, the degradation of RhB and MO over the 0.5Fe/100Bi sample is typically
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characterized by TOC (Fig. S4 of ESI). The TOC removal for RhB and MO after 120 min of UV light irradiation is 49% and 53%, respectively. However, it does not reach
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a complete conversion to CO2 + H2O after 2 hours. Two possible reasons can be considered. One may be that more time is needed to complete the mineralization; the other may be that the dye has been decomposed into small organic molecule.
CB= -1.1eV
e
e e e
hv
VB=2.4eV
e e
e
CB=0.28eV
Eg=2.2eV
Eg=3.5eV
e
h+
hv
h+
BiOCl
VB=2.48 eV
Fe2O3 11
Page 11 of 17
Fig. 6. Schematic diagram of photogenerated carriers transportation over the xFe/yBi heterojuctions
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The activity of photocatalyst is strongly dependent on the generation and separation of photogenerated charges [29-31]. The transfer rate and the separation
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efficiency of interfacial charges can be effectively improved by the Fe2O3/BiOCl heterojunction. In Fig. 6, the conduction band (CB) (-1.1eV) of BiOCl lies above the
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CB (0.28 eV) of Fe2O3, and the valence band (VB) (2.48 eV) of Fe2O3 lies below that
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(2.4eV) of the BiOCl. Due to their matched energy bands, the stable heterojunction can form between n-typed Fe2O3 and p-typed BiOCl, leading to the formation of an
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internal electric field built in the Fe2O3/BiOCl interface. Driven by the internal electric field, the photogenerated electrons at the CB of BiOCl can migrate to the CB
d
of the Fe2O3, while the generated holes in the VB of Fe2O3 can move along the
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opposite direction to that of BiOCl. As a result, the recombination of photogenerated electrons and holes can be reduced effectively leading to a greatly improved
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photocatalytic activity, compared with BiOCl or Fe2O3. In order to further explore the reaction mechanism of the degradation of dye over the Fe2O3/BiOCl heterojunction,
the effects of various radical scavengers are studied. As shown in Fig. S5, the hydroxyl radicals is primarily responsible for the dye degradation.
4.
Conclusions
The uniform Fe2O3 nanocubes can be well dispersed on BiOCl nanosheets by a simple hydrothermal method. The Fe2O3/BiOCl composites exhibit fairly higher activities for RhB and MO dyes than BiOCl or Fe2O3. This is mainly attributed to the formation of a stable p/n Fe2O3/BiOCl, which can refrain greatly the recombination of 12
Page 12 of 17
photogenerated carriers.
Appendix
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Supporting materials can be available freely at http://www.elsevier.com
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Acknowledgements
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This work is financially supported by National Science Foundation of China (21377060, 21103049), Six Talent Climax Foundation of Jiangsu (20100292), Jiangsu
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Science Foundation of China (BK2012862), Jiangsu Province of Academic Scientific Research Industrialization Projects (JHB2012-10, JH10-17), the Key Project of
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Environmental Protection Program of Jiangsu (2013016, 2012028), the Project Funded by the Science and Technology Infrastructure Program of Jiangsu
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(BM2013139, 201380277), A Project Funded by the Priority Academic Program
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Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu Province Innovation Platform for Superiority Subject of Environmental Science and
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Engineering sponsored by SRF for ROCS, SEM (2013S002) and “333” Outstanding Youth Scientist Foundation of Jiangsu (2011015).
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Graphic abstract: Uniform Fe2O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity
0.8
much higher activities than
0.6
BiOCl or Fe2O3 for the
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The xFe2O3/yBiOCl shows
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1.0
degradation of rhodamine B
0.4
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C/C0
Na Li, Yujian Jin, Xia Hua, Kai Wang, Jingjing Xu, Mindong Chen, Fei Teng
RhB
and methyl orange
MO
0.2 0.0 10
20
30
an
0
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te
d
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Time/min
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Highlights 1.
The Fe2O3/BiOCl p/n heterojunctions are prepared by an in-situ hydrothermal
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method. The Fe2O3 nanocubes uniformly deposit on the BiOCl nanosheets.
3.
The Fe2O3/BiOCl heterojunctions have a higher activity than bare Fe2O3 or
cr
2.
Ac ce p
te
d
M
an
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BiOCl for the degradation of dye.
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Page 17 of 17
S1381-1169(14)00402-6 http://dx.doi.org/doi:10.1016/j.molcata.2014.08.045 MOLCAA 9260
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
19-6-2014 28-8-2014 28-8-2014
Please cite this article as: N. Li, Y. Jin, X. Hua, K. Wang, J. Xu, M. Chen, F. Teng, Uniform Fe2 O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.08.045 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.
Uniform Fe2O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity
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Na Li, Yujian Jin, Xia Hua, Kai Wang, Jingjing Xu, Mindong Chen, Fei Teng
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Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and
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Pollution Control, School of Environmental Sciences and Engineering, Nanjing
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University of Information Sciences and Engineering, Nanjing 210044, China
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Abstract
The xFe2O3/yBiOCl (x/y, molar ratio) composites are prepared via a facile
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hydrothermal process, in which the uniform Fe2O3 nanocubes are well deposited on
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the BiOCl nanosheets. The xFe2O3/yBiOCl shows much higher activities than BiOCl or Fe2O3 for the degradation of rhodamine B (RhB) and methyl orange (MO).
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Specifically, the degradation rate of the 5Fe2O3/100BiOCl catalyst is 7.2 and 3.7 times as high as those of the bare BiOCl for MO and RhB, respectively. This has been mainly attributed to the formation of p/n Fe2O3/BiOCl heterojunctions.
Keywords: Uniform Fe2O3 nanocubes; BiOCl nanosheets; p/n heterojunction; Photocatalysis
Corresponding author. Email: [email protected] (F. Teng); Phone/Fax: 0086-25-58731090 1
Page 1 of 17
1.
Introduction
BiOCl, as an efficient, inexpensive photocatalyst, has been widely studied in the degradation of organic pollutants [1-5]. Its layered structure favors for the separation
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of the photo-generated carriers, thus leading to a high activity [6]. So far, semiconductor coupling is usually used to further improve its performances, for
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example, BiOCl/Bi2O3 [7,8], PANI/BiOCl [9], BiOI/BiOCl [10], WO3/BiOI [11],
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NaBiO3/BiOCl [12], BiOCl/BiOBr [13], Bi2S3/BiOCl [14,15], Ag/AgCl/BiOCl [16], Ag/AgX/BiOX [17], BiOCl/Ag3PO4 [18], etc. Among the coupled materials,
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nevertheless, Ag, AgX, Ag3PO4 and BiOI are fairly expensive, which limits their practical applications. Thus, it is desirable to explore the low-cost coupling materials.
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Furthermore, some of the preparation methods are cumbersome, for example, using toxic solvents [9] or expensive surfactant [11,16,17], or finely controlling pH values
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highly needed.
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[7,8,12,14]. Hence, a simple, economical and environmentally friendly method is
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Herein, Fe2O3/BiOCl is prepared by a facile hydrothermal method. Fe2O3 is selected as a coupling component with the BiOCl on base of its following advantages: First, Fe2O3 is an abundant, low-cost and environmentally friendly chemical in nature, which is widely applied in magnetism, sensors and biologics and photocatalysis [19-25]. Secondly, its energy bands can well match those of the BiOCl, which favor for the transfers of photogenerated charges from one material to another. Thirdly, a stable heterojunction is easy to form between the n-typed Fe2O3 and the p-typed BiOCl semiconductors, which favors for the separation of photogenerated electrons and holes thus refrain effectively their recombination. Furthermore, more than one kind of pollutants (e.g., different organic compounds, heavy metal ions, alkaline or acidic substances, etc.) is generally contained in 2
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practical industrial wastewaters. It is needed to develop an efficient and inexpensive photocatalyst that can effectively degrade industrial wastewaters containing multiple organic dyes.
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Herein, the Fe2O3/BiOCl heterojunctions are, for the first time, prepared by a simple hydrothermal method. We have investigated the activities of the catalysts at
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different molar ratios of the Fe2O3 to BiOCl. Furthermore, our main attention is paid to their photodegradation performances for the simulated the RhB-MO mixture
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wastewater. Here RhB is considered to be a cationic dye while MO is an anionic dye.
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The influences of different dye degradation have been briefly discussed. This contribution is aimed to extend photocatalysis technology to the practical
Experimental
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2.
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applications.
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2.1. Synthesis of xFe2O3/yBiOCl sample
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The BiOCl employed was prepared according to previously reported method [26]. The preparation method of Fe2O3/BiOCl is as follows: 0.4 mmol of the FeCl3·6H2O was added into 50 mL of distilled water. After the FeCl3·6H2O was dissolved fully, 1 mmol of the as-prepared BiOCl powder was then added to the solution and stirred for 30-min by a magnetic stirrer. After that, the mixture was transferred into a Teflon-lined stainless steel autoclave and maintained at 160 °C for 12 h. The as-obtained sample was then washed with distilled water for several times and dried at 60 °C for 5 h. In order to achieve the optimum photocatalysts，the molar ratio of the Fe2O3 to BiOCl varies from 5/100, 10/100, 20/100, 30/100 to 40/100 in our experiment. Furthermore, the phase-pure Fe2O3 sample was also prepared under the
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same conditions without adding the BiOCl. Herein, the as-prepared pure Fe2O3, xFe2O3/yBiOCl and the phase-pure BiOCl samples were labeled as 0Fe/100Bi, 5Fe/100Bi, 10Fe/100Bi, 20Fe/100Bi, 30Fe/100Bi, 40Fe/100Bi and 100Fe/0Bi,
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respectively.
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2.2. Characterization
The crystal structures of the samples were determined by X-ray powder diffractometer
(Rigaku
D/max-2550VB),
using
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polycrystalline
graphite
monochromatized CuK radiation (λ= 0.154 nm), operating at 40 kV and 50 mA. The
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XRD patterns were scanned in the range of 20-80o (2θ) at a scanning rate of 5o min-1.
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The samples were characterized on a scanning electron microscope (SEM, Hitachi SU-1510) with an acceleration voltage of 15 keV. The samples were coated with
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5-nm-thick gold layer before observations. The fine surface structures of the samples
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were determined by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) equipped with an electron diffraction (ED) attachment with an
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acceleration voltage of 200 kV. The texture properties of the samples were measured by nitrogen sorption isotherms. The surface areas and pore size distribution of the samples
were
calculated
by
the
Brunauer-Emmett-Teller
(BET)
and
Barret-Joyner-Halender (BJH) methods, respectively. UV-vis diffused reflectance spectra (UV-DRS) of the samples were obtained using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).
2.3. Photocatalytic degradation reactions Photocatalytic activities of the samples were evaluated by photocatalytic decomposition of rhodamine B (RhB) and methyl orange (MO), respectively. Typically, 0.05 g of powder was respectively added into the solution (200 mL, 10 4
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mg/L), which was irradiated with a 300 W Xe arc lamp equipped with ultraviolet light (λ ≤ 420 nm). The suspension was stirred for 30 min to reach an adsorption–desorption equilibrium of dye molecules on the surface of photocatalyst.
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During photoreaction, 4 mL of suspension was collected at a given interval time and
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centrifuged to remove the particles. The concentration of dye remained in the solution
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was determined by using UV–vis spectrophotometer. For the RhB-MO mixture wastewater, 200 mL of 10 mg/L RhB and 10 mg/L MO are employed. Results and discussion
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3.
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3.1. Characterization of the samples
Fig. 1(a,b) shows the representative SEM image of the typical BiOCl sample.
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Moreover, these microspheres are well separated one another, although they have
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discrepant diameters. It is interesting that the microspheres are assembled by
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numerous nanosheets. Besides, the phase-pure BiOCl is obtained (Fig. 2). After being coupled with the Fe2O3, some of microspheres have broken into pieces, the others
microspheres have become loose, as shown in Fig. 1(c,d). It is perhaps on this account that the surface area of xFe/yBi samples is smaller than that of the pure BiOCl (Table 1). In Fig. 2, the diffraction peaks of the Fe2O3 can not clearly be observed for
10Fe/100Bi sample, which may be due to some diffraction peaks of Fe2O3 overlap with those of the BiOCl (e. g., the diffraction peaks at 33.5° and 54°).
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(b)
(c)
(d)
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(a)
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Fig. 1. Scanning electron microscopy (SEM) images of the xFe/yBi samples (x/y, the
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molar ratio of Fe2O3 to BiOCl) samples: (a,b) x/y = 0/100; (c,d) x/y = 10/100
Table 1 Texture properties of the xFe/yBi samples (x/y, the molar ratio of Fe2O3 to
BiOCl )
Sample
0Fe/100Bi 5Fe/100Bi 10Fe/100Bi 20Fe/100Bi 30Fe/100Bi 40Fe/100Bi
[a]
BET surface area (m2/g) 15.9 1.6 1.8 1.7 1.1 2.1
[b]
Pore volume (cm3/g) 0.056 0.009 0.009 0.015 0.012 0.023
[b]
Pore size (nm) 10.7 36.8 35.7 34.3 39.5 40.7
Notes: [a], calculated by the Brunauer-Emmett-Teller (BET) method; [b], calculated by the Barret-Joyner-Halender (BJH) method
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Fig. 2 shows the XRD patterns of the samples. At higher x/y values than 10/100 (molar ratios), besides the diffraction peaks of the BiOCl, the diffraction peaks of the Fe2O3 are observed. Moreover, the (110) peak intensity of the Fe2O3 increases
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gradually with increasing the amount of the Fe2O3 added. Furthermore, Fig. 3 displays the HRTEM images and SAED pattern of the 10Fe/100Bi sample. It is clearly
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observed from Fig. 3a that the cubic Fe2O3 nanoparticles uniformly distribute on the BiOCl nanosheets. It is clear that the sides lengths of the Fe2O3 cubes are about 30~60
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nm. Fig. 3b shows SAED pattern of the sample. The clear diffraction spots result from
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the (200) and (110) Bragg reflections of the BiOCl and the (110) Bragg reflections of the Fe2O3, respectively. Moreover, the interface between the Fe2O3 and the BiOCl is
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observed by HRTEM (Fig. 3c). The lattice spacing of 0.248 nm is well indexed to the (110) crystal plane of the Fe2O3, while that of 0.273 nm matches the (110) crystal
d
plane of the BiOCl. The HRTEM results firmly confirm the formation of xFe/yBi
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heterojunctions.
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(110)
Intensity (a.u.)
Standard Fe2O3
100Fe/0Bi
40Fe/100Bi
*
30Fe/100Bi
*
20Fe/100Bi
*
10Fe/100Bi 5Fe/100Bi
0Fe/100Bi Standard BiOCl
10
20
30
40
50
60
70
80
2 Theta (degree)
Fig. 2. XRD patterns of the xFe/yBi samples, standard Fe2O3 and BiOCl 7
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(b)
(a)
BiOCl (200)
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BiOCl (100)
Fe2O3 (100)
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5 1/nm
0.248nm Fe2O3 (110)
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(c)
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0.274nm BiOCl (110)
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d
5 nm
Fig. 3. High-resolution electron emission microscopy (HRTEM) images and selected
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area electron diffraction pattern (SAED) of the 10Fe/100Bi sample: (a) TEM; (b) SAED; (c) Lattice fringe images
0.25
0Fe/100Bi 5Fe/100Bi 30Fe/100Bi 100Fe/0Bi
Absorbance(a.u.)
0.20
0.15
0.10
0.05
0.00 200
300
400 500 600 Wavelength(nm)
700
800
Fig. 4. UV–vis diffuse reflectance spectra (UV-DRS) of the typical xFe/yBi samples
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Besides, Fig. 4 shows the UV–DRS spectra of the typical xFe/yBi samples. When the Fe2O3 were loaded on the surface of the BiOCl sample, the absorption ability of light was enhanced in the wavelength range of 340 – 540 nm. It was also found that
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the absorption of xFe/yBi samples in the ultraviolet light range apparently increased
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and a red shift appeared upon the addition of the Fe2O3. The absorption ability of light
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of the 30Fe/100Bi was greater than that of the 5Fe/100Bi, but the degradation rates of dyes were lower than that of the 5Fe/100Bi. The possible reason may be that too
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much amount of Fe2O3 would agglomerate or grow into large particles, resulting in the decease of active sites. The UV–DRS spectra demonstrate that the light absorption
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ability of the xFe/yBi samples has also been improved than BiOCl or Fe2O3, also
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d
favoring for the improvement of activity.
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3.2. Photodegradation activities of RhB and MO Under visible light irradiation, the xFe/yBi heterojunctions almost have no catalytic
activity (Fig. S1 of SI (Supporting information)), which may be closely relative to the visible light responsive Fe2O3. It has been reported that the photogenerated electron–hole pairs can not be separated easily but easily recombine again, due to the short diffusion length of photogenerated holes in the Fe2O3 [23,27,28]. Thus, all the photocatalytic activities of the samples were evaluated under ultraviolet light irradiation.
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1.1
(a)
1.0
(c)
1.0 0.9 0.8
0.8
0.3 0.2 0.1 0.0
0
20 Time/min
30
2
0
40 4
(b)
A B C D E F G
3
ln(C0/C)
10
A B C D E F G
3
1
5
2
10
15 20 Time/min
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0.0
0.4
25
30
cr
0.2
A B C D E F G
0.5
(d)
an
0.4
0.6
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C/C0
A B C D E F G
ln(C0/C)
C/C0
0.7
0.6
1
0
0 5
10 Time/min
15
20
0
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0
5
10 Time/min
15
20
d
Fig. 5. Degradation curves and kinetic curves of RhB and MO dyes over the xFe/yBi
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samples under ultraviolet light irradiation (≤ 420 nm): (a,b) RhB (200 mL, 10 mg/L); (c,d) methylene orange (MO) (200 mL, 10 mg/L); A: 0Fe/100Bi; B: 5Fe/100Bi; C:
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10Fe/100Bi; D: 20Fe/100Bi;
E: 30Fe/100Bi; F: 40Fe/100Bi; G: 100Fe/0Bi
Observed from Fig. 5(a,b) that the xFe/yBi heterojunctions have the improved
photocatalytic activities. Specifically, the 10Fe/100Bi sample exhibits the highest photocatalytic activity for the degradation of RhB among the samples. With increasing the amount of Fe2O3, the activity of the xFe/yBi catalyst decreases. It may
be that at a too high x/y ratio, more Fe2O3 nanoparticles have agglomerated and even separated from the surface of BiOCl (Fig. S3d). Furthermore, it has been reported [23,27,28] that the photogenerated electron–hole pairs of Fe2O3 are easy to recombine again. So the photocatalytic degradation efficiencies of the xFe/yBi samples decrease
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at too high Fe2O3 amounts. Further, the photodegradation of anionic mehylene orange dye (MO) is also investigated. In Fig. 5(c,d), the xFe/yBi heterojunctions also show the improved activity for the degradation of MO, compared with pure BiOCl.
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Moreover, the optimum molar ratio of Fe2O3 to the BiOCl is 5/100. Fig. 5b and 5d present the degradation reaction kinetic curves of RhB and MO, respectively. The
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xFe/yBi heterojunctions show the high reaction rates than pure BiOCl or Fe2O3. Table S1 summarize the calculated apparent rate constants (ka). For the decomposition of
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RhB, 10Fe/100Bi sample has the highest reaction rate among them, whose ka value is
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4 and 16 times higher than those of pure BiOCl and Fe2O3, respectively. For the degradation of MO, the ka value of the 5Fe/100Bi sample is the largest among them,
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which is 7 and 19 times higher than those of pure Fe2O3 and BiOCl, respectively. In order to determine whether the xFe/yBi heterojunctions can mineralization of
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dyes, the degradation of RhB and MO over the 0.5Fe/100Bi sample is typically
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characterized by TOC (Fig. S4 of ESI). The TOC removal for RhB and MO after 120 min of UV light irradiation is 49% and 53%, respectively. However, it does not reach
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a complete conversion to CO2 + H2O after 2 hours. Two possible reasons can be considered. One may be that more time is needed to complete the mineralization; the other may be that the dye has been decomposed into small organic molecule.
CB= -1.1eV
e
e e e
hv
VB=2.4eV
e e
e
CB=0.28eV
Eg=2.2eV
Eg=3.5eV
e
h+
hv
h+
BiOCl
VB=2.48 eV
Fe2O3 11
Page 11 of 17
Fig. 6. Schematic diagram of photogenerated carriers transportation over the xFe/yBi heterojuctions
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The activity of photocatalyst is strongly dependent on the generation and separation of photogenerated charges [29-31]. The transfer rate and the separation
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efficiency of interfacial charges can be effectively improved by the Fe2O3/BiOCl heterojunction. In Fig. 6, the conduction band (CB) (-1.1eV) of BiOCl lies above the
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CB (0.28 eV) of Fe2O3, and the valence band (VB) (2.48 eV) of Fe2O3 lies below that
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(2.4eV) of the BiOCl. Due to their matched energy bands, the stable heterojunction can form between n-typed Fe2O3 and p-typed BiOCl, leading to the formation of an
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internal electric field built in the Fe2O3/BiOCl interface. Driven by the internal electric field, the photogenerated electrons at the CB of BiOCl can migrate to the CB
d
of the Fe2O3, while the generated holes in the VB of Fe2O3 can move along the
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opposite direction to that of BiOCl. As a result, the recombination of photogenerated electrons and holes can be reduced effectively leading to a greatly improved
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photocatalytic activity, compared with BiOCl or Fe2O3. In order to further explore the reaction mechanism of the degradation of dye over the Fe2O3/BiOCl heterojunction,
the effects of various radical scavengers are studied. As shown in Fig. S5, the hydroxyl radicals is primarily responsible for the dye degradation.
4.
Conclusions
The uniform Fe2O3 nanocubes can be well dispersed on BiOCl nanosheets by a simple hydrothermal method. The Fe2O3/BiOCl composites exhibit fairly higher activities for RhB and MO dyes than BiOCl or Fe2O3. This is mainly attributed to the formation of a stable p/n Fe2O3/BiOCl, which can refrain greatly the recombination of 12
Page 12 of 17
photogenerated carriers.
Appendix
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Supporting materials can be available freely at http://www.elsevier.com
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Acknowledgements
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This work is financially supported by National Science Foundation of China (21377060, 21103049), Six Talent Climax Foundation of Jiangsu (20100292), Jiangsu
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Science Foundation of China (BK2012862), Jiangsu Province of Academic Scientific Research Industrialization Projects (JHB2012-10, JH10-17), the Key Project of
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Environmental Protection Program of Jiangsu (2013016, 2012028), the Project Funded by the Science and Technology Infrastructure Program of Jiangsu
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(BM2013139, 201380277), A Project Funded by the Priority Academic Program
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Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu Province Innovation Platform for Superiority Subject of Environmental Science and
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Engineering sponsored by SRF for ROCS, SEM (2013S002) and “333” Outstanding Youth Scientist Foundation of Jiangsu (2011015).
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Graphic abstract: Uniform Fe2O3 nanocubes on BiOCl nanosheets and its improved photocatalytic activity
0.8
much higher activities than
0.6
BiOCl or Fe2O3 for the
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The xFe2O3/yBiOCl shows
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1.0
degradation of rhodamine B
0.4
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C/C0
Na Li, Yujian Jin, Xia Hua, Kai Wang, Jingjing Xu, Mindong Chen, Fei Teng
RhB
and methyl orange
MO
0.2 0.0 10
20
30
an
0
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te
d
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Time/min
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Highlights 1.
The Fe2O3/BiOCl p/n heterojunctions are prepared by an in-situ hydrothermal
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method. The Fe2O3 nanocubes uniformly deposit on the BiOCl nanosheets.
3.
The Fe2O3/BiOCl heterojunctions have a higher activity than bare Fe2O3 or
cr
2.
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te
d
M
an
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BiOCl for the degradation of dye.
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Page 17 of 17