BiOBr Composites with

Subscriber access provided by NEW YORK MED COLL

Article

Microwave-assisted synthesis of BiOCl/BiOBr composites with improved visible-light photocatalytic activity Shujuan Zhang, and Jinfeng Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on September 30, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Microwave-assisted synthesis of BiOCl/BiOBr composites with improved visible-light photocatalytic activity Shujuan Zhang*, Jinfeng Yang College of Science, Tianjin University of Science & Technology, Tianjin, 300457, P.R. China *Corresponding author. Tel./Fax: +86-22-60600656 E-mail address: [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT BiOCl/BiOBr composite photocatalysts with different Cl-to-Br molar ratios were synthesized by a simple microwave-assisted coprecipitation (MWAC) method. The structures, morphology and photocatalytic properties of the samples were characterized by X-ray diffraction, scanning electron microscopy, and UV–vis absorption spectrometry. The photocatalytic activities of these BiOCl/BiOBr composites were evaluated by their ability in degradation of Rhodamine B under visible-light

irradiation.

Results

reveal

that

the

MWAC-synthesized

1:5

BiOCl/BiOBr composite is much more active than that synthesized by the hydrothermal method. The former composite obviously maintains the high photocatalytic activity after five cycles and thus is the best one among all these photocatalysts.

The

visible-light

photocatalytic

activity

enhancement

of

BiOCl/BiOBr composites could be attributed to their strong absorption in the visible region and the low recombination rate of electron-hole pairs. Among different scavengers, the main active species during the photocatalytic process are h+ and •O2−. The possible photocatalytic mechanism was proposed based on the experimental results. Keywords: Microwave; BiOCl/BiOBr; Photocatalytic; Rhodamine B 1 Introduction Semiconductor photocatalysis has been regarded as the most potential resolution to environment purification and solar energy conversion in the past years.1-4 As a traditional photocatalyst, TiO2 can be activated by UV irradiation ( λ < 390 nm).5 However, its efficiency in solar energy conversion is limited by its large band gap (3.2 eV).1-6 So far, composite photocatalysts may be more photocatalytically active than a single photocatalyst, as they can reduce the recombination and enhance the separation of electron-hole pairs.7-9 Hence, works should been done to enhance the efficiency of composite photocatalysts. Bismuth-based oxyhalides (BiOX, X=Cl, Br, I) have attracted growing attention as potential application in photocatalytic degradation of organic pollutants. To further improve the photocatalytic efficiency of BiOX (X = Cl, Br and I), efforts 2


Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


should be devoted to develop novel photocatalysts.10-11 BiOX/BiOY (X, Y=Cl, Br, I) composites are much higher photocatalytically active than single BiOX.1,12,13 Because BiOCl/BiOBr generates more photoexcited electron–hole pairs and gains stronger photocatalytic activity, synthesis of BiOCl/BiOBr can improve the photocatalytic activity. The traditional synthesis methods include chemical etching,1 hydrothermal (HT) method,9 deposition–precipitation,13and ethylene glycol- assisted solvothermal process.12 However, these methods are usually limited by longer time consumption and higher operational difficulty. Microwave-assisted coprecipitation (MWAC) should be one novel method for preparation of heterostructured BiOCl/BiOBr photocatalysts. This method attracts many researchers due to its simpleness, cleanness, low cost, low energy consumption, environmental friendliness and enormous potential for pollutant degradation.15-16 Microwave energy can enhance organic chemical reaction in the heating process. The merits of this method are controlling the simultaneous growth of crystals and the recombination of interparticles by the microwave heating.17 In this study, a simple MWAC approach was used to prepare BiOCl/BiOBr composite for the first time. The photocatalytic activities of the obtained samples were evaluated via the ability in degradation of Rhodamine B (RhB) under visiblelight irradiation. The structures of the BiOCl/BiOBr composites were also characterized. Results indicate that the 1:5 BiOCl/BiOBr is much more photocatalytically active than the single-phase pure BiOCl or BiOBr powder. The possible mechanism of photocatalytic activity enhancement in BiOCl/BiOBr was proposed on basis of characterization results. 2 Experimental 2.1 Preparation BiOCl/BiOBr powder composites of different ratios were synthesized by a simple MWAC method. All the chemicals were analytical grade and used as received without further purification. In

a

novel

procedure,18

cetyltrimethylammonium

chloride

(CTAC)

and

cetyltrimethylammonium bromide (CTAB) (with Br/Cl molar ratio of 1, 2.5, 5, 7.5, 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

or 10) were added in order and slowly into an ethylene glycol (EG) solution containing stoichiometric amounts of Bi(NO3)3·5H2O. The mixtures were constantly stirred at 40 ℃ and in air for 1 h. Then the resulting mixtures were transferred into a JK-MCR-205 microwave reactor for 10 min with the stable operation power of 200 W, and then cooled down to room temperature. The microwave system has the possibility of continuous microwave irradiation to enhance the temperature. The final product in each mixture was separated by centrifugation with rotating speed of 7500 r/min. The resulting precipitates were washed with deionized water and absolute ethanol for three times, and vacuum-dried at 60 °C. For comparison, pure BiOCl or BiOBr powder was synthesized by the same method, while the 1:5 BiOCl/BiOBr (HT) were synthesized as follows: the corresponding mixtures were transferred

into 100 mL Teflon-lined autoclave up to 80% of the total. The autoclave was then heated at 140 ◦C for 8 h and cooled down to room temperature.5 The final product was separated as the above steps. The as-prepared samples were used for

further

characterizations. The Br/Cl molar ratio of the final BiOCl/BiOBr composites were detected by chromatography ( DIONEX ICS-1100) . The result displayed that the actual Br/Cl molar ratio is closed to 1.4, 2.2, 6.3, 8.1, 9.6. 2.2 Characterization All samples were analyzed by an X-ray powder diffraction meter (Equipment model XRD-6100, Shimadzu) with high-intensity Cu-Kα (λ=1.5406 Å). The morphologies of BiOCl/BiOBr samples were observed by a scanning electron microscope (SEM, Hitachi). Ultraviolet–visible (UV–vis) absorption spectra were recorded on an HP8453 spectrophotometer with BaSO4 used as a reference and at wavelength of 200–800 nm. The synthetic method of samples was by the microwave reactor. 2.3 Measurement of photocatalytic activity Photocatalytic activities of the BiOCl/BiOBr photocatalysts as-prepared were evaluated by testing their ability in degradation of Rhodamine B (RhB) under visible light irradiation in a photocatalytic reactor. The visible light was irridated from a 300 4


Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


W Xe lamp. In each photocatalytic test, 20 mg of a BiOCl/BiOBr composite was dissolved in 200 mL of an RhB solution (20 mg/L). The suspension was magnetically stirred in the dark for 30 min to achieve adsorption–desorption equilibrium. Under visible light irradiation, 4 mL from the suspension of RhB and catalyst was collected every 10 min and separated by centrifugation. The solution after photodegradation was detected by the UV-vis spectrophotometer at the absorbance of 554 nm. According to the Langmuir-Hinshelwood (L-H) kinetic model, the experimental data were sent into a kinetic equation to describe photocatalytic RhB degradation:19

kapp = −

ln(c/ c 0 ) t

(1)

3 Results and discussion 3.1 Structural characterization of catalysts The XRD patterns of the BiOCl/BiOBr composites with different Cl/Br ratios are shown in Fig. 1. Clearly, all the catalysts are highly crystallized. BiOCl (JCPDS 06-0249) and BiOBr (JCPDS 73-2061) with tetragonal structures and high purity can be observed, because no other specific diffraction peak was detected, which observation is in good consistent with the literature20. The 1:5 BiOCl/BiOBr composite (HT) shows the same result. The diffraction peaks of BiOCl/BiOBr composites possess BiOBr and BiOCl two-phase substance. With the addition of BiOBr, the characteristic peaks of BiOCl were weakened in the diffraction patterns of BiOCl/BiOBr compounds. These results were caused by the fact that BiOBr particles inhibited the crystal growth of BiOCl. Figure 2 displays the SEM images of all the tested samples. Figure 2a and b shows that BiOCl and BiOBr are composed by microstructure with the size about 500 nm and hierarchical microspheres (diameter about 0.8-1 µm). In comparison, the SEM images of BiOCl/BiOBr composites with different Cl/Br ratios are composed by hierarchical microspheres and a few irregular structures (Fig. 2c-g). Figure 2h shows hierarchical microspheres (diameter of 2–3 µm) and some nanostructures. The loose structure of BiOBr particles may result in larger surface area and enhanced 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

catalytic activity. 3.2 Optical absorption of catalysts The results of BiOCl/BiOBr optical characters with different Cl/Br ratios in Fig. 3 were detected by UV–vis absorption spectrometry. Results show that the pure-phase BiOCl exhibits strong absorption at λ

BiOBr Composites with

Recommend Documents