BiOCl microflowers

Journal of Colloid and Interface Science 546 (2019) 32–42

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Enhancing photocatalytic activity on gas-phase heavy metal oxidation with self-assembled BiOI/BiOCl microflowers Xiaoming Sun a,e,1, Jia Lu a,1, Jiang Wu a,b,⇑, Dayong Guan c, Qizhen Liu d, Naiqiang Yan e a

College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China c College of Electric and Information Engineering, Tongji University, Shanghai 200092, China d Shanghai Environment Monitoring Center, Shanghai 200030, China e Environmental Science and Engineering College, Shanghai Jiao Tong University, Shanghai 200240, China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 9 February 2019 Revised 13 March 2019 Accepted 14 March 2019 Available online 15 March 2019 Keywords: BiOI/BiOCl Microflower heterostructure Template-free coprecipitation Heavy metal Wet electrostatistic precipitator

a b s t r a c t A one-pot synthetic approach to prepare self-assembled BiOI/BiOCl microflowers by a template-free coprecipitation method at room temperature has been developed. The physicochemical structure of BiOI/BiOCl microflowers were characterized using transmission electron microscopy (TEM), high resolution TEM (HRTEM), scanning electron microscopy (SEM). The composition information and bonding energy structure of the BiOI/BiOCl microflowers were studied by X-ray diffraction (XRD) and highresolution X-ray photoelectron spectra (XPS), Fourier Transform Infrared Spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy (DRS) analysis and photoluminescence (PL) spectra. The photocatalytic performance of as-prepared BiOI/BiOCl microflowers was tested through photocatalytic oxidation of gasphase mercury, as a useful catalyst (or additive) in wet electrostatistic precipitator (WESP) to capture heavy metals including mercury. The results show that the prepared BiOI/BiOCl samples demonstrate higher photocatalytic efficiency than pure BiOI or BiOCl. By optimizing the component ratio of the BiOI and BiOCl, up to 72.2% oxidation efficiency can be achieved in BiOI/BiOCl microflowers. Finally, the photocatalytic influence of BiOI/BiOCl microflowers on gas-phase mercury oxidation had been proposed. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding author at: No. 2103 Pingliang Road, Shanghai 200090, China. E-mail address: [email protected] (J. Wu). These authors contributed to the work equally and should be regarded as co-first authors. 1

https://doi.org/10.1016/j.jcis.2019.03.049 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

With the unreasonable development of industry, environmental pollution become more and more serious [1]. The industrial pollution mainly including organic pollutant (VOCs, POPs etc.), inorganic

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

pollutant (NOX, SOX, dusts etc.) and hazardous heavy metal pollutant (mercury, arsenic, selenium and lead etc.) [2]. Heavy metals, different from organic pollutants and inorganic pollutants, are indestructible and undecomposable [3]. Even worse, heavy metal can concentrate and accumulate by food chain and absorbed by human body because of the human are the top of the food chain [4]. Heavy metal, for example the mercury, mainly emitted from petrochemical plant, coal-burning power plant and smelting plant due to its volatility and low solubility, are not achieve environmental protection request by the common air pollution control devices (APCDs) [5–7]. Mercury has three forms, particle mercury (Hgp), elemental mercury (Hg0) and bivalent mercury (Hg2+) [8]. Hgp and Hg2+ can be easily captured by wet electrostatic precipitator (WESP) and/or wet flue gas desulfurization (WFGD), but Hg0 is hard to control [9,10]. A feasible technical solution is to photocatalytically oxidize Hg0 by catalysts into divalent mercury Hg2+, and then capture it by APCDs [11–13]. Photocatalytic technology is green, high efficiency, cheap and no secondary pollution and is widely used in energy conversion and pollution control fields [14,15]. The traditional photocatalysts are TiO2 and ZnO, obtained a high energy bandgap (Eg), tardy electron transfer velocity and low light absorption efficiency, thus are not enough to meet the human needs of the energy conversion and pollution control [16]. The BiOX (X represent F, Cl, Br, I) crystal has a tetragonal lattice structure, consisting of alternating double halogen layers and (Bi2O2)2+ layers [17,18]. Therefore, BiOX compounds tend to form three-dimensional (3D) flower-like structures, which lead to more convenient on the photogenerated electron-holes pairs separated. The Eg of the BiOX can be controllable by the atomic number of the X (X represent F, Cl, Br, I). Due to elemental F and fluorine compound are highly toxic, BiOF is not the best candidates in common catalysts. BiOCl is obtained a broad Eg (3.46 eV) and highly oxidative activity but is hard to generated electron-hole pairs on the catalysts surface by the visible light [19,20]. BiOI can be generated by visible light, but is hard to photocatalytically oxidize Hg0 to Hg2+ because of the narrow Eg (1.85 eV) and the fast recombination of the electron-hole pairs [21,22]. It seems to adjust the concentration of the I
and Cl
to form BiOI/BiOCl heterostructure can attain an appropriate Eg and acceptable recombination velocity of the electron-hole pairs. Meanwhile, the alternating double halogen layers and (Bi2O2)2+ layers may make the I
and Cl
homogeneous growth, which can form a three-dimensional hierarchical architectures. The 3D hierarchical architectures may form porous structure, which can enhance the visible light absorbance, increase the active sites, prolong the photoinduced electrons transfer distance and reduce the rate of the recombination of the photogenerated electron-hole pairs. In the last few years, there are some articles studied the photocatalytic activity of the BiOI/BiOCl. For example, BiOI/BiOCl/RGO photocatalytic degradation of rhodamine B [23], exposed different facets on BiOI/BiOCl to degradation of methyl orange (MO) or rhodamine B [24–26], BiOI/BiOX (X = BiOBr or BiOCl) photocatalytic degradation of MO, rhodamine B or bisphenol-A [27–30], ions modified BiOI/BiOCl to photocatalytic degradation of rhodamine B [31], etc. It is clearly to see that most of these articles are focus on photocatalytic degradation of the organic pollutants (rhodamine B, MO and bisphenol-A) except the one synthesize the BiOI/BiOCl to photocatalytically oxidize nitric oxide (NO) [32]. In this work, BiOI/BiOCl microflowers was synthesized using a straightforward template-free coprecipitation method to photocatalytically oxidize Hg0. Detailed studies of the formation mechanism and characterization of BiOI/BiOCl heterojunctions were conducted. Meanwhile, the catalytic efficacy of the as-prepared BiOI/BiOCl microflowers on mercury oxidation was investigated. Moreover, the catalytic mechanism of BiOI/BiOCl microflowers was proposed based on these experimental and characterization results.

2. Experimental methods 2.1. Chemical and materials Bismuth nitrate pentahydrate (Bi(NO3)35H2O), Potassium iodide (KI), Sodium chloride (KCl) were all obtained from Guoyao Chemical Reagent Co. Ltd. 2.2. Synthesis of BiOI/BiOCl microflowers BiOI/BiOCl microflowers were prepared at room temperature through a simple template-free coprecipitation method. In a typical synthetic procedure, 5 mmol Bi(NO3)35H2O was suspended in 50 ml deionized water and was stirred vigorously until completely dissolved. KCl and KI were added to the above solution and continuously stirred for 5 h. Then the resulted products were centrifuged with deionized water and ethanol until no NO
3 . As shown in Table 1, depending on the molar ratio of KCl to KI (19:1, 9:1, 4:1, 2:1, 1:1), different BiOI/BiOCl were synthesized and donated as B-19, B-9, B-4, B-2, B-1, respectively. In addition, pure BiOI and BiOCl were prepared under the same condition without adding KI or KCl. 2.3. Photocatalytic tests The bench scale experimental system is shown in Fig. 1, which was similar to that used in our previous study [33]. The system included four major parts: source gas (N2), mercury generator, photocatalytic raction tank and mercury on-line analyzer. The N2 rate was controlled by mass flowmeter and passed through the mercury generator to introduce the Hg0 to the system. Then the flow passed through the photocatalytic reaction tank to photocatalytically oxidize the Hg0, the reaction flow passed through the mercury on-line analyzer to monitor the outlet Hg0 concentration. The bypass was set to monitor the inlet Hg0 concentration. In the experiment, the flow rate is 1.2 L/min, the reaction temperature is 40 °C, the environment temperature is 25 °C and the relative humidity is 80%. The initial Hg0 concentration was controlled at around 50–60 lg/m3. The photocatalyst (50 mg) was dissolved in ethanol, coated on a quartz glass sheet and then dried at 80 °C. The quartz slide was placed in the photocatalytic reactor and illuminated by visible light from an LED lamp. The photocatalytic oxidation efficiency was calculated as follows. Hg0in and Hg0out represented the mercury concentration at the outlet of the bypass and reaction path, respectively.

gð%Þ ¼

Hg0in - Hg0out Hg0in

 100%

ð1Þ

3. Results and discussions 3.1. Phase structure The XRD patterns of the prepared BiOI/BiOCl photocatalysts are shown in Fig. 2. The position of diffraction peaks corresponds to Table 1 Synthesis of BiOI/BiOCl composites. Samples

Cl/I(mol:mol)

Content of BiOI(%)

B-19 B-9 B-4 B-2 B-1 BiOCl BiOI

19:1 9:1 4:1 2:1 1:1 1:0 0:1

5 10 20 33.3 50 0 100

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

Fig. 1. Schematic diagram of the experimental system.

BiOI, diffraction peaks of the BiOI/BiOCl complexes are detected. The diffraction peaks exhibit a shift to lower angles, which may have resulted from the incorporate of I
into the BiOCl. The I
ionic radius is greater than that of Cl
(0.22 nm vs 0.18 nm), thus increasing the lattice constant. The introduction of I
ions leading to the shift of XRD peaks indicates the successful synthesis of the BiOI/BiOCl composites [24].

102

(a)

110

104 114

002

212

BiOI

Intensity (a.u.)

B-1 B-2 B-4

3.2. Morphology analysis

B-9 B-19 101 110 102 112 200 002

001

10

20

30

40

212

50

60

BiOCl

70

80

90

2θ (degree) 102

(b)

110

BiOI

Intensity (a.u.)

B-1 B-2 B-4 B-9

B-19 101

20

25

110

30

102

BiOCl

35

40

2θ (degree) Fig. 2. XRD patterns of the BiOI/BiOCl photocatalysts (a) and partial magnified details (b).

tetragonal phase BiOCl (JCPDS #06-0249) and tetragonal phase BiOI (JCPDS #10-0445) perfectly. Narrow sharp peaks indicate a high degree of crystallization in the synthesized samples [34]. With increasing BiOI amounts in the composites, the intensities of the (1 0 1), (1 1 0), (1 0 2), (2 1 2) peaks of BiOCl at 2h angles of 25.9°, 32.5°, 33.4° and 58.6° decreased gradually. In addition, as shown in Fig. 2a, the sample B-4 starts to have the (1 0 2), (1 1 0), (1 0 4), (1 1 4) and (2 1 2) peaks of BiOI at 29.6°, 31.7°, 45.7°, 51.3° and 55.2°. Fig. 2b is a partial pattern of the diffraction angle from 20° to 40°. As shown in Fig. 2b, after the addition of

SEM was used to study the surface morphologies of the BiOI/ BiOCl microflowers. As shown in Fig. 3a–h, all samples exhibit similar flower-like structures except the pure BiOCl exhibits irregular lamellar nanoplate structure with smooth surface. The XRD patterns indicate that BiOI nanosheets gradually formed with the increase of KI added in the precursors. However, it is difficult to distinguish BiOCl and BiOI nanosheets in the SEM figures. The formation of 2D nanosheets structure is mainly attributed to the internal structure of BiOX, i.e., the (BiO)2+ 2 layer is sandwiched between two layers of I
and Cl
atoms, leading to anisotropic growth of BiOI and BiOCl in a certain axial direction to form 2D nanosheets [35]. Nanosheets connect and aggregate at the center to form flower-like structures (Fig. 3(a)–(e)). The number of flower structures increases and the size of flowers reduces gradually as the amount of BiOI increases during the preparation process, due to minimization of total energy in the process of crystal growth. These primary nanosheets tend to self-assemble into flower-like structures in the absence of template agent [36,37]. To further analyze the morphology and crystallinity, B-9 and B1 specimens were selected to conduct TEM and HRTEM characterization. Fig. 4 shows the representative microstructures of B-9 and B-1. It is further revealed that the as-prepared samples have irregular flower-like architectures (Fig. 4a and b), which is consistent with the SEM observation. Fig. 4c and d show high-resolution TEM images of B-9. A lattice spacing of 0.275 nm is observed (Fig. 4d), which corresponds to the (1 1 0) plane of BiOCl. At the same time, the latticed fringe of 0.199 nm corresponding to the (1 0 4) plane of BiOI is observed, as shown in Fig. 4d. The crystal planes corresponding to the measured latticed fringes are consistent with XRD patterns, which indicates the co-existence of mixed phases of BiOCl and BiOI in the B-9 sample. As illustrated by EDS spectrum (Fig. 4e and f), O, Bi, I and Cl are detected in the B-9 and B-1 samples, which are consistent with the elements determined in the BiOI/BiOCl composites. The proportion of elements measured is also similar to the proportion of theoretical elements. EDS dot-mapping images of B-9 sample are shown in Fig. 5. In the flower-like structure, all theoretical elements are evenly distributed, indicating that all the elements in the sample are highly dispersed.

X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

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Fig. 3. SEM images of (a) B-19, (b) B-9, (c) B-4, (d) B-2, (e) B-1, (f) BiOCl, (g) BiOI, (h) low resolution of the B-9.

Based on the above discussion, we propose a three-step formation mechanism of BiOI/BiOCl microflowers, as shown in Fig. 6. In step 1, Bi3+ ions react with I
or Cl
to form BiOX nuclei, with O supplied by water. After the nucleation stage, the initial BiOX par-

ticles form a two-dimensional lamellar structure to reduce surface energy [38]. Lastly, according to the Ostwald Ripening mechanism, the relatively thin nanosheets crosslink and self-assemble to form microflowers [39].

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Elem O Bi I Cl

Nanocataly 53.1 25.9 2.17 18.83

O

Bi

I II I

0

2

4

6

8

10

Bi

Cl Bi

I Bi

12

Bi

14

16

I I

I

Bi

Energy (keV)

Nanocataly 50.85 25.79 12.01 11.35

Bi O

Bi Cl

Elem O Bi I Cl

(f) B5 B-1 (b)

Counts

Counts

(e)B2 B-9 (a)

0

2

4

Bi

6

8

10

Bi

Bi

Bi

12

14

16

Energy (keV)

Fig. 4. TEM images of (a) B-1 and (b),(c) B-9; HRTEM images of (d) B-9; EDS spectrum of (e) B-9 and (f) B-1.

3.3. XPS analysis In order to further investigate the surface chemical component and surface element valence state in surface chemical bonding of the photocatalysts, XPS characterization was carried out, as shown in Fig. 7a–e. As expected, the binding energy peaks of Bi, O, Cl, I are present in the survey spectrum of B-9. The extra C element is derived from the adventitious carbon in the instrument. As shown in Fig. 7b, two peaks at binding energy of 164.74 eV and 159.40 eV are attributed to Bi 4f5/2 and Bi4f7/2, respectively [30]. In the XPS spectra of O 1s region (Fig. 7c), it can peak split into two peaks. An obvious peak at 530.66 eV is assigned to the Bi-O bonds in (BiO)2+ slabs of the BiOX layers, and the other peak at 532.66 eV is attributed to surface hydroxyl groups [40]. As to the Cl 2p spectra (Fig. 7d), the peaks of Cl 2p3/2 and Cl 2d1/2 are located at 198.26 eV and 199.90 eV, indicating that Cl ions present a valence of
1 [41]. The peaks at 619.62 eV and 631.03 eV can be defined as I 3d5/2 and I 3d3/2 respectively, which can be attributed to the I
in BiOI/BiOCl [42].

3.4. FTIR analysis The FTIR spectra of BiOI/BiOCl with different BiOI and BiOCl contents are shown in Fig. 8. The peaks at around 1640 cm
1 and

3450 cm
1 can be assigned to the bond-stretching vibrations of OAH because of the adsorption of water molecules or the surface hydroxyl groups on the catalyst surface. The peak at about 520 cm
1 correlates to BiAO bond-stretching vibrations [43]. 3.5. Optical properties The band structure of photocatalysts is an important indicator of the semiconductors’ photocatalytic performance. UV–vis DRS spectra of the prepared samples are shown in Fig. 9. The pure BiOCl only exhibits an absorption edge in the UV light region, located at 363 nm. The pure BiOI shows a broad absorbance in the visible spectral region, and its absorbance edge is 691 nm. The energy bandgap of the semiconductor can be calculated by the equation: Eg = 1240/k. The Eg of BiOCl is 3.41 eV and the Eg of BiOI is 1.79 eV. 3.6. Photocatalytic activity of BiOI, BiOCl, and BiOI/BiOCl composites In this paper, self-assembled flower-like structure BiOI/BiOCl photocatalysts prepared by a one-pot template-free coprecipitation synthesis method is used for photocatalytic oxidation of the gas phase Hg0. As shown in Fig. 10(a), the adsorption efficiency of BiOCl is 6.2%. The adsorption efficiency of other samples is a litter higher than BiOCl and is approximately 9.3%. The reason of this

X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

37

Fig. 5. EDS dot-mapping images of B-9 sample.

may be that BiOCl is nanoplate structure and other samples are microflower-like structure. The microflower-like structure has a 3D stereochemical structure obtained higher specific surface area than the 2D nanoplate structure result in higher adsorption efficiency. The photocatalytic efficiency of pure BiOI and BiOCl samples are 16% and 12% respectively at 18 W and 420 nm LED illumination, which is unsatisfactory. Compared with the pure samples, BiOI/BiOCl composite photocatalysts show improved photocatalytic activities. The photocatalytic efficiency of composite photocatalysts change with different BiOI content. Among them, the B-9 sample shows the best photocatalytic performance, reaching up to 72.2%.

In addition to photocatalytic activity, the stability of photocatalyst is crucial for their practical application. Under constant conditions, the B-9 sample was applied as a representative sample to perform a cyclical test for removing Hg0 to study its stability. In each cycle, the B-9 sample loaded on the quartz plate was exposed to visible light for the first cycle of the experiment. After each set of experiments, the LED lights were turned off after the mercury concentration was stabilized, and then the next set of experiments was started when the mercury concentration reached up to a stable value again. It can be clearly seen from Fig. 10(b) that BiOI/BiOCl composite photocatalysts shows little deactivation after six cycles of testing. It still exhibits superior photocatalytic performance,

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

Bi

Bi

O

Cl

I

Schematic representation of the crystal structure of BiOX

Cl I Nucleation process

Nanosheets

self-assembly

Fig. 6. Schematic illustration of the possible formation process of BiOX microflowers.

which confirmed that B-9 possesses high stability. XPS analysis was conducted to investigate the combination property between the mercury and the catalysts, as shown in Fig. 10(c) and (d). In Fig. 10(c), both fresh and used BiOI/BiOCl composites were analyzed. The chemical composition of BiOI/BiOCl showed no change after photocatalytic oxidation of Hg0, indicating that the asprepared BiOI/BiOCl has a high photocatalytic stability. The Hg 4f spectra of the used BiOI/BiOCl composites were recorded, which showed that some oxide mercury is absorbed on the surface of the photocatalysts. As shown in Fig. 10(d), in the spectrum of the Hg 4f, the peaks at about 100.93 and 105.26 eV can be ascribed to the HgAO bond. This indicated that the Hg0 did not directly react with I
or Cl
to form the HgI2 or HgCl2, but was oxidized by superoxide radical or free hydroxyl to form HgO. Under normal circumstances, the semiconductor photocatalysts could generate electrons and holes after illumination, and these photo-generated carriers are easily recombined in the photocatalysts [44]. The recombined energy is emitted by fluorescence. The photoluminescence (PL) characterization is conducted to investigate the transfer and separation efficiency of the photogenerated electron hole pairs. Lower peak intensity of the PL spectral curve correlates with higher photo-generated carriers migration and separation efficiency, and vice versa. PL spectra of BiOI, BiOCl, and BiOI/BiOCl composites are shown in Fig. 11. Compared with BiOI and BiOCl, BiOI/BiOCl nanocomposites exhibit lower emission intensities of PL spectra, which proves that the BiOI/BiOCl nanocomposites have excellent effect on inhibiting photoelectron and electron recombination. The B-9 sample (10% BiOI/BiOCl) has the weakest emission intensity, indicating that it has the strongest potency of suppressing photo-generated electrons and holes. The above conclusions are consistent with the results of photocatalytic performance tests. 3.7. Mechanism of improved photocatalytic activities The photocatalytic properties of semiconductors are influenced by crystal morphology, light absorption, energy band structure and reactive oxygen species. 3.7.1. Crystal morphology In this paper, the photocatalytic activities of BiOI/BiOCl nanocomposites are higher than that of single component, which indicates that the flower-like structure of the composite photocatalysts contribute to the improvement of photocatalytic perfor-

mance. With the increase of BiOI content, self-assembled flowerlike structures gradually increase, which lead to an increase of heterojunction interfaces and photocatalytic active sites. Visible light can be reflected by the surfaces of the flower-like structures, which could enhance utilization of light. When the ratio of BiOI is increased, the photocatalytic effect is reduced. An explanation may be that the excessive BiOI has a narrow band gap as the recombination center of electrons and holes, thus inhibits further improvement of photocatalytic performance. Therefore, the microflower structure is helpful for the BiOI/BiOCl composites to absorb visible light and enhance the separation efficiency of the photo-induced electron-hole pairs. 3.7.2. Light absorption It is well known that the photocatalytic reaction mainly depends on the generation of photogenerated electron hole pairs in the valence band and conduction band of catalyst. The UV–vis DRS spectra confirmed that BiOI can absorb visible light, but BiOCl can not absorb visible light. However, BiOI has a narrow band gap, resulting in a low redox potential, while the BiOCl has a high redox potential because of its broader band gap. The BiOI/BiOCl flowerstructure composites are formed through recombining the BiOI and BiOCl, so that the optimal redox potential is obtained. Hence, the photocatalytic activity of the BiOI/BiOCl composites is higher than that of the single BiOI or BiOCl. 3.7.3. Energy band structure Some of the photoinduced electrons and holes can migrate to the surface of the catalyst and react with the adsorbed reactants, while the other ones recombine within the catalyst. Therefore, the key factors affecting the photocatalytic activity are the effective separation and decrease the velocity of recombination of photogenerated carriers. The self-assembly structure between BiOI and BiOCl as a p-type semiconductor play an important role of excitation and effective separation of electron holes under visible light. The heterostructure formed by the BiOI/BiOCl nanocomposites can effectively promote the separation of photogenerated electrons and holes, and has better photocatalytic performance than singlecomponent samples, which is confirmed by the PL characterization. 3.7.4. Reactive oxygen species Reactive oxygen species, such as superoxide radicals (O2
) and hydroxyl radicals (OH) have strong oxidative ability and take cru-

X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

39

Fig. 7. XPS spectra of B-9: (a) survey spectrum; (b) Bi 4f; (c) O1s; (d) Cl 2p; (e) I 3d. (All the peak maxima were calibrated to C 1s at 284.60 eV, which was mainly attributed to remnant organic precursors not completely removed from the BiOI/BiOCl heterostructure.)

cial role in the photodegradation reactions [45,46]. The Eg of BiOI is 1.79 eV, thus is easily excited by the visible light (k > 420 nm, energy less than 2.95 eV) to generate the electron-hole pairs [47]. Under the visible light irradiation, the photogenerated electrons (e
) accumulate on the conduction band of BiOI can reduce the O2 to produce a few amounts of the O2
according to the electron transport theory. BiOI cannot reduce the H2O or OH
to OH which is testified by our previous report and others research [48–50]. For BiOCl, the Eg is 3.41 eV, thus it cannot generate the electrons-hole pairs by visible light. However, when BiOI hybrid with BiOCl forming the interface heterostructure can efficiently enhance the photoactivity. The photogenerated electrons and holes can transfer

between BiOI and BiOCl, result in O2 is reduced to O2
and H2O or OH
is reduced to OH. The O2
and OH can oxidize the Hg0 to Hg2+. A proposed mechanism for BiOI/BiOCl photocatalytically oxidize Hg0 is shown in Fig. 12. BiOI can excited by the visible light to generate the electron-hole pairs. Although the sandwich structures can help reduce the recombination rate of the photo generated carriers, the distance between EVB and ECB is so short that the photo generated carriers recombine to the EVB or ECB before the photo generated carriers react with the absorbed Hg0. Thus, the pure BiOI photocatalytic activity is not good. On the other hand, the BiOCl obtained a good oxidability on Hg0 oxidation pro-

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

Fig. 8. FTIR spectra of different samples: B-9 and B-1, BiOCl, BiOI.

cess because of the broad Eg. But a broad Eg indicates that the BiOCl cannot generate by the visible light, which inhibit its photocatalytic activity. Therefore, Hg0 cannot be effectively oxidize by

Fig. 9. UV–vis diffuse reflectance absorption spectra of the BiOCl and BiOI.

BiOCl. The BiOI/BiOCl heterostructures have more suitable bandgap energy than the BiOI and BiOCl component samples. When BiOI/BiOCl microflowers were illuminated by visible light, BiOI

Fig. 10. (a) Photocatalytic efficiency of gas-phase Hg0 under 18 W LED light irradiation by as-prepared photocatalysts; (b) cycle test efficiency diagram of the B-9. (c) XPS survey spectra of fresh and used BiOI/BiOCl composites. (d) Hg 4f spectra.

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42 þ

BiOI=BiOCl þ hv ! BiOI=BiOClðh þ e
Þ

ð2Þ

OH
+ hþ !  OH

ð3Þ

O2 + 2Hþ + 2e
! H2 O2

ð4Þ

H2 O2 + hv ! 2 OH

ð5Þ

O2 (g) + e
!  O2
(ad)

ð6Þ

Hg0 (g) ! Hg0 (ad)

ð7Þ

3Hg0 (ad) + 2 O2
(ad) + 2Hþ ! 3HgO(ad)+H2 O

ð8Þ

Hg0 (ad) + 2 OH(ad) ! HgO + H2 O

ð9Þ

Fig. 11. Photoluminescence spectra of the BiOI/BiOCl, BiOI and BiOCl.

could act as a sensitizer and electrons in the valence band of BiOI could be excited up to a higher potential edge. Thus, the valence band and conduction band position of BiOI are more negative than that of BiOCl. The photogenerated electrons will transfer from conduction of BiOI to the conduction of BiOC, leaving the holes on the BiOI valence band. Meanwhile, due to the photosensitizing effect of BiOI, the photogenerated holes will transfer from the valence of BiOCl to the valence of BiOI. Holes are enriched in the valence band of BiOI and electrons are enriched in the conduction band of BiOCl. The photogenerated carriers could be effectively used. High-energy electrons migrate to the ECB of the BiOCl and react with O2 adsorbed on the surface of the BiOI/BiOCl photocatalyst to form  2
O . Photo-generated holes in the EVB of BiOI can form OH with surface hydroxy, and then the highly active OH and O2
oxidize Hg0 to Hg2+. Due to these effective separation of photoelectrons and holes and their low recombination rates, the BiOI/BiOCl heterostructures exhibit better photocatalytic performance in the removal of Hg0 under visible light than BiOI and BiOCl. The mechanism of photocatalytic oxidation can be described as follows.

4. Conclusions In summary, self-assembled BiOI/BiOCl microflowers were synthesized using a template-free coprecipitation method at room temperature. BiOI/BiOCl microflowers possesses visible light absorption under LED light illumination. Under the irradiation of an 18 W LED lamp, the as-prepared BiOI/BiOCl microflowers showed higher photocatalytic efficiency than pure BiOI or BiOCl. The excellent photocatalytic activity of the BiOI/BiOCl microflowers could be attributed to the self-assembled structure, the favorable band gap and the formation of BiOI/BiOCl heterojunction interface, which could greatly enhance the composites visible light excitability, enhance the transfer rate of the photocarriers and inhibit the recombination of the photo electron-hole pairs. This work could provide some insight into the controllable synthesis of nanosheet-assembled structures to photocatalytically oxidize inorganic pollutants.

Fig. 12. The mechanism of photocatalytic removal of gas-phase Hg0 under visible light.

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X. Sun et al. / Journal of Colloid and Interface Science 546 (2019) 32–42

Acknowledgments This work was partially sponsored by National key research and development program (2018YFB0605103).

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