SDS-assisted solvothermal synthesis of BiOBr microspheres with highly visible-light photocatalytic activity

Author’s Accepted Manuscript SDS-assisted solvothermal synthesis of BiOBr microspheres with highly visible-light photocatalytic activity Yang Zhao, Xin Tan, Tao Yu, Shucong Wang www.elsevier.com

PII: DOI: Reference:

S0167-577X(15)30804-1 http://dx.doi.org/10.1016/j.matlet.2015.10.155 MLBLUE19811

To appear in: Materials Letters Received date: 27 January 2015 Revised date: 26 October 2015 Accepted date: 31 October 2015 Cite this article as: Yang Zhao, Xin Tan, Tao Yu and Shucong Wang, SDSassisted solvothermal synthesis of BiOBr microspheres with highly visible-light photocatalytic activity, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.10.155 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 galley proof before it is published in its final citable 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.

SDS-assisted solvothermal synthesis of BiOBr microspheres with highly visible-light photocatalytic activity Yang Zhaoa，Xin Tanb, Tao Yua,c,d*, Shucong Wange a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Science, Tibet University, Lhasa 850000, PR China c Tianjin University-National Institute for Materials Science Joint Research Center, Tianjin University, Tianjin 300072, PR China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P.R. b

China e

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China

*Corresponding author. Tel.:/Fax +86 2223502142. E-mail addresses: [email protected], [email protected] (T.Yu). Abstract Hierarchical BiOBr microspheres were firstly synthesized via a facile solvothermal route in the presence of sodium dodecyl sulfate (SDS). The products were characterized by XRD, XPS, FESEM and TEM. The results showed that the BiOBr microspheres between 2 μm and 4 μm in diameter were constructed by interlaced nanoflakes with the thickness of about 25 nm, and the 2D nanoflakes were composed of nanocrystals of about 8 nm in size. Photocatalytic activity of the as-prepared BiOBr was evaluated by the photocatalytic degradation of RhodamineB (RhB) under visible light irradiation. Within 30 min, the degradation rate reached almost 100%. Keywords: Bismuth oxybromide; Photocatalytic activity; SDS; Microstructure; Solvothermal; X-ray techniques

1. Introduction As alternative photocatalysts, Bi-based compounds have attracted considerable attention owing to their low toxicity, earth abundance, strong visible light absorption and excellent photocatalytic activity. Bismuth oxyhalides BiOX (X=Cl, Br, I), as an important category of Bi-based compounds, have layered tetragonal matlockite structure with [Bi2O2] slabs interleaved by double slabs of halogen atoms along the [001] direction. The internal static electric fields between the [Bi2O2]2+ and halogen anionic layers are believed to improve the separation of photo-generated electron-hole pairs [1]. Among bismuth oxyhalides, BiOBr is an active and stable catalyst with the band energy of ~2.7 eV morphologies of BiOBr, including 2D nanosheets

[3-6]

[2]

. To date, various

and 3D hierarchical architectures [7-11], have been

successfully synthesized. Compared with 2D nanostructures, 3D architectures could improve light harvesting, shorten diffusion pathways and increase the reactive sites of semiconductors, hence improve their photocatalytic efficiencies

[2]

. Organic solvents, ionic liquids (ILs) and surfactants have

been used to tailor the self-assembly process of the BiOX (X =Cl, Br, I) hierarchical architectures. It was found that cationic surfactant CTAB

[4, 6, 12-13]

and nonionic surfactant PVP [14-15] have been applied

to the morphology control of BiOX. However, to the best of our knowledge, it is still rare in the literature about the preparation of BiOX with the assistance of anionic surfactant sodium dodecyl sulfate (SDS). Herein, we report for the first time the synthesis of BiOBr hierarchical microspheres in the presence of SDS via a facile and simple solvothermal route. The effects of SDS on the morphology of BiOBr 1

were investigated. In addition, the photocatalytic activity of as-prepared BiOBr was evaluated by the photodegradation of RhB under visible light irradiation.

2. Experimental All the chemicals were of analytic grade and used without further purification. In a typical synthesis, Bi(NO3)3·5H2O (5 mmol) and KBr (5 mmol) were added into 20 mL of EG respectively, then sonicated until well dissolved. After that, the two kinds of solution were mixed, followed by the addition of 0.2 g SDS under vigorous stirring. The final transparent solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 100℃ for 8 h. After cooling down to room temperature (ca. 30℃) naturally, the obtained white precipitate was collected by centrifugation and washed with water and ethanol several times. Finally, the product was dried at 60℃ for 6 h. Additionally, the contrast sample was obtained without the addition of SDS under the same conditions. The as-prepared samples were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2500) with monochromatized Cu Kα radiation (λ = 0.1542 nm). The scanning rate was 0.06°s-1 and scanning range was 5º– 80°. X-ray photoelectron spectroscopy (XPS) was employed for the measurement of composition and chemical states of the samples. Field emission scanning electron microscope (FESEM, FEI Nanosem 430) was used to observe the morphology of the samples with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were taken over a Tecnai G2 F20 transmission electron microscope with an accelerating voltage of 200 kV. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of RhB under visible light irradiation from a 300 W Xe lamp (HXS-F/UV 300, Beijing NBet Technology Co., Ltd) with a 420 nm cutoff filter. Typically, 0.05 g photocatalyst was added into 50 mL RhB aqueous solution (10 mg L-1). Before illumination, the suspension was magnetically stirred in the dark for 30 min to ensure the adsorption/desorption equilibrium. During the degradation, about 4 mL of the suspension was taken and centrifuged every 5 min for the following analysis. The dye concentration of each sample was determined using a UV-vis spectrophotometer (T6, Beijing Purkinje General Instrument Co., Ltd).

3. Results and discussion The purity and crystallinity of the as-synthesized BiOBr were investigated by powder XRD analysis, as shown in Fig. 1. It can be seen that all the diffraction peaks of the two samples are perfectly indexed to the tetragonal phase of BiOBr (JCPDS Card No. 09-0393) with lattice parameters a = b = 3.926 Å, c = 8.103 Å. No other diffraction peaks are found, indicating that both the prepared samples have high purity and single phase. The peaks of the sample with SDS are broadened relative to those without SDS, which may be ascribed to the small crystalline size of BiOBr. The full-width at half-maximum (FWHM) of the (110) plane after subtracting the instrumental broadening was applied in Scherer formula to estimate the average size of crystallites. The crystal sizes of the as-synthesized BiOBr in the absence and presence of SDS are about 13 nm and 9 nm, respectively.

2

20

(214)

(310)

(204) (220) (204) (220)

(214)

(212) (105) (105)

(211) (114)

(200)

40

50

60

(310)

(212) (104) (211) (114)

(200) (004)

(112)

(111)

30

(104)

(004)

(102)

(110) (112)

(110)

(111)

(101) (101) (002)

(001)

(102)

(002)

(001)

intensity(a.u.)

10

with SDS without SDS

70

80

2degree

XPS analysis shown in Fig. 2 was used to investigate the composition and chemical states of the as-prepared BiOBr. The relevant XPS peak position is calibrated against the C1s signal of contaminant carbon at a binding energy of 284.6 eV. From Fig. 2a, it can be seen that only Bi, O, Br and a trace amount of C are detected, confirming the composition of the samples. Two strong peaks at 159.06 eV and 164.28 eV (Fig. 2b) are readily assigned to Bi 4f7/2 and Bi 4f5/2, respectively, which are corresponding to Bi3+. The peak at the binding energy of 68.19 eV (Fig. 2c) is ascribed to Br 3d, which is the characteristic of Br-. Fig. 2d and e show O 1s peaks of the samples obtained in the absence and presence of SDS, respectively. The O 1s signal in Fig. 2d can be fitted by two peaks at 530.0 eV and 532.5 eV; the former is ascribed to lattice oxygen atoms in BiOBr crystal and the latter to surface hydroxyl(or crystal water)[16]. It should be noted that the O 1s signal in Fig. 2e is fitted to a superposition of three Gaussian components at 529.9 eV, 531.4 eV and 532.7 eV, because attempts to resolve the signal into only two peaks resulted in an unreasonably large full-width half-maximum (FWHM > 2 eV). The peak at 531.4 eV can be attributed to the O-atoms in the vicinity of an O-vacancy [17-18]. The others located at 529.9 eV and 532.7 eV, similar to those in Fig. 2d, are deemed as the oxygen in BiOBr and M-OH (or H2O), respectively. Hence, the results of XRD and XPS confirm that pure BiOBr can be obtained with the proposed method.

a -Bi 4d5 -Bi 4d3 -O 1s

200000

100000

b 159.06 eV Bi 4f 7/2

Counts per second

-O KLL

-Bi 4p3

Bi 4f5 -Br 3s -C 1s

-Br 3p

300000

100000

Survey

-Bi 5d -Br 3d

Counts per second

Bi 4f7

400000

80000

164.28 eV Bi 4f 5/2

60000

40000 20000

0 0

200

400

600

800

Binding energy/eV

1000

0 154

156

158

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166

168

Binding energy/eV

3

15000

c

38000

Counts per second

68.19 eV

Counts per second

d

Br 3d

12000

9000

6000

36000

O 1s without SDS

34000 M-O 32000

H-O

30000 28000

60

62

64

66

68

70

72

74

76

Counts per second

44000

520

524

528

532

536

540

Binding energy/eV

Binding energy/eV

e

42000

O 1s with SDS M-O

40000 Ovac 38000

H-O

36000 34000 32000 520

524

528

532

536

540

Binding energy/eV

The morphology and microstructure of the products were investigated by FESEM and TEM. In the absence of SDS, the product is almost entirely composed of flower-like structures with the diameter of 3–5μm (Fig. 3a). Fig. 3a inset shows an individual micro-flower assembled by many square slices with even thickness of about 160 nm. When SDS is added, the as-prepared BiOBr is of sphere-like hierarchical structure with the diameter from 2μm to 4μm (Fig. 3b). High magnification image (Fig. 3b inset) reveals that the microsphere is constructed by plenty of nanoflakes. Further observation shows that the 2D nanoflakes with a thickness of about 25 nm are densely packed with each other. It can be concluded that the addition of SDS is a key factor for the formation of microsphere structure. Further observation from HRTEM image (Fig. 3c) reveals that the slices of the flower-like structure are composed of many nanocrystals with a narrow size distribution of 10–12 nm. HRTEM image(Fig. 3d) taken from the edge of a microsphere also reveals that the nanoflakes are composed of smaller nanocrystals of ~8 nm in size relative to the product without SDS , which agrees well with the XRD results. The corresponding SADE (Fig. 3d inset) shows the polycrystalline nature of the as-prepared BiOBr. Based on the TEM results, we infer that the formation of nanosheets can be ascribed to imperfect oriented attachment (OA). OA is described as spontaneous self-organization of adjacent particles so that they share a common crystallographic orientation at the planar interface [19].

4

According to the above characterizations, with the addition of SDS, the diameter of nanocrystals decreased, the thickness of nanosheets reduced from hundreds of nanometers to dozens of nanometers, and more oxygen vacancies were obtained as well. SDS is an anionic surfactant containing a sulfate hydrophilic segment and a hydrocarbon hydrophobic segment. When a certain amount of SDS was added into the reaction system, sulfate groups and Bi3+ formed ion-pairs due to electrostatic interactions. Then long alkane groups provided external steric repulsion to help the particles overcome the van der Waals attraction, which prevented the aggregation [20]. This may be the reason why we can obtain the smaller nanocrystals and thinner nanosheets in the presence of SDS, which further resulted in more oxygen vacancies. Photocatalytic degradation of RhB was employed as a probe reaction to investigate the activities of as-synthesized BiOBr under the irradiation of visible light (λ≥420 nm). As shown in Fig. 4a, the sphere-like BiOBr exhibits excellent photocatalytic activity, and can completely degrade RhB within 30 min, whereas 85% of RhB is degraded with the flower-like sample. During 30 min dark adsorption, 62% and 24% of RhB was adsorbed on the surface of sphere-like and flower-like samples, respectively. To eliminate the adsorption effect, the pseudo-first order model expressed by ln(C0/C)=kt was applied to the evaluation of the degradation rate, as shown in Fig. 4b. The k value of BiOBr sample prepared with the assistance of SDS is 0.111 min-1(R2=0.9765), which is larger than that prepared without SDS (0.059 min-1, R2=0.9885). We consider that more oxygen vacancies and smaller nanocrystals of sphere-like BiOBr provide more active sites [21], hence result in stronger adsorption capacity which contributes to the injection of photo-excited electrons into the conduction band of the semiconductor, and consequently enhance the photocatalytic activity. 5

Active species trapping experiments were carried out to investigate the photodegradation process of RhB. Since BiOBr with the band energy of ~2.7 eV is an active catalyst under visible light irradiation, both of processes, i.e., direct semiconductor photoexcitation and indirect dye photosensitization, would occur in the photodegradation experiment. Considering that the oxidation potential of photo-generated holes in BiOBr photocatalyst (1.59 eV) [22] is lower than the standard redox potentials of ·OH /OH—（1.99 eV）[23], the photo-generated holes in BiOBr cannot oxidize OH— to ·OH. Therefore, it can be inferred that VB holes or

radicals are mainly responsible for the

oxidation of RhB over the BiOBr catalyst. To further make the reaction mechanism clear, we carried out

and holes trapping experiments for the BiOBr catalyst prepared with SDS. As shown in Fig.

4c, the RhB degradation was significantly suppressed by the addition of benzoquinone (BQ, a scavenger). However, the addition of EDTA (a hole scavenger) has a slight effect on the degradation process. Therefore, it can be deduced that

was the dominative active species for the RhB

photodegradation. The possible reaction mechanism is summarized as Fig. 4d.

b

a

5

without SDS with SDS

1.0 light on

with SDS without SDS

4

0.111 min-1

0.8

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c/c0

ln(ce/c)

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t /min

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t /min

4. Conclusions In summary, we have successfully developed a facile SDS-assisted solvothermal route for synthesizing 3D sphere-like BiOBr. Meanwhile, flower-like BiOBr was obtained without SDS. Although both of the products are of 3D structure, the sphere-like BiOBr possessed smaller nanocrystals, thinner self-assembled nanosheets and more oxygen vacancies compared with the flower-like BiOBr, which could be ascribed to the effect of SDS. Moreover, the as-prepared sphere-like BiOBr showed higher photocatalytic activity relative to flower-like BiOBr under visible light. The photocatalytic degradation rate of RhB with sphere-like BiOBr reached almost 100% within 30 min. Acknowledgements 6

This work was supported by the National Natural Science Foundation of China (21406164， 21466035), the National Key Basic Research and Development Program of China (973 program, No. 2014CB239300, and 2012CB720100), Research Fund for the Doctoral Program of Higher Education of China (No. 20110032110037, 20130032120019).

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Figure captions Fig. 1 XRD patterns of the as-prepared BiOBr samples. Fig. 2 XPS spectra of BiOBr prepared in the presence of SDS (a) survey scan, (b) Bi4f, (c) Br3d, (e) O1s and (d) O1s in the absence of SDS. Fig. 3 SEM and TEM of BiOBr synthesized in the absence of SDS (a) and (c) and in the presence of SDS (b) and (d). Fig. 4 (a) Photocatalytic degradation efficiencies of RhB, (b) kinetics of RhB decolorization on the different BiOBr catalysts, (c) trapping experiments of BiOBr prepared with SDS, and (d) photodegradation mechanism of RhB on BiOBr under visible light (λ≥420 nm).

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Highlights: 1. Hierarchical BiOBr microspheres were synthesized in the presence of anionic surfactant SDS for the first time.

2. The as-prepared BiOBr microspheres were constructed by interlaced nanoflakes which were composed of many nanocrystals.

3. The sphere-like BiOBr exhibited excellent photocatalytic activity under visible light irradiation.

8