BiOBr Heterojunction

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Energy, Environmental, and Catalysis Applications

One-pot Hydrothermal Synthesis of SnO2/BiOBr Heterojunction Photocatalysts for the Efficient Degradation of Organic Pollutants Under Visible Light Haijin Liu, Cuiwei Du, Meng Li, Shengsen Zhang, Haokun Bai, Lin Yang, and Shanqing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09617 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

One-pot Hydrothermal Synthesis of SnO2/BiOBr Heterojunction Photocatalysts for the Efficient Degradation of Organic Pollutants Under Visible Light Haijin Liu1, 3*, Cuiwei Du1, Meng Li3, Shengsen Zhang3 Haokun Bai2, Lin Yang1, Shanqing Zhang3* 1

School of Environmental Science, Henan Normal University, Key Laboratory for Yellow

River and Huaihe River Water Environment and Pollution Control, Ministry of Education, Henan Key Laboratory for Environmental Pollution Control, Xinxiang 453007, P. R. China. 2Faculty

of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, P. R. China.

3

School of Environment and Science, Gold Coast Campus, Griffith University QLD 4222 Australia. (email*: [email protected] (H L); [email protected](S Z))

ABSTRACT: The establishment of p-n heterojunction between semiconductors is an effect means to improve the performance of semiconductor photocatalysts. For the first time, we synthesize SnO2/BiOBr heterojunction photocatalysts using a one-step hydrothermal method. Systematic material characterizations suggest that the photocatalysts consist of irregular BiOBr nanosheets with the length about 200 nm and width about 150 nm, and SnO2 nanoparticles are anchored uniformly onto the nanosheets. Most importantly, electrochemical characterization including transient photocurrent profiles and electrochemical impedance spectra suggest that SnO2/BiOBr heterojunctions are created, which facilitates the charge separation and transfer efficiency of photogenerated charge carriers. As such, SnO2/BiOBr photocatalysts exhibit remarkable photocatalytic activities in terms of degrading a series of organic pollutants. Radical trapping experiments and ESR spectra suggest that superoxide radicals (•O2-) and hydroxyl radicals (•OH) are primary medium species running through photocatalytic degradation process, and enhanced photocatalytic performance.

Keywords: SnO2; BiOBr; heterojunction; radical; one-step hydrothermal

1

ACS Paragon Plus Environment

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1. Introduction During the last decades, photocatalysis technologies have been widely used in different areas, such as water splitting, organic contaminant decomposition, dye sensitization, sterilization, air purification, and cosmetic products, etc.1-4 Over the last few years, TiO2 has been considered as the most popular photocatalytic material due to its low toxicity, cost-effectiveness, relatively high activity, and stability. However, the use of TiO2 is restricted owing to its wide bandgap; hence, it only responds to ultraviolet light, which makes up merely ∼4% of the whole solar spectrum. In this case, research into the modification of TiO2, and the research for new visible-light photocatalysts

have been hot issues. Recently, many studies have focused on bismuth oxyhalides (BiOX, X = Cl, Br, and I), which have unique stratified structures and excellent photocatalytic activities.5, 6 Specifically, in view of its suitable band gap (2.58 eV), the semiconductor BiOBr has garnered considerable interest. At present, variously shaped BiOBr have been reported, including nanoflakes, nanospheres, nanoflowers, 3-D architectures, etc.7-9 To lessen the recombination rate about photoinduced electron-hole pairs of pure BiOBr, researches have investigated several heterojunctions, such as BiPO4/BiOBr, Cu2S/BiOBr, and Bi2O3/BiOBr.10-13 Currently, BiOBr has been extensively applied to the photocatalytic degradation of dyes, antibiotics, phenols, and more.7, 14, 15 Tin dioxide (SnO2) has been widely employed in photoelectrochemistry and photodetection because of non-toxicity, reliable stability, and outstanding optical electricity properties.16-18 However, its large energy gap energy (Eg = 3.56–3.66 eV19) and high recombination probability of e- – h+ pairs restrict its application in photocatalysis. Thus, researchers have attempted, via different methods, to overcome this barrier. The modification with narrow bandgap semiconductors, such as SnS2/SnO220, SnO2/g-C3N421, ZnO/SnO222, has been proven to be an effective strategy for expanding the light absorbing region. Further, the combination of SnO2 with Bi photocatalysts, SnO2/Bi2O323 and SnO2/BiOI24 for instance, has become a new trend toward improving the photocatalytic properties of SnO2. However, most of the synthesis processes in the studies were involved many steps, leading to long reaction time, high energy cost, low purity, low production yield, and expensive materials cost. Simple and cost-effective method for SnO2 based composites is of great significance. Although BiOBr and SnO2 have been studied extensively, the synthesis and characterization 2


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of the SnO2/BiOBr composite has not been reported yet. Herein, SnO2/BiOBr heterojunction photocatalysts were prepared via a simple and cost-effective one-step hydrothermal method, which was much milder than those of conventional methods without the use of expensive chemicals, numerous cumbersome procedures, and high energy consuming calcination steps. More importantly, electrochemical characterization demonstrates that efficient heterojunctions were established between the BiOBr and SnO2 in the resultant catalyst could accelerate the separation of charge carriers. Consequently, the as-prepared SnO2/BiOBr delivered significantly improved performance in contrast to pure BiOBr and SnO2 in the degradation of Rhodamine B (RhB), crystal violet (CV), and triclosan (TCS) under broad-spectrum sunlight irradiation. Furthermore, the potential enhanced mechanism and relationship between the SnO2 and BiOBr were proposed and verified by radical trapping experiments and ESR spectra.

2. Materials and methods 2.1. Chemicals and reagents Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and ethylene glycol (EG) were obtained from Aladdin (China), sodium hydroxide (NaOH) and potassium bromide (KBr) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Stannic chloride pentahydrate (SnCl4·5H2O) and ethanol (EtOH) were purchased from Tianjin Chemical Reagent Co., Ltd, China (Tianjin, China). 2.2. Sample preparation Volumes of 4.8 mmol Bi(NO3)3·5H2O and 4.8 mmol KBr were dissolved in 12 mL H2O and 28 mL EG (ethylene glycol) under magnetic stirring for 0.5 h. Meanwhile, a certain amount of SnCl4·5H2O (the weight ratios of SnO2 to BiOBr were 0, 20, 30, and 40 wt%, respectively) was added into a mixed 10 mL EtOH and 10 mL NaOH aqueous solution (Sn4+/OH- molar ratio of 1/4) to form a Sn(OH)4 suspension. Subsequently, it was added cautiously into the above solution in conjunction with ultra-sonication for 30 min. The obtained mixtures were then transferred into a capacity of 100 mL stainless steel Teflon-lined autoclave, maintained at 110 ºC for 10 h. The proposed formation process of SnO2/BiOBr composites is described in Scheme 1. The obtained precipitates were separated by centrifugation and rinsing with pure water and ethanol, and finally dried overnight at 60 ºC, resulting in a white and light powder. 2.3. Characterization 3



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The samples’ crystal structures were determined by X-ray diffraction (XRD) on a Bruker-D8AXS diffractometer using Cu-Kα source (λ = 0.15406 nm). The particle size and morphologies of obtained materials were investigated via using a JSM-6390LV scanning electron microscope (SEM). Energy-dispersive spectroscopy (EDS) was also tested during the SEM measurements. The morphology, element, and crystal lattice of the samples were measured by JEM-2100 transmission electron microscope (TEM). The surface composition was determined by X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Ka source (Escalab-250Xi X-ray spectrometer). FTIR spectrum was performed using a Perkin-Elmer Spectrum 400 in the range of from 400–4000 cm−1 using KBr disks. UV–Vis diffuse reflectance spectra (DRS) were obtained by UV–Vis spectrophotometer (Lambda 950, PerkinElmer), in which BaSO4 acted as a reference. Photoluminescence (PL) spectra were recorded on fluorescence spectrophotometer (FP-6500, Japan) with a Xenon lamp as excitation wavelength setting at 340 nm. The free radicals electron spin resonance (ESR) measure was conducted via DMPO solution on a Bruker spectrometer. The photocurrents and electrochemical impedance spectroscopy (EIS) were measured in a three-electrode configuration (CHI-660B, Shanghai, China). Ag/AgCl and Pt wire worked as reference and counter electrodes, respectively, and the electrolyte was 0.5 mol L-1 Na2SO4 aqueous solution. The work electrodes were fabricated as following: a slurry containing the as-prepared samples was prepared by mixing 10 mg of the material powder with 100 μL of 25% PVA solution under ultrasonication for 1 h. The slurries were put onto a cleaned FTO slice with (1 cm × 2 cm) and dried at 80 °C in vacuum overnight. 2.4. Evaluation of photocatalytic activity In order to evaluate the photocatalytic activities of the as-prepared catalysts systematically, a series of organic compounds, CV, RhB and TCS were selected as the representative pollutants. The concentrations of CV, RhB and TCS were 20, 20 and 30 mg L−1, the volumes were all 50 mL. Nine parallel 5 W LED lights (λ>400 nm) were set as light sources in a PCX50B multi-channel catalytic reaction system. The light sources were cooled down during the reaction by a cold air stream and there was no light difference between the bottles. In experiment, 50 mg photocatalyst was dispersed in 50 mL CV, RhB, or TCS solution under magnetic stirring, respectively. And then the suspension was stirred under dark for 40 min in order to acquire adsorption-desorption equilibrium. During each photocatalytic process, 3 mL suspensions were withdrawn at regular intervals and analyzed 4


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with UV-2900 spectrometer at wavelengths of 582 nm, 554 nm, and 280 nm for the CV, RhB, and TCS solutions, respectively. The degradation rates were calculated on the basis of following equation:

η=

C0 − Ct ×100% C0

Where C0 is the initial concentration (following exposure to darkness for 40 min.) and Ct is the concentration at time t during the photocatalytic degradation reaction.

3. Results and discussion 3.1. Materials synthesis and characterization As illustrated in Scheme 1, the SnO2/BiOBr photocatalysts were prepared through a one-step hydrothermal process. Briefly, a certain quantity of SnCl4•5H2O was dissolved into mixture of EtOH and NaOH solution to form white floc i.e., Sn(OH)4, in the reaction vessel. The as-prepared Sn(OH)4, served as a precursor of the SnO2 nanoparticles. In parallel, Bi(NO3)3•5H2O was dissolved into the solution of water and ethylene glycol, thereafter KBr was slowly poured into the solution, resulting in a yellow solution. Subsequently, the white floc Sn(OH)4 solution was added dropwise in the yellow solution and transferred into autoclave and maintained at 110℃ for 10 h. Afterwards, the precipitate was separated and rinsed several times, and a white SnO2/BiOBr composite powder was obtained following drying. The synthesis process was simple, time-saving, and energy-efficient, which are in strong favor for potential commercialization. To directly observe the morphologies and microstructures of pristine SnO2, BiOBr, and SnO2/BiOBr, SEM and TEM analyzes were conducted in Figure 1. The SEM images in Figure 1a of pure SnO2 reveal the nanoparticle aggregates (the nanoparticles easily adhered to one another). The morphology of pure BiOBr in Figure 1b was composed of thin irregular nanosheets with smooth surfaces, which had a length of 200 nm, a width of 150 nm, and a thickness of < 50 nm. The SnO2/BiOBr composites in Figure 1c retained the same irregular nanosheet morphologies. Numerous nanoparticles were evenly deposited across the surfaces of the nanosheets, which roughened the surface and thickened the nanosheets. The elements and structures of the SnO2/BiOBr composites were further examined by energy-dispersive X-ray spectroscopy (EDS) and HRTEM, with images displayed in Figures 1d–f. Figure 1d indicates that SnO2/BiOBr composite was comprised of Bi, Br, O, and Sn, which suggested the coexistence of SnO2 and BiOBr. In Figure 1e, 5



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SnO2 nanoparticles may be seen dispersed across the surface of BiOBr with an average diameter of ~5 nm. As confirmed in the HRTEM of SnO2/BiOBr composites (Figure 1f), two sets of different lattice images are observed at the interfaces. The lattice fringes of 0.34 nm and 0.28 nm were consistent with the (110) crystal facet of SnO2 and (102) crystal facet of BiOBr, respectively. The crystal structure of the SnO2, BiOBr, and SnO2/BiOBr composites were presented by XRD patterns and displayed in Figure 2. BiOBr (JCPDS No. 78-0348) was a typical crystalline material that exhibited strong reflections in its XRD spectra. The strong diffraction peaks at 2θ=10.91°, 21.91°, 25.20°, 31.72°, 32.24°, 39.33°, 46.24°, 50.66°, 53.38°, and 57.16° corresponded to (001), (002), (101), (102), (110), (112), (200), (104), (211), and (212) crystal planes of tetragonal BiOBr. SnO2 exhibited a tetragonal phase (JCPDS No. 01-0625), and the main diffraction peaks at 26.19°, 33.54°, and 51.59° were assigned to the (110), (101), and (211) crystal planes, respectively. For the SnO2/BiOBr samples, most of the diffraction peaks were assigned to BiOBr in the spectra, which was likely because the intensities of SnO2 peaks were quite weak, and the peak positions were just overlapped by the BiOBr peaks. However, a weak peak located at 2θ of 33.54°could be observed in all the SnO2/BiOBr samples, which was attributed to the (101) facet of SnO2, which demonstrated that the SnO2/BiOBr composites were successfully synthesized. Furthermore, we could still find interactions between the SnO2 and BiOBr through changes in their peaks intensities. After the incorporation of SnO2, the peak at 10.91° increased dramatically; while most of the other peaks decreased by different degrees in conjunction with increasing amounts of SnO2. The chemical environment of the BiOBr and SnO2/BiOBr composite were further analyzed using XPS. As shown in Figure 3a, five elements, namely, Bi, Br, Sn, O, and C, are identified in a 30 wt% SnO2/BiOBr composite, revealing that SnO2 was successfully incorporated into the BiOBr. The C 1s peak in the spectrum emerged from adventitious carbon. In the Bi 4f XPS spectrum (Figure 3b), two peaks located at 165.30 and 160.10 eV are attributed to the Bi 4f5/2 and Bi 4f7/2 of pure BiOBr, respectively, indicating that chemical valence of Bi was +3. Further, the peak features of Bi3+ shifted slightly (~0.55 eV) to lower energy positions in the SnO2/BiOBr composite, which is likely caused by the strong interactions between the SnO2 and BiOBr.25 Similar binding-energy shifts could be also witnessed in the high-resolution spectra of the Br 3d and O 1s. The two peaks of Br 3d in Figure 3c at 68.43 and 69.31 eV are ascribed to Br 3d5/2 and Br 3d3/2, respectively.26 In Figure 3d, the peaks of Sn 3d5/2 and Sn 3d3/2 located at 487.35 and 495.80 eV respectively, indicating 6


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that Sn is primarily in the state of Sn4+ 27. In Figure 3e, the peaks of O 1s are resolved as two peaks at 530.4 and 531.7 eV for BiOBr, which corresponds to Bi-O and O-H bonds, respectively.12 The peaks of O 1s in the composites are fitted to three peaks in Figure 3f; the peak at 531.52 eV corresponds to the Sn−O bonds24; whereas the other two peaks are consistent with those of pure BiOBr. The FTIR spectra of the SnO2, BiOBr, and 30% SnO2/BiOBr are displayed in Figure 4a. As for the pure BiOBr sample, the characteristic peak at ~501 cm-1 is assigned to the Bi–O vibrations28. The peaks centered at 1636 cm-1 and 3443 cm−1 corresponds to the vibrations of O–H29, and the band at 2361 cm−1 is associated with the C=O vibrations of absorbed CO2. The FTIR spectrum of the SnO2 nanoparticles consists of an intense broad peak at ~594 cm-1 and 648 cm-1, which correspond to Sn–O–Sn and O–Sn–O stretching vibrations, respectively30. Furthermore, the SnO2/BiOBr show similar spectra with BiOBr; however, the stretching vibration of O–Sn–O at 648 cm-1 appears in the spectra verifies the successful synthesis of the SnO2/BiOBr composites. The PL signal of pure BiOBr and SnO2/BiOBr samples are shown in Figure 4b. The pristine BiOBr demonstrates strong emission intensity at from 450-550 nm, which could be put down to the recombination of electron–hole pairs. It was obvious that the intensities of the PL emissions for the SnO2/BiOBr with different mass ratios decreased in contrast to pristine BiOBr. The detailed sequence of PL intensity is shown as follows: BiOBr > 20 wt% SnO2/BiOBr > 40 wt% SnO2/BiOBr > 30 wt% SnO2/BiOBr, which implies that the formation of the SnO2/BiOBr heterojunction could be favorable for restraining the recombination of photo-induced charge carriers in BiOBr. The optical properties were investigated by UV-vis-DRS. In Figure 5a, the BiOBr shows an absorption edge at 420 nm. And the exhibited absorption edge of SnO2 is located at ~370 nm, whereas the absorption edges of the SnO2/BiOBr heterojunctions with different mass ratios are situated between SnO2 and BiOBr. The optical band edge in Figure 5b was determined using the Tauc equation: αhγ=A(hγ −Eg)n/2, where A is constant, α, h, γ, and Eg are the absorption coefficient, Planck’s constant, photon energy, and band gap, respectively; n was defined by optical transition of a semiconductor (n=1 for direct transition and n=4 for indirect transition).31,

32

As reported in

previous research, the n values of SnO2 and BiOBr were 1.24, 32 From the plot of (Ahγ)1/2 versus (hγ), the band gaps of SnO2 and BiOBr are found to be 3.48 and 2.58 eV,24, 33 respectively. The band edge energy of BiOBr and SnO2 could be further evaluated by empirical equation: 7



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EVB=X - E0 + 0.5Eg; ECB= EVB - Eg, where X is the semiconductor electronegativity , and X values for SnO2 and BiOBr are calculated to be 6.2534 and 6.18 eV26; E0 is the free electrons energy on hydrogen scale (~4.5 eV); EVB and ECB are the valence band (VB) and conduction band (VB) edge potential. Thus, VB and CB of SnO2 are ca. 3.49 eV and 0.01 eV, and those of BiOBr were 2.97 and 0.39 eV, respectively. Based on the above results, a possible photocatalytic mechanism of the SnO2/BiOBr sample is depicted in Figure 6. On the basis of the Mott−Schottky plots, the BiOBr is testified as a p-type semiconductor, and SnO2 was an n-type semiconductor. Further, pure SnO2 and BiOBr showed a nested band array might have inhibited the transfer of photoinduced electron-hole pairs. The Fermi level (Ef) of SnO2 was close to the CB and that of BiOBr was near the VB, and charges diffusion or transfer would cease when the Fermi level of the SnO2 and BiOBr attained equilibrium. At the same time, the energy bands of BiOBr were increased, and for SnO2 decreased, on account of their Fermi levels, shown in Figure 6b. Under white light irradiation, BiOBr can be stimulated to produce electrons and holes, then the electrons in the CB of the BiOBr migrated to the CB of the SnO2. Meanwhile, holes are gathered in the VB of the BiOBr. Subsequently, the electrons translated O2 to •O2−, and the h+ could oxidize H2O to give •OH. Therefore, the separation of the photogenerated electron–hole pairs could be strengthened due to the formation of an internal electric field and p-n heterojunctions. This mechanism can be further confirmed by electrochemical measurement, including transient photocurrent response and EIS Nyquist plots. Figure 7a displays the photocurrent (Iph) response plots of the pure SnO2, BiOBr and SnO2/BiOBr composites for three on-off cycles at 0.0 V. Almost no photocurrent generated in the five samples in dark, because the photocatalysts are nonconductive due to no free charge carriers. In strong contrast, upon the illumination of visible light (λ>400 nm), significant photocurrents were produced, suggesting the charge carriers, (photoelectrons and holes) were generated. In particular, the photocurrent intensities of the SnO2/BiOBr composites were clearly stronger than the pure SnO2 and BiOBr. The maximum photocurrent of 30% SnO2/BiOBr reaches 8.8 μA/cm2 as indicated in the potocurrent response curves at 40 s, approximately 7.3 times as high as that of pure BiOBr. These results from photocurrent response demonstrate that the recombination of the photogenerated charge carriers is well suppressed by p-n heterojunction. Electrochemical impedance spectra (EIS) were subsequently applied to investigate the interface charge carriers.35 8


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Figure 7b shows Nyquist plots of the SnO2, BiOBr and SnO2/BiOBr heterojunction samples. It is well established that the semicircle radius presents charge transfer efficiency. The radius in the plots of the SnO2/BiOBr composites is smaller than that of the pure SnO2 and BiOBr electrodes. Nevertheless, the 30 wt% SnO2/BiOBr sample has the smallest radius among all the catalysts, demonstrating that the SnO2/BiOBr p-n heterojunction is beneficial to the separation of photogenerated charge carriers, thus bring about enhanced photocurrent response and better electron transfer efficiency. 3.2. Photocatalytic degradation of organic pollutants In order to evaluate the photocatalytic performance of prepared pure BiOBr, SnO2, and SnO2/BiOBr complexes, the degradation of RhB, CV, and a typical personal care product (PPCP) triclosan (TCS), were carried out under 5 W LED light irradiation. Before irradiation, the mixture of as-prepared photocatalysts and RhB solution were stirred for 40 min in dark to achieve adsorption-desorption equilibrium. The variations in RhB concentrations (manifested as a normalized concentration of C/C0) are shown in Figure 8a. Particularly, different mass ratio SnO2/BiOBr composites exhibited significantly enhanced photocatalytic efficiencies over bare SnO2 and BiOBr, which indicated that the formation of heterojunctions was an effective way to improve photocatalytic activity. In particular, the degradation rate of RhB attained ~98.2% in 20 min. using 30 wt% SnO2/BiOBr, which was almost twice than that of pure BiOBr, and 10 times that of pure SnO2. For comparison, a blank experiment was conducted without the addition of a photocatalyst, revealing that only ~5% RhB was degraded without a photocatalyst, which could be ascribed to self-photolysis under visible light irradiation. The obtained reaction kinetic data were depicted using the pseudo-first-order equation in Figure 8b: −ln(C/C0) = kt, where k is the reaction rate constant, C is the RhB concentration at time t, C0 is the initial RhB concentration following adsorption. According to the equation, the photocatalytic degradation constants for 20 wt% SnO2/BiOBr (0.1007 min−1), 30 wt% SnO2/BiOBr (0.2110 min−1), and 40 wt% SnO2/BiOBr (0.0596 min−1) were much higher than SnO2 (0.0060 min−1) or BiOBr (0.0447 min−1). Therefore, the 30 wt% SnO2/BiOBr catalyst exhibited the highest photocatalytic activity. Another widely used dye solution (CV, 20 mg/L) was selected as objective pollutant to appraise the photocatalytic activities. The light was switched on following the establishment of adsorption9



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desorption balance in 40 min. As shown in Figure 8c, all of the composites exhibited strong capacities to degrade CV in 70 min. Similarly, the SnO2/BiOBr composites showed significantly enhanced activities over bare BiOBr and SnO2. The degradation rate of CV over 30 wt% SnO2/BiOBr was 95%, whereas for pure BiOBr or SnO2, it was only 30% and 10%, respectively. In contrast to pure BiOBr and SnO2, the degradation rate showed a ~3 and 9 fold increase for the removal (%) of CV with 30 wt% SnO2/BiOBr. As potential dye photosensitization might influence photocatalysis, we further evaluated the photocatalytic activity of the samples using TCS as a probe molecule, with the results shown in Figure 8d. It is clear to see that the TCS was degraded efficiently in 180 min. with SnO2/BiOBr composites, where the photocatalytic abilities were as follows: 30 wt% SnO2/BiOBr > 20 wt% SnO2/BiOBr > 40 wt% SnO2/BiOBr > BiOBr > SnO2. The apparent rate constant of 30 wt% SnO2/BiOBr was 3.2 times higher than that of pure BiOBr. These results confirmed that the dyesensitization effect was not the primary factor on enhanced photocatalytic activity of the SnO2/BiOBr composites, as TCS solution was colorless and could not absorb visible light. The recyclability of the 30 wt% SnO2/BiOBr heterojunction was also investigated under identical conditions by the recycling degradation of RhB under white light irradiation. For each run, the photocatalyst was recycled, cleaned, and dried. As shown in Figure 8e, there was no obvious deactivation of the photocatalytic performance after four recycling runs. Further, no obvious changes were observed in the XRD spectra (Figure 8f) of the the 30 wt% SnO2/BiOBr prior to and following the four cycle reactions. Above results indicated that the SnO2/BiOBr composites demonstrated prominent recyclability and structural stability. To distinguish the functions of different active species in photocatalytic reaction mechanism, three active scavenger species (1,4-benzoquinone (BQ), ethylenediamine tetraacetic acid disodium (EDTA-Na2), and isopropyl alcohol (IPA)) were adopted to the RhB degradation process, which are commonly used as scavengers of superoxide radicals (•O2-), holes (h+) and hydroxyl radicals (•OH), respectively. The results wre shown in Figure 9a, the addition of EDTA-Na2 had a subsidiary effect on the degradation of RhB, indicating that h+ played an auxiliary role in the decomposition of RhB. Nevertheless, the degradation rate of RhB was significantly decreased through adding BQ and IPA trapping agents, it is no doubt that •O2-and •OH played vital roles in the photocatalytic degradation of RhB. 10


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To further explore the radicals in the trapping experiment, spin-trapping ESR was conducted to probe the generation of active oxygen species on catalysts surfaces, where 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) was employed as trapping agent. As shown in Figure 9b and 9c, all samples were no ESR signals in the dark, and relatively weak ESR signals of DMPO •O2- and DMPO •OH over pure BiOBr were observed under light irradiation. Nonetheless, the strong peaks of DMPO •O2- and DMPO •OH were detected over the 30 wt% SnO2/BiOBr heterojunction following light illumination, and the four characteristic ESR signals for DMPO •O2- and DMPO •OH with intensity ratios of 1:1:1:1 and 1:2:2:1 were observed under white light irradiation. Obviously, the 30 wt% SnO2/BiOBr has much stronger response than that of pure BiOBr, which revealed that the •O2- and •OH radicals are generated in a much more efficient way due to the introduction of SnO2. The ESR results further suggest that •O2- and •OH are produced in photocatalytic process and play a significant role in the photocatalytic degradation process. 4. Conclusions An efficient SnO2/BiOBr photocatalyst is successfully fabricated via a one-step hydrothermal method. The obtained SnO2/BiOBr heterogeneous catalysts exhibited remarkable photocatalytic activity in the degradation of organics such as RhB, CV, as well as TCS, in contrast with individual SnO2, BiOBr and their mixture. This can be ascribed to the fact that the SnO2/BiOBr p-n heterojunctions

could

boost

the

charge

separation

efficiency in

photocatalytic

and

photoelectrocatalytic processes and therefore facilitates the production of more •O2- and •OH radicals upon light irradiation and subsequent degradation efficiency. Such a radical reaction mechanism is supported by results from the ESR analysis while the establishment of p-n junction is supported by the transient photocurrents and EIS spectra. As the SnO2/BiOBr heterojunction photocatalyst is low cost, stable, and recyclable, the proposed photocatalyst is promising for the photocatalytic degradation of organic compounds and disinfection of bacteria.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21677040), the China Postdoctoral Science Foundation (No. 2015M582188), the Natural Science Foundation of Henan Province (No. 182300410121) and the 111 Project (No. D17007).

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Scheme 1. Schematic of the proposed synthesis process of the SnO2/BiOBr heterojunction composites.

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Figures and Figure captions

Figure 1. SEM images of pristine SnO2 (a), BiOBr (b), and SnO2/BiOBr composites (c); EDS of SnO2/BiOBr (d); TEM image of SnO2/BiOBr (e-f)

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Figure 2. XRD patterns of BiOBr, SnO2 and as-prepared SnO2/BiOBr of different ratios.

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Figure 3. XPS spectra of the pure BiOBr and 30 wt% SnO2/BiOBr composite. Survey scan of 30 wt% SnO2/BiOBr (a). High-resolution spectra of Bi 4f (b), Br 3d (c), Sn 3d (d) and O1s (e-f).

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Figure 4. FTIR spectra of the SnO2, BiOBr, and 30% SnO2/BiOBr samples (a) and photoluminescence (PL) spectra of BiOBr, and SnO2/BiOBr with different mass ratios (b).

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Figure 5. UV–vis diffuse reflection spectra of SnO2, BiOBr, and different mass ratio SnO2/BiOBr composites (a). A plot of (Ahγ)1/2 versus the bandgap (eV) for SnO2, BiOBr, and 30 wt% SnO2/BiOBr (b).

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Before contact

a

After contact e-

CB

0.39 eV

CB

e- e- e-

0.01 eV

2.58 eV

Ef

3.48 eV

2.97 eV

P-type BiOBr

VB

3.49 eV

n-type SnO2

O2

e- e- e-

•O2−

—+ + h+ h+ h+ +

•OH

Ef VB

— — —+

Ef

e-

b

H2O

BiOBr

SnO2

Figure 6. The energy position, bandgaps and fermi levels of the individual p-type BiOBr and ntype SnO2 (a) and the as-prepared heterojunction SnO2/BiOBr (b). CB: Condction band;VB: Valence band; Ef: Fermi energy level; h+: photohole; e-: electron.

21



12

(a)

SnO2 BiOBr

30

20 wt% SnO2/BiOBr 40 wt% SnO2/BiOBr 30 wt% SnO2/BiOBr

8 6 4 2

(b) SnO2 BiOBr 20 wt% SnO2/BiOBr 40 wt% SnO2/BiOBr 30 wt% SnO2/BiOBr

25

-Z′′ (kΩ/cm2)

10 Iph(µA/cm2)

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 22 of 25

20 15 10 5

0

0

0

20

40

60 80 100 120 140 160 Time (s)

0

5

10

15

Z′ (kΩ/cm2)

20

25

30

Figure 7. Transient photocurrent response (a) and EIS Nyquist plots (b) of the SnO2, BiOBr and SnO2/BiOBr composites.

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Figure 8. Photocatalytic activities of different photocatalysts for the degradation of RhB (a), CV (c) and TCS (d) degradation under white light irradiation (λ> 400 nm), respectively. (b)The first-order kinetics of RhB with different samples. (e) Cycling runs of 30 wt% SnO2/BiOBr sample for the degradation of RhB and (f) XRD patterns prior to and following cyclic photocatalytic experiments for RhB degradation under white light irradiation (λ> 400 nm).

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Figure 9. Photocatalytic degradation of RhB with the addition of different scavengers using 30 wt% SnO2/BiOBr photocatalyst (a); ESR spectra of the DMPO •O2- (b) and DMPO •OH (c) adducts over the pristine BiOBr and 30 wt% SnO2/BiOBr with and without white light irradiation.

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The TOC graphic

Before contact

a

After contact e-

CB

0.39 eV

CB

e- e- e-

0.01 eV

2.58 eV

Ef

3.48 eV

2.97 eV

P-type BiOBr

VB

3.49 eV

n-type SnO2

O2

e- e- e-

•O2−

—+ + h+ h+ h+ +

•OH

Ef VB

— — —+

Ef

e-

b

H2O

BiOBr

SnO2

25


BiOBr Heterojunction

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