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Homogeneous {001}-BiOBr/Bi heterojunctions: facile controllable synthesis and morphology-dependent photocatalytic activity Yuxi Guo, Yihe Zhang, Hongwei Huang, and Na Tian ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00884 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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ACS Sustainable Chemistry & Engineering

Homogeneous

{001}-BiOBr/Bi

heterojunctions:

facile controllable synthesis and morphologydependent photocatalytic activity Yuxi Guo†, Yihe Zhang*,†, Na Tian†, Hongwei Huang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes,

National Laboratory of Mineral Materials,School of Materials Science and Technology, China University of Geosciences, No.29 Xueyuan Street, Beijing 100083, China Corresponding Authors * Y. H. Zhang. E-mail: [email protected]. * H. W. Huang. E-mail: [email protected].

KEYWORDS. photocatalyst, BiOBr, metallic Bi, in situ reduction, PVP-assisted, plasmon resonance effect

ABSTRACT. The homogeneous BiOBr/Bi heterojunctions photocatalyst was synthesized from {001} facet dominated BiOBr flakes via a PVP-assisted in situ reduction reaction at room temperature. The high {001} facet exposure of BiOBr could induce the homogeneous distribution of metallic Bi on the surface of BiOBr. The introduction of PVP not only effectively protected the uniform structure, but also largely promoted the photocatalysis properties. Compared to the bare BiOBr, an obviously enhanced photochemical performance was achieved

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over the homogeneous BiOBr/Bi pertaining to Methyl Orange (MO) degradation and photocurrent generation. The highly enhanced photocatalytic activity can not only be attributed to the surface plasmon resonance effect and efficient separation of electron–hole pairs by the metallic Bi, but also largely depended on its uniform and regular structure. The present work provided a new approach to the development of attractive bismuth-based-photocatalysts/metallic Bi heterostructures with controllable structures and photocatalytic performance.

INTRODUCTION In view of making full use of the renewable solar energy source, the search of semiconductor photocatalysts for environment purification and energy production remains one of the most intense research interest.1-4 TiO2 is widely studied as an promising photocatalytic material.5-6 However, it requires UV light to be activated. Therefore, the development of novel visible-lightdriven (VLD) photocatalysts has drawn considerable attention among researchers.7-11 The tetragonal BiOX compounds has been widely applied as photocatalysts, pharmaceuticals, catalysts, ferroelectric materials.12-26 Among the bismuth oxyhalides, the BiOBr becomes attractive because it displays the visible-light responsive activity and good stability.

27-30

However, the light absorption of BiOBr with a band gap of 2.69 eV is still limited.31 Therefore, studies on further enhance its light responsive ability and photocatalytic efficiency is still important. Among various techniques, the surface plasmon resonance (SPR) noble-metal nanostructures have been proved to be effective to enhance the photocatalytic performance of the substrate semiconductors.32-42. Similar to noble metal, bismuth as a typical semimetal material, has also been discovered to exhibit plasmonic properties.43-44 In comparison to noble metals, the metallic


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Bi is inexpensive. Concerning the unique properties of Bi, they can be perfect alternative SPR nanostructures in the photocatalytic process. The reported Bi-deposited semiconductor photocatalysts, including Bi/Bi2O3, Bi/(BiO)2CO3, all exhibit enhanced photocatalysis efficiency.45-49 Given the behavior of Bi, developing BiOBr/Bi structured nanocomposites to enhance photocatalytic activity has been attracting immense research efforts. Some groups synthesized BiOBr/Bi photocatalysts with improved visible photocatalytic activity, but they largely ignored the effect of Bi-rich {001}facet of BiOBr on the performance of Bi deposited and did not investigate further to gain uniform and regular morphology of samples.50-51 It is widely accepted that properties of composite photocatalytic materials are rather sensitive to their morphology. Until now, no research about controllable synthesis of desired morphology to improve BiOX/Bi’ photocatalysis has been reported. Herein, we report a novel PVP-assisted in situ reduction reaction strategy to synthesize uniform BiOBr/Bi heterojunctions from high {001} facets exposed BiOBr flakes at room temperature. During the reduction reaction, the precise control of distribution of metallic Bi was the key point in determining desirable physicochemical properties. Programming in the direction, we used the BiOBr flakes with high exposure of {001} facets which consist of [B2O2]2+ slabs as the substrates. As the Bi element is mainly contained in the [B2O2]2+ slabs on the {001} facets, the reduced metallic Bi may maximize its distribution area and form a homogeneous coating on the surface of the BiOBr crystals. Moreover, in order to control the synthesis process, we introduced PVP into the reaction system to maintain a peaceful reduction environment.

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We

studied how to control the reaction condition for the formation of uniform and regular BiOBr/Bi structures. A possible mechanism for the formation of regular BiOBr/Bi samples was proposed.


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Moreover, the morphology-dependent photocatalytic activities of BiOBr/Bi was evaluated on the degradation of methyl orange under visible light (λ > 420nm) irradiation. EXPERIMENTAL SECTION Synthesis: The method for the synthesis of BiOBr flakes with high exposure of {001} facets is similar to that described in the previous work. 0.97g Bi(NO3)3•5H2O and 0.238g KBr was dispersed and dissolved into 25ml distilled water, respectively. The KBr solution was then added dropwise into the stirring Bi(NO3)3 suspension. After being stirred for 0.5 h at room temperature, the mixture was transferred into 100-mL Teflon-lined stainless autoclave and then heated at 120 ℃ for 24 hours. The resulting BiOBr precipitates were centrifuged, washed thoroughly with de-ionized water and ethanol and eventually dried at 60 ℃ for 12h. An in situ reduction was used to prepare Bi coated BiOBr flakes, as depicted in Scheme 1. 0.5 g of BiOBr flakes was dispersed in 20 mL of deionized water containing 7.5g PVP, to which 20 mL NaBH4 solution with different concentrations ranging from 5 to 30 mmol L-1 was dropped slowly under constant magnetic stirring. The BiOBr/Bi flakes prepared with different NaBH4 solutions were defined as BiOBr/Bi-5, BiOBr/Bi-10, BiOBr/Bi-15, BiOBr/Bi-20 and BiOBr/Bi30. The samples prepared with 5, 20 ,30 mmol L-1 without the use of PVP was named as NP BiOBr/Bi-5, NP BiOBr/Bi-20, NP BiOBr/Bi-30. After being reacted for 10 min, the obtained solid powder was collected by several centrifugation and washing steps and then dried at 60 ℃ for 12h. Characterization


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The X-ray diffraction (XRD) （X/max-rA Advance diffractometer with Cu Kα radiation） were collected to analyze the crystal phase composition of samples. The nanostructure and size distribution of the as-obtained samples were observed by scanning electron microscopy (SEM) on a Hitachi S–4800. N2 adsorption–desorption isotherms were conducted to measure the specific BET surface areas of the BiOBr/Bi photocatalysts. The detailed information of the morphology was detected by the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) using a JEOL JEM-2100. The optical properties of prepared hybrids were studied with a Varian Cary 5000 diffuse reflectance UV-vis spectrometer by employing BaSO4 as a standard. X-ray photoelectron spectra (XPS) (A VGMK II X-ray photoelectron spectrometer) were applied to explore surface elemental compositions and oxidation states of the sample. A Bruker ER200-SRC-10/12 electron paramagnetic resonance spectrometer equipped with visible-light irradiation system was operated to acquire the electron spin resonance (ESR) signals of radicals. The photoluminescence (PL) was recorded on a fluorescence

spectrophotometer

(RAMANLOG

6).

A

ChenhuaInstrument

CHI660C

electrochemical workstation was used to conduct the photoelectrochemical experiments by using a 300 W xenon lamp as the visible light source and 0.1 M Na2SO4 as supporting electrolyte. A carbon electrode and a platinum-saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. The indium-tin oxide (ITO) glass coated with samples worked as the working electrode. All of the above measurements were conducted at room temperature. Photocatalytic Experiment. For the measurement of photocatalytical activity of the as-prepared samples, the degradation of MO under 500 W Xe arc lamp as the light source using a 420 nm cutoff filter. Typically, 50


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mg samples and 50 mL MO aqueous suspension were placed in a quartz test tube. Prior to irradiation, to reach the adsorption/desorption equilibrium the suspensions were fiercely magnetically stirred for 1 h in the dark. Then the visible light illumination was carried out. At given irradiation time intervals, 3 ml of the mixed solution was taken out and centrifuged, and the concentration of MO in the supernatant was monitored by a U-3010 UV-vis spectroscopic spectrophotometer by checking its characteristic absorbance at 464 nm. Determination of reactive species For investigating the active species that are responsible for the photocatalytic activity, severer scavengers, including 10 mmol L−1 isopropanol (IPA, •OH scavenger), 1 mmol L−1 1,4benzoquinone (BQ, •O2− scavenger), 1 mmol L−1 ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na, h+ scavenger), were individually added into the 50 mL MO solution. The testing was similar to the above photocatalytic experiment.

RESULTS AND DISCUSSION. TEM and HRTEM were performed on the as-synthesized pure BiOBr and BiOBr/Bi-20 photocatalyst to investigate the structure hybrid BiOBr/Bi (Figure 1). As displayed in Figure 1a, the surfaces of the pristine BiOBr flake were very smooth and neat. By mixing the BiOBr flakes with NaBH4 aqueous solution, an in situ reduction reaction was carried out instantly. From Figure 1c, we can obviously see that there was a layer of shell surrounding the BiOBr flake. From the enlarged TEM microscopy of BiOBr/Bi-20 in Figure 1d, the clear boundary between the BiOBr core and the shell was also observed, providing a strong evidence for the reduction of BiOBr surface. The HRTEM image of pure BiOBr sample was depicted in Figure 2a. The lattice


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space was measured to be 0.277 nm, which corresponded to the (110) lattice plane of BiOBr, revealing that the prepared BiOBr was mainly enclosed by the {001} facets. The typical HRTEM image of BiOBr/Bi-20 was shown in Figure 2b, which was taken on the edge of a single flake. The fringe spacing was 0.294 nm, which was well indexed into the interplanar spacing of (111) of metallic Bi. It confirmed that the metallic Bi was formed on the surface around the BiOBr flakes as a result of partial reduction.

Figure 1. TEM images: (a) pure BiOBr flake before coating, (b) the magnified image,(c) BiOBr/Bi-20, (d) the magnified image of (c)


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Figure 2. (a) HRTEM images of pure BiOBr, (b) HRTEM images of edge of BiOBr/Bi-20.

The morphological information of the samples was also investigated by SEM. The pure BiOBr consisted of regular rectangle flakes with diameters of about 1.5 um and smooth surface (Figure 3a). Figure 3b–e showed the morphologies of the BiOBr/Bi composites with different NaBH4 concentrations of 10 (b), 15 (c), 20 (d) and 30 (e) mmol L-1. This samples are all prepared by adding 7.5 g PVP during the reduction process. We can see that all BiOBr/Bi samples still remained the regular rectangle flake structure. However, with the increase of the NaBH4 concentrations, the surface of the samples became more and more rough, indicating that the surface of BiOBr samples had been reduced by NaBH4. The schematic illustration under the SEM images clearly showed the change that occurred on the surface of these samples (Figure 3fj).


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Figure 3. SEM images: (a) pure BiOBr, (b)BiOBr/Bi-10, (c) BiOBr/Bi-15, (d) BiOBr/Bi-20 and (e) BiOBr/Bi-30; (f-j) Schematic illustration of the surface of samples: (f) pure BiOBr, (g)BiOBr/Bi-10, (h) BiOBr/Bi-15, (i) BiOBr/Bi-20 and (j) BiOBr/Bi-30.

As a useful organic surfactant, PVP played very crucial roles in forming uniform BiOBr/Bi structure here. Figure 4 displayed the SEM images of the as-obtained BiOBr/Bi composites by in situ reduction process in 20 mmol L-1 NaBH4 solution with different amounts of PVP. As shown in Figure 4a and b, in the absence of PVP, the flakes suffered serious damage compared to the untreated pristine BiOBr and the surface of the BiOBr became extremely rough. Intriguingly, with the addition of PVP (From 2.5-7.5g) (Figure. 4b-f), the shape of the BiOBr became more and more regular and the surface of the sample gradually turned uniform. Accordingly, we supposed that it was the particular stabilization of PVP that enabled the formation of the resultant BiOBr/Bi flakes with a perfect and uniform structure.


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Figure 4. SEM images of BiOBr/Bi prepared with different content of PVP, (a-b) 0g, (c-d) 2.5g, (e-f) 5g, (g-h) 7.5g.


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Figure 5a displayed the unit cell of BiOBr. The [Bi2O2] layers are combined by two slabs of bromine to form a ‘sandwich’ structure along the c-axis. We can clearly see that the Bi elements are regularly distributed on the [Bi2O2] layers, and the arrangement of Bi atoms on this layer is the closest. In other words, the distribution density of Bi atoms is the largest and most homogeneous on [Bi2O2]2+ layer in the BiOBr crystal. As the {001} facets of BiOBr consists of [Bi2O2]2+ layers (Figure 5b), the metallic Bi can hence form an excellent coating on the BiOBr crystal with exposed {001} facets.


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Figure 5. (a) Atomic structure of the BiOBr sample, (b) Atomic structure of the (001) facets. (c) XRD patterns (d) the value intensity ratios of the (002) and (200) peaks of the samples: BiOBr, BiOBr/Bi-10, BiOBr/Bi-15, BiOBr/Bi-20 and BiOBr/Bi-30. XRD patterns of the pure BiOBr and BiOBr/Bi products synthesized in NaBH4 solution with different concentrations of 10, 15, 20, and 30 mmol/L were shown in Figure 5c. The diffraction peaks of as-prepared BiOBr were in perfectly good agreement with tetragonal phase of BiOBr (JCPDS 73-0733). Fascinatingly, comparing to the standard spectrum of BiOBr, the (001), (002)， (003) and (004) peaks belonging to the {001} facets of as-prepared BiOBr/Bi displayed overwhelmingly higher relative intensities (Figure S1). The above results revealed that the main exposed surfaces of the as-prepared BiOBr were {001} facets. After being reduced by NaBH4 solution, all peaks of BiOBr/Bi samples can also be assigned to the BiOBr phase. However, probably because of its low amount, there were no characteristic peaks of metallic Bi detected in the XRD patterns. Meanwhile, no changes in the 2θ position of the peaks of BiOBr can be observed in the BiOBr/Bi samples, which implied that Bi appeared on the surface of BiOBr instead of entering its lattice. It can also be found that in the XRD patterns of the BiOBr/Bi, compared to the intensity of pure BiOBr, the relative intensity of (001), (002), (003) and (004) peaks decreased with the of NaBH4. The intensity ratios of the (002) versus (200) peaks were 4.54, 3.92, 3.58, 2.87 and 2.58 for pure BiOBr, BiOBr/Bi-10, BiOBr/Bi-15, BiOBr/Bi-20 and BiOBr/Bi-30, respectively(Figure 5d). This change in peak intensity also reflected the gradually decreased peaks of (001) series facets with increasing the NaBH4 content, confirming that the Bi nanoparticles were derived from the reduction of the exposed {001} facets of BiOBr.


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The DRS spectra of the pure BiOBr, Bi simple substance and the as-prepared BiOBr/Bi samples were shown in Figure 6. The absorption edge of the pristine BiOBr was estimated to be at 450 nm. After reduction by NaBH4 solution, the BiOBr/Bi hybrids exhibited more absorption within the visible light range and the absorption of BiOBr/Bi samples was gradually close to that of pure Bi simple substance with increasing the amount of NaBH4. This significant enhancement of visible light absorption derived from the SPR effect of metallic Bi. Combined with the HRTEM result in Figure.2b it can be further confirmed the presence of Bi0 species on the surface of BiOBr flakes.

Figure 6. (a) UV–vis diffuse reflectance spectra of the samples.

XPS spectrogram was analyzed on BiOBr/Bi-20 to elucidate the surface elemental compositions and oxidation states of the sample. The XPS survey spectrum of the sample was displayed in Figure 7a. The binding energy peaks at 158 eV (Bi 4f), 529eV (O 1s) and 681 eV (Br 3d) can be assigned to Bi, O and Br, which were the dominant composition of the BiOBr/Bi20. The high-resolution XPS spectrum of Bi4f of the sample was shown in Figure 7b. The core


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level binding energies at 164.08 eV and 158.8 eV were the feature of Bi3+ in BiOBr.

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Apart

from the two peaks, there were two peaks that were located at 163.07 eV and 157.5 eV with low intensity, which can be assigned to the metallic Bi.55 The XPS result was consistent with that of DRS and TEM, proving the existence of metallic Bi as a result of in situ reduction of the surface of BiOBr flakes. As displayed in Figure 7c, the XPS spectra of the Br3d region can be fitted into two peaks at 68.9 eV and 67.9 eV.56 The O1s spectra of the BiOBr/Bi-20 (Figure. 7d) can be deconvoluted into two peaks. The peak located at 528.9 eV can be ascribed to the Bi-O bonds in the BiOBr layered structure and the binding energy value of 530.0 eV is attributed to hydroxyl groups on the surface. 19


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Figure 7. XPS spectra of BiOBr/Bi-20 photocatalyst (a)survery pattern (b)Bi4f, (c) O1s and (d)Br3d.

Based on the above results, a potential reaction process was proposed to explain formation of uniform BiOBr/Bi structure, as illustrated is in Scheme 1. First, BiOBr flakes with exposed {001} facets were obtained by a modified hydrothermal method. When NaBH4 was added to the suspension of BiOBr flakes in the presence of the PVP, the redox reaction occurred between BiOBr and NaBH4, and the primary Bi nanocrystals nucleate firstly formed. PVP was known to play a significant role in the syntheses of metallic nanoparticles57. With the assistance and protection of PVP molecules acting as the capping agent, the damage to the surface of BiOBr by the reduction reaction was largely alleviated and the aggregation of nanoparticles was also suppressed. Thus, the metallic Bi homogeneously grew and coated in situ on the smooth surface of the BiOBr flakes. Scheme 1. Schematic Illustration of the Possible Formation Process of the uniform BiOBr/Bi structure


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In our experiments, the visible light photocatalytic performances of these BiOBr/Bi samples were evaluated for degradation of MO dye (Figure 8a). The pseudo-first-order kinetics of pure BiOBr and BiOBr/Bi samples were compared in Figure 8b. After NaBH4 treatment, the photoactivities of BiOBr/Bi samples gradually improved with the increase of NaBH4 concentrations. When the concentration of NaBH4 reached to 20mmol L-1, the BiOBr/Bi-20 showed the highest photocatalytic activity with a degradation rate of 70% in 150 min, whereas only 26% of MO had been decomposed by the pure BiOBr in the same time. The photocatalytic efficiency of BiOBr/Bi-20 (0.0081 min−1) was about 4 times higher than that of pristine BiOBr (0.0021 min−1). Therefore, the appropriate amount of metallic Bi can act as an excellent nonnoble metal cocatalyst for the enhancement of the photocatalytic activity under visible light irradiation. With further increasing the concentration of NaBH4, a decrease of degradation rate was observed. The excessive Bi metals might become the recombination centers of photoinduced charge carriers, decreasing the photocatalytic activity.58-59 The photodegradation of colorless organic pollutant bisphenol A (BPA) of the above samples under visible light was also monitored (Figure S2). The BiOBr/Bi hybrides still exhibited much higher photocatalytic activity than the pure BiOBr. Therefore, the photosensitization role of MO during photocatalysis in the manuscript can be ruled out. To gain a deep insight about the relationship between the property and microstructure, the visible light photocatalytic activities of the BiOBr/Bi composites prepared with different PVP amount were investigated in terms of MO decomposition. From Figure 8c, we can see that a proportional correlation was found between the PVP amount and the photocatalytic efficiency of the BiOBr/Bi samples. When no PVP was added, the BiOBr/Bi sample displayed a poor photocatalytic performance and the degradation rate was only 29% after 150 min. It is because of


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the severely damaged structure of BiOBr that led to the low photodegradation efficiency. The BiOBr/Bi hybrid with a uniform surface structure prepared in the presence of 7.5g PVP exhibited the best photocatalytic activity, which could degrade 71% of MO in 150 min. These results obviously elucidated that the photocatalytic activity of the obtained hybrids largely relied on the uniform structure. Evidently, the improved structure and properties were resulted from the use of PVP. The specific surface areas of our photocatalysts was measured via N2 adsorption– desorption isotherms. The BET-specific surface area for the as-obtained BiOBr/Bi composites by in situ reduction process in 20 mmol L-1 NaBH4 solution with different amounts of PVP is 9.58 m2/g (0g PVP), 9.44 m2/g (2.5g PVP), 8.93 m2/g (5g PVP), 8.51 m2/g (7g PVP), respectively. Even thought the specific surface area of samples increased with the decreased amount of PVP used in the preparation process, the BiOBr/Bi hybrid with a uniform surface structure prepared in the presence of 7.5g PVP still exhibited the best photocatalytic activity (see Figure 8c). This is because by adding PVP, the surface of BiOBr is protected to avoid further damage of the inner part of the BiOBr crystal. Therefore, the intact uniform BiOBr/Bi samples possess good performance than the damaged rough ones.


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Figure 8.(a) Photocatalytic degradation curves of MO for BiOBr/Bi samples prepared with different concentrations of NaBH4 under visible light irradiation (λ > 420 nm). (b) Apparent reaction rate constants for photocatalytic degradation of MO.(c) Photocatalytic degradation curves of MO for BiOBr/Bi samples with different amount of PVP under visible light irradiation (λ > 420 nm). (d) Photocurrent generation in the BiOBr and BiOBr/Bi-20 photocatalysts under visible-light irradiation (λ > 420 nm, [Na2SO4] = 0.1 M). The transient photocurrent generations were investigated for BiOBr flakes and BiOBr/Bi-20 electrodes to evaluate the mutual interaction between the metallic Bi and the BiOBr flakes. As shown in Figure 8d, under simulated solar irradiation a prompt and steady photocurrent generation was obtained during the on and off cycles. The BiOBr/Bi-20 sample exhibited much enhanced photocurrent intensity in comparison to the pure BiOBr. It was obvious that the in situ


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deposition of metallic Bi resulted in the significant enhancement of the photocurrent. The apparently higher photocurrent meant the acceleration of photoinduced charge carriers transfer process in the BiOBr/Bi hybrid. In our study, PVP is indispensable to synthesize uniform BiOBr/Bi structure with desired photocatalytic activity. Figure 9 showed the the images of samples prepared by 5, 20 and 30 mmol L-1 NaBH4 with and without the use of PVP. From Figure 9, the powder colors shift from shallow yellow to grey and dark black with the increase of concentration of NaBH4 solution. This phenomenon could be attributed to the different amount of metallic Bi reduced on the surface of BiOBr. However, by using the same concentration NaBH4 solution the colors of the samples prepared without PVP were nearly the same as their counterpart prepared with PVP, indicating that the BiOBr/Bi samples prepared with the use of PVP and without PVP exhibited the similar light absorption. So the samples with and without the use of PVP should have similar amount of reduced metallic Bi on their surface. The color of sample prepared by using low concentration of NaBH4 solution (5 mmol L-1) was nearly the same as the pure BiOBr samples, which meant that there was not enough metallic Bi on the surface of samples to improve the photocatalytic activity. To confirm this, we also measured the photocatalytic activity of NP BiOBr/Bi-5. (Figure S3). The sample almost showed no improvement of photocatalytic activity in comparison with the pure BiOBr. The SEM images of the NP BiOBr/Bi-5 were shown in Figure S4. Although the surface of BiOBr was not severely damaged in the reduction process because of the low concentration of NaBH4 solution, the reduced metallic Bi was not enough to reach the desired photocatalytic activity. If more NaBH4 is needed in the reaction to gain more metallic Bi on the surface of BiOBr, PVP is indispensible to in the formation of homogeneous BiOBr/Bi heterojunctions.


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Figure 9. Images of the BiOBr samples. (A) BiOBr/Bi-5; (B) BiOBr/Bi-20; (C) BiOBr/Bi-30; (D) NP BiOBr/Bi-5; (E) NP BiOBr/Bi-20; (F) NP BiOBr/Bi-30. Scavenger trapping experiments were carried out to detect the main active radicals in the degradation process. As illustrated in Figure 10, the degradation efficiency of MO was nearly not inhibited after addition of IPA (•OH scavenger), indicating that •OH was not the main active species responsible for the degradation of MO. By adding the MeOH (h+ scavegner), only 45% of MO was degraded after 150 min, less than the 73% degradation rate of MO without scavenger. This showed that the holes participated in the degradation process of MO, but it was still not the main active species. In the presence of BQ (•O2− scavenger), degradation efficiency of MO was significantly inhibited, suggesting that •O2− should be the main active species in MO oxidation. The electron spin resonance (ESR) analysis on BiOBr/Bi-20 and BiOBr was performed to investigate the production of active radical groups in the photocatalytic process. As shown in Figure 11 a and b, the typical ESR spectra of the DMPO-•O2- adduct and DMPO-•OH adduct were observed both in the light irradiated suspension of BiOBr/Bi and pure BiOBr. This indicated that the •O2- and •OH radicals were both produced in the photocatalytic process. As the •OH could not be formed on the VB of Bi-based photocatalyst60, the present •OH in this research


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may be derived from superoxide anion radicals. However, according to the scavenger experiments, the degradation efficiency of MO is nearly not inhibited after addition of IPA, indicating that •OH is not active specie responsible for the degradation of MO. Theoretically, the standard redox potential of O2/•O2− is –0.28 eV, which is more negative than that of the CB edge potential of BiOBr (0.29 eV), the electrons on the edge of CB could not reduce the O2 to generate •O2−.61 However, this cannot exclude the formation of •O2− by photo-generated electrons from the higher conduction bands. This may due to the structural defects of BiOBr that help to transfer the electrons on the edge of the CB to the higher conduction bands. The reactants absorbed on the surface of the BiOBr may also lead to the movement of electrons from the edge of the CB to higher CB. Some previous literatures also reported the generation of •O2− over pure BiOBr.62-63 The intensity of the formed •O2- signals in BiOBr/Bi-20 were stronger than those in pure BiOBr, thus accounting for a better photocatalytic activity of BiOBr/Bi than BiOBr toward the degradation of MO. The increase of •O2- signal intensity in BiOBr/Bi also indicated that there are more electrons that react with the surface adsorbed O2 in the conduction band of BiOBr, which may come from the metalic Bi.


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Figure 10. (a) Plots of trapping experiment of active species of BiOBr/Bi-20 during the photocatalytic reaction of MO under visible light irradiation (λ > 420 nm). The typical Nyquist plots of BiOBr/Bi-20 and BiOBr before and after visible light irradiation were shown in Figure 11c. It was obvious that the arc radius of BiOBr/Bi-20 was smaller than the pure BiOBr. The smaller arc radius indicated the higher efficiency of separation and transfer process of photogenerated charge carriers, thus the photocatalytic activity of BiOBr/Bi was improved. As seen in Figure S5, the inverse minimum frequency of BiOBr/Bi electrode shifted to a lower frequency in comparison to the pure BiOBr sample. The effective lifetime of injected electrons (τ ) in the conduction band of semiconductor can be calculated by equation as follows: τ=1/2πf, where f is the inverse minimum frequency.

64

From this equation, the lifetime of

injected electron of the BiOBr/Bi (13 µs) is estimated about 1.4 times higher than that of the BiOBr (9 µs). This prolonged lifetime should be ascribed to fast charge transfer that can efficiently inhibit the recombination of electron-holes carriers.


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Figure 11. ESR spectra: (a) DMPO–•O2− and (b) DMPO–•OH adducts on BiOBr/Bi photocatalyst under visible light irradiation (λ > 420 nm). (c) EIS Nynquist plots of pure BiOBr and BiOBr/Bi composites in the light on/off cycles under visible-light irradiation (λ > 420 nm, [Na2SO4] = 0.1 M) and (d) PL spectra of pure BiOBr and BiOBr/Bi composite. In a semiconducting material, the efficiency of charge migration and transfer can be reflected by the PL spectra. Figure 11d showed the comparison on PL spectra of BiOBr/Bi-20 and BiOBr under excitation wavelength of 233 nm. Compared with the bare BiOBr, the BiOBr/Bi sample showed obviously lower PL intensity, revealing its higher separation rate of the photoexcited charge carriers, thus the higher photocatalytic activity. Combined with the EIS results, the PL spectra analysis clearly demonstrated that the deposited metallic Bi can contribute to greatly inhibition in the recombination rate of charge carriers.47


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According to the above characterizations, a combination of mechanisms shown in Figure 12 had been proposed to justify the enhanced photocatalytic activity of BiOBr/Bi. Firstly, the SPR effect of deposited metallic Bi on the BiOBr enhanced the visible light absorption, as confirmed by the UV-vis DRS spectra. Meanwhile, the photoexcited electron–hole pairs of metallic Bi were preferentially formed by the SPR light absorption. Secondly, because the conduction band of BiOBr (0.29 eV) was lower than the Fermi level of Bi metal (–0.17 eV)48, the photoexcited electrons on the Bi surface would be injected to the conduction band of the adjacent BiOBr. The Bi shifted to more positive potentials to produce positive charges after the electrons were released. Then the metallic Bi would turn to its original state once it accepted the electrons from the valence band of BiOBr. This electron transfer process at the interface was beneficial to inhibit the recombination of photoinduced charge carriers. The photoexcited electrons on the higher conduction band of BiOBr, together with electrons from the metallic Bi, would react with the adsorbed O2 on the surface of BiOBr/Bi to generate more •O2−. Meanwhile, the holes generated at the valence band. The •O2− and holes then degraded the pollutants in the photocatalysis process .


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Figure 12. Schematic diagram of the BiOBr/Bi hybrid photocatalyst under visible light illumination.

CONCLUSIONS In summary, homogeneous heterostructured BiOBr/Bi photocatalysts were fabricated by a facile PVP-assisted in situ reduction route from {001} facets dominated BiOBr flakes at room temperature. In the process, Bi3+ on the {001} facets of BiOBr was in situ reduced to metallic Bi


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by BH4- ions and then deposited on the surface of the BiOBr flakes. The introduction of PVP had a significant effect on the uniform morphology as well as the photocatalytic activities. The significantly improved visible light photocatalytic efficiency of as-prepared regular BiOBr/Bi is not only attributed to the combined contribution of SPR effect, efficient separation of charge carriers by Bi nanoparticles, but also largely dependent on the perfect homogeneous structural property. Our work not only provides a highly effective plasmon-induced BiOBr/Bi uniform structural hybrid photocatalyst, but also elucidates a designed approach to rational bismuthbased-photocatalysts/metallic Bi with controllable structures and photocatalytic performance.

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Y. H. Zhang. E-mail: [email protected]. *H. W. Huang. E-mail: [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was jointly supported by National High Technology Research and Development Program (863 Program 2012AA06A109) of China, National Natural Science Foundations of China (Grant No. 51302251 and No. 51172245), Fundamental Research Funds for the Central Universities (2652013052),special co-construction project of Beijing city education committee, Key Project of Chinese Ministry of Education (No:107023).

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For Table of Contents Use Only Homogeneous

{001}-BiOBr/Bi

heterojunctions:

facile controllable synthesis and morphologydependent photocatalytic activity Yuxi Guo†, Yihe Zhang*,†, Na Tian†, Hongwei Huang*,†

Homogeneous {001}-BiOBr/Bi heterojunction are fabricated by facile controllable strategy and it shows morphology-dependent photocatalytic activity toward water purification.


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Bi Heterojunctions ... - ACS Publications

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