Bi2Fe4O9

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Facial Synthesis and Photoreaction Mechanism of BiFeO3/Bi2Fe4O9 Heterojunction Nanofibers Ting Zhang, Yang Shen, Yun-Hang Qiu, Yong Liu, Rui Xiong, Jing Shi, and Jianhong Wei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b03138 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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ACS Sustainable Chemistry & Engineering

Facial Synthesis and Photoreaction Mechanism of BiFeO3/Bi2Fe4O9 Heterojunction Nanofibers Ting Zhang, Yang Shen, Yunhang Qiu,Yong Liu, Rui Xiong, Jing Shi, Jianhong Wei*

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Luojiashan Road, Wuhan, 430072, P. R. China

* Corresponding author: Jianhong Wei, E-mail: [email protected] Tel: +86-27-68754613, Fax: +86-27-68752569

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ABSTRACT:

Pure BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterostructure nanofibers were successfully synthesized by a facile wet chemical process followed by electrospinning technique. Compared with the pure BiFeO3 and Bi2Fe4O9 nanofibers, the introduction of Bi2Fe4O9 in the BiFeO3 makes its absorption edge red shift to absorb much more visible light, and improves its separation efficiency of photo-generated carrier. Besides, the as-obtained BiFeO3/Bi2Fe4O9 nanofibers exhibit higher photocatalytic activity in both the degradation of Rhodamine B and H2 evolution from water under visible light irradiation. The BiFeO3/Bi2Fe4O9 nanofibers exhibited about 2.7 times and 2.0 times higher H2 evolution than that of pure BiFeO3 and pure Bi2Fe4O9 samples, respectively. The possible photoreactive mechanism of the BiFeO3/Bi2Fe4O9 nanofibers was carefully investigated according to the results of photocatalytic and photoelectric performance and a Z-scheme mechanism was proposed. Such BiFeO3/Bi2Fe4O9 heterostructure and its composing strategy

may

bring

new

insight

into

the

designing

of

highly

efficient

visible-light-responsible photocatalysts.

Keywords：：BiFeO3/Bi2Fe4O9 heterojunction nanofiber, photoelectric performance, water splitting Z-scheme mechanism.

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Introduction Recent years, much attention has been paid on the semiconductor photocatalyst for their

potential utilization of solar energy to solve the increasing environmental and energy crisis.1-3 In general, TiO2 is widely employed in the field of photocatalytic remediation of organic pollutants and photocatalytic water-splitting. However, its wide band gap (3.2 eV) and low quantum efficiency limit its widely application. For the above reason, it’s significant to search a stable, high efficiency and visible-light response photocatalysts. Until now, many metal oxides, such as Bi2WO6, BiFeO3, BiOCl, WO3, Ag3PO4 and so on had been developed and researched as visible-light-driven photocatalysts.4-8 Among them, Multiferroic BiFeO3, with optical, magnetic and electric properties parameters coupling in the same phase, has attracted a great deal of attention due to their potential applications in data storage, sensor and quantum electromagnets, etc. Furthermore, much attention has been paid on the BiFeO3 as a kind of visible light-driven photocatalyst because of its narrow bandgap (~2.2eV) and magnetic property at room temperature. These two properties make it easily to be driven by the visible light and recovered from the treated water via a magnetic separation technique. Nevertheless, the rapid electron-hole recombination and the low quantum yield limited its widely application.9,10 Until now, much effort had been performed to improve its photocatalytic performance by metal or nonmetal doping,11,12 conducting polymer sensitization13 and construction of multicomponent heterojunctions, etc. 14-16 As another important bismuth-based semiconductor oxide, Bi2Fe4O9 has also been widely used as a visible-light-driven photocatalyst because of its narrow double bandgaps

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of 1.9eV and 2.3eV. 17,18 In general, Bi2Fe4O9 is apt to be formed as the second phase during the process of preparing BiFeO3 powder or ceramics, making the phase purity of BiFeO3 to be very processing-sensitive.19 In our work, we found that BiFeO3 containing small amounts of Bi2Fe4O9 shows a better photocatalytic activity and photoelectric performance than that of pure BiFeO3. However, although some researchers had reported the visible-light driven photo-degradation activities of BiFeO3/Bi2Fe4O9 microspheres until now,20-22 there has little report on its photocatalytic hydrogen evolution from water, and their photoelectric performance and photoreactive mechanism of BiFeO3/Bi2Fe4O9 heterojunctions still needed further investigated. In this work, a facial method was used to prepare BiFeO3/Bi2Fe4O9 heterojunction nanofibers, their photoelectric conversion process photoelectric and photocatalytic hydrogen evolution process were carefully studied. A possible photocatalytic mechanism of BiFeO3/Bi2Fe4O9 heterojunction nanofibers was proposed.

Experimental section Pure BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterostructure nanofibers were prepared by a facile wet chemical process followed by electrospinning technique. The prepared process,

characterization,

photoelectrochemical

properties

measurement

and

photocatalytic activity evaluation were described in detail in Supporting Information.

Results and discussion Figure 1 presents the XRD patterns of the as-prepared BiFeO3, Bi2Fe4O9 and BiFeO3/Bi2Fe4O9 heterojunction nanofibers with different weight percentage of Bi2Fe4O9.

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The pure BiFeO3 nanofibers (Figure1a) were obtained at annealing temperature of 500 oC. According to the JCPDS card no. 20-0169, the diffraction peaks in Figure1a can be assigned to the rhombohedra structure of bismuth ferrite and no other impurity phases were observed. The Bi2Fe4O9 phase was detected accompanied by the formation of the BiFeO3 phase when the annealing temperature up to 550 oC ( Figure 1b~e ), the relatively content of Bi2Fe4O9 in the as-spun samples increased gradually with the increasing of annealing temperature changed from 550 oC to 700 oC. According to the Le Bail whole-pattern analysis method,22 the relative contents of Bi2Fe4O9 in the as-spun samples were calculated to be about 3.9%, 12.3 %, 28.8% and 70.7 %, and they were named as BB01, BB02, BB03, and BB04, respectively. The pure Bi2Fe4O9 nanofibers with an orthorhombic structure were obtained at annealing temperature of 750 oC according to the JCPDS card no. 25-0090 (Figure 1f). The controlled morphology of the as-prepared BiFeO3/Bi2Fe4O9 heterojunction samples were shown in Figure 2. Figure 2a presents the typical SEM images of the BB02 nanofibers. As shown, the BB02 nanofibers with a diameter of about 100 - 400 nm were aligned in random orientation due to the drum collector.

After annealed at 600 oC, the

nanofibrous morphology of the BB02 nanofibers was remained (Figure 2b). A typical TEM image of BB02 nanofibers was shown in Figure 2c. It clearly shows that a single nanofiber was compactly packed with nanoparticles, which agglomerated to form BiFeO3/Bi2Fe4O9

nanofibers.

To

better

investigate

the

crystal

structure

and

micro-composition of the BiFeO3/Bi2Fe4O9 nanofibers, a HRTEM image of as-obtained BB02 sample was performed and shown in Figure 2d. The interplanar spacing of

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approximately 0.319 nm was indexed to (121) crystal plane of the Bi2Fe4O9 phase, whereas the interplanar spacing of approximately 0.288 nm correspond to (110) crystal plane of the BiFeO3 phase. These lattice fringe parameters match well with the corresponding lattice parameters calculated by XRD. It confirms that a well-defined heterojunction formed in the interface of BB02 nanofibers. The other BiFeO3/Bi2Fe4O9 heterojunction samples have the similar structure with the BB02 because of the similar preparation process (omitted here). Figure 3 shows the UV-vis diﬀuse reﬂectance absorption spectra of different samples. As shown in Figure 3a, the pure BiFeO3 nanofibers possessed photo-absorption from the UV to visible light until 576 nm, corresponding to the band gap of 2.15 eV, which was calculated from the tangent line in the plot of the K-M function ( (α hv)2 ) vs photo energy (hv) by extrapolating the tangentlines to α = 0 . For pure Bi2Fe4O9, an obvious absorption edges at ~ 632 nm is corresponding to optical bandgap of 1.96 eV, another small absorption edges at ~ 810 nm corresponding optical bandgap of 1.55 eV. The absorption edge at ~ 632 nm can be ascribed to electronic transitions from valence band to conduction band, and the absorption at ~ 810 nm is ascribed to the d-d transitions of Fe.23,24 For BiFeO3/Bi2Fe4O9 heterojunction samples (from BB01 to BB04), compared with pure BiFeO3, the absorption edges of all heterojunction samples exhibit a red shift after coupled with Bi2Fe4O9, and the amount of red-shifting increases with the increasing the Bi2Fe4O9 content, which suggest that they can absorb more visible light than that of pure BiFeO3 nanofibers. Besides, with the enhancement of Bi2Fe4O9 phase in the composite, an

6


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obvious absorption in the range of 600-800 nm was observed for the BB03 and BB04 samples, it’s the characteristic of the d-d electronic transitions of Fe element coming from Bi2Fe4O9 phase. According to Figure 3b, the bandgaps from BB01 to BB04 were estimated to be 2.12 eV, 2.10 eV, 2.02eV, 2.06 eV, respectively. The decreased bandgap is benefit to broaden the absorption range of visible-light, correspondingly result in enhancement of photocatalytic activity in the visible-light region. The Band-gaps, absorption edges and corresponding BET surface areas (SBET) of the catalysts were shown in Table S1 . The order of SBET was as follows: BB02 > BB03> BB04 > Bi2Fe4O9 > BB01> BiFeO3. In our experiment, with the increasing of heat-treated temperature, the BiFeO3 phase gradually transformed into Bi2Fe4O9 phase along with the growing up of particle size. The specific surface area is related to the particle size and particle stacking way, as a result, BB02 samples exhibits the optimum specific surface area in our case. In general, specific surface area is one of the most important factors which significantly affect the activities of the photocatalysts. Since most photocatalytic reactions occur at the surface of the photocatalyst, a larger surface area can provide more available surface active sites and faster interfacial charge transfer for the reaction,

correspondingly

leading to higher photocatalytic activity.25-27 the results of SBET of BiFeO3/Bi2Fe4O9 heterojunction samples are basically consistent with the photocatalytic performance. The photocatalytic performance of as-synthesized samples was estimated by photocatalytic degradation of RhB under visible light irradiation (Figure 4). It was demonstrated that the photolysis of RhB without a photocatalyst could be ignored according to the blank experiments(it was not shown in the Figure 4), which means that

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the degradation of RhB by as-prepared photocatalysts was mainly due to photocatalysis(omitted here). To evaluate the reaction kinetics of the degradation of RhB, the plot of ln(C0/C) versus time is plotted in Figure 4a, which shows an approximately linear dependence. The observed rate constants for pure BiFeO3 and pure are Bi2Fe4O9 are 0.001121 and 0.00152/min respectively. As indicated, the BB02 heterojunction nanofibers exhibited better photocatalytic activities than those of pure BiFeO3 and Bi2Fe4O9 nanofibers, the observed rate constants for BB02 is 0.00916/min, which is about 6.57 times and 5.02 times higher than that of pure BiFeO3 and pure Bi2Fe4O9 samples, respectively. On the other hand, although all of the BiFeO3/Bi2Fe4O9 heterojunctions exhibit much higher photodegradation activity than that of pure samples, the photodegradation rate first increases with the increasing of the Bi2Fe4O9 content in the nanocomposites, and reaches the optimal photocatalytic activity when Bi2Fe4O9 content is about 12.3 wt% (BB02),

then, the photodegration rate decreases obviously with the

further increasing of Bi2Fe4O9 content. The obvious different of photocatalytic activities should not attributed to their morphology or surface area (Table S1), for there are no noticeable changes of them for these samples . So, the presence of BiFeO3/Bi2Fe4O9 heterojunction and the exposure of both phases on the surface are maybe the main cause for the enhancement of photocatalysts.17,28 From XRD, we know that the content of Bi2Fe4O9 in the nanocomposites gradually increases with the increasing of heat-treated temperature. The photocatalytic activity is firstly increased with the increasing of BiFeO3/Bi2Fe4O9 heterojunction in the surface of nanofibers. Then it decreased gradually when further increasing the content of Bi2Fe4O9 in the nanocomposites, which due to the

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factor that the surface of BiFeO3/Bi2Fe4O9 nanofibers are covered by much more Bi2Fe4O9 nanoparticles, resulting in the decreasing of the amount of exposed BiFeO3/Bi2Fe4O9 heterojunction, corresponding leading to poor photocatalytic performance. Figure 4b shows the rate of photocatalytic H2 evolution rate from the water splitting and triethanolamine over as-synthesized samples. The result shows that the BiFeO3/Bi2Fe4O9 samples exhibit much higher H2 evolution rate, and the H2 evolution rate firstly increases with the increasing of the Bi2Fe4O9 content and then decreases with the further increasing of Bi2Fe4O9 content in the BiFeO3/Bi2Fe4O9 heterojunctions. Among them, the BB02 sample exhibits a maximum H2 evolution rate (~ 800 µmol/g), which was about 2.0 times that of the pure Bi2Fe4O9 samples. Besides, the BiFeO3/Bi2Fe4O9 heterojunction photocatalysts exhibits favorable RhB degradation stability and H2 evolution stability (Figure 4c & d). The rates for RhB degradation and H2 evolution had no noticeable deterioration after the fifth run, which means that the BiFeO3/Bi2Fe4O9 heterojunction photocatalysts possess excellent recycling stability. To further understand the role of BiFeO3/Bi2Fe4O9 heterojunction, the charge transport efficiency and the amount of surface hydroxyl radicals of different samples were investigated by transient photocurrent (Figure 5a) and photo-luminescence (PL) measurement (Figure 5b), respectively.

In general, charge carrier density and charge

mobility are key parameters to evaluate the charge transport efficiency. An enhancement in the photocurrent indicates a more efficient charge transfer and a slower recombination of photoelectron and holes occurred, which is beneficial to relevant photocatalytic activity. As indicated in Figure 5a, the photocurrent density of BB02 sample is 1.8 µA/cm2, which

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is far large than that of the pure BiFeO3 (0.6 µA/cm2) and pure Bi2Fe4O9 (0.9 µA/cm2) samples, respectively. Meanwhile, the photocurrent densities change in the order of BB02 > BB03> BB01> BB04 > Bi2Fe4O9 > BiFeO3. The higher photocurrent density of BB02 indicated more efficient photogenerated electrons and holes separation and less recombination of the charge carriers at their interface. The analysis of •OH radical’s formation on the sample surface was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule.

Figure

5b shows the changes of PL spectra of different samples in terephthalic acid solution with irradiation time. Usually, PL intensity is proportional to the amount of produced hydroxyl radicals.29,

30

As indicated, at a fixed time (60min), the BB02 sample still shows the

strongest PL intensity, infers that there are more •OH radicals on its surface. The result can be attributed to the large amounts of unpaired electrons and holes on the surface of the BB02 samples, as shown in the results of I- t analysis. The unpaired electrons and holes existed on the surface of BB02 sample are easily caught by surface adsorbed O2 and H2O to form more •OH radicals to participate the photocatalytic reaction, correspondingly resulting in higher photocatalytic activity. The results agreed well with the photocatalytic degradation of RhB and photocatalytic H2 evolution measurements. The above result also infers that •OH radicals maybe the main active species in the photocatalytic system. To confirm the speculation about main active species, the trapping experiments were designed

with

disodium

ethylenediamine

tetraacetate

(EDTA-2Na,

0.1mmol),

1,4-benzoquinone (BQ, 0.1 mmol) and tert-butyl alcohol (TBA, 0.1 mmol) used as the hole (h+) scavenger, superoxide radical (O2-) scavenger and hydroxyl radical (•OH)

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scavenger, respectively. Besides, in terms of the nanoscaled BiFeO3/Bi2Fe4O9 perovskite, the interfacial Fe atoms partially exist as Fe3+ with extremely high surface potential, and they are apt to react with hydrogen peroxide (H2O2) to form •OH radicals through the combined effects of conduction electron scavenging and the Fenton reaction by the iron species31,32 ( the main reaction process can be found in supporting information). H2O2 ( 0.1 mmol)was also added to detect the role of •OH radicals. As displayed in Figure 6a, when 1 mmol of BQ was added, the degradation percentage of RhB was reduced from 65% to 51%, indicating that superoxide radical play a relatively important role in the photocatalytic oxidation process. Besides, the adding of TBA caused an obvious suppression in the photodegradation of RhB, the degradation percentage of RhB was reduced from 65% to 30%. Besides, with the aid of H2O2, RhB was almost completely degraded after visible-light irradiation for 1.5 h, which means that •OH was main active species. The above results agree well with the terephthalic acid photoluminescence probing analysis. On the contrary, the photocatalytic activity of BiFeO3/Bi2Fe4O9 heterojunctions had been improved slightly by the addition of EDTA-2Na, it means that the holes were not the main active species to the degradation process. The time-resolved transient fluorescence spectroscopy was performed to further examined the lifetime of charge carriers in the presence of different components, as shown in Figure 6b. It shows a longer decay time value for BiFeO3/Bi2Fe4O9 heterojunction nanofibers than that of pure BiFeO3 or Bi2Fe4O9 samples, which is caused by the effective charge transfer across the interface of BiFeO3 and Bi2Fe4O9.33 The

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ultrafast charges transfers, separating the charges in spatially, inhibit the recombination and prolong the lifetime of charge carriers, thereby promotes the photocatalytic reactions for photodegradation of RhB and H2 evolution. Based on the above results, the photocatalytic mechanism for the BiFeO3/Bi2Fe4O9 is proposed and illustrated in Scheme 1.

According to the test result of XPS (Figure S1),

the valence band (VB) of BiFeO3 and Bi2Fe4O9 were 2.58 eV and 1.19 eV, respectively. Our results are agree well with the Li and Wu’s reports.34,35 The bandgaps obtained from UV–vis curves are 2.15 and 1.96 eV for BiFeO3 and Bi2Fe4O9, respectively, and therefore the conductor band(VB) positions of BiFeO3 and Bi2Fe4O9 can be calculated to be 0.43 eV and -0.77 eV, respectively. Now that the CB band edge of BiFeO3 is located at 0.43 eV, indicating that the photogenerated electrons shows poor ability to reduce H+ into H2. On the other hand, the CB band edge of Bi2Fe4O9 is located at -0.77 eV, indicating that the photogenerated electrons in the Bi2Fe4O9 CB possess stronger reducing power to split water than that of pure BiFeO3. The bandgap analysis is agreed well with our experiment result. Now that the CB edges of BiFeO3 (0.43eV) is much close to the VB of Bi2Fe4O9 (1.19eV), the potential difference is about 0.76 eV (See scheme 1) and the BiFeO3/Bi2Fe4O9 samples show much higher H2 evolution performance than that of pure BiFeO3 and Bi2Fe4O9 samples. A Z-Scheme mechanism is more suitable for this photoreactivity system36,37. In this case, the photo-generated electrons in the CB of BiFeO3 maybe first shift to the VB of Bi2Fe4O9 and combine with the holes in its VB. This kind of charge transfer process efficiently enhances the separation of electron/hole pairs and suppress their recombination,

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meanwhile, reserve the photogenerated electrons still on the CB of Bi2Fe4O9, thus leading to much stronger photodegradation performance and higher H2 evolution performance of BiFeO3/Bi2Fe4O9 samples under visible-light irradiation.

Conclusions The BiFeO3/Bi2Fe4O9 heterojunction nanofibers were successfully synthesized by an electrospinning technique combined with a facile wet chemical process. The ratio of Bi2Fe4O9 to BiFeO3 in the composites could be controlled by adjusting the sintering temperature of reactive process. The investigation of optical absorption and photocatalytic activity demonstrated that the BiFeO3/Bi2Fe4O9 heterojunction nanofibers samples showed significantly higher photocatalytic activity than that of BiFeO3 and Bi2Fe4O9 nanofibers on the degradation of RhB and photocatalytic splitting water to produce H2 under visible-light irradiation. It was found that formation of well-defined heterojunctions between BiFeO3 and Bi2Fe4O9 are account for the enhancing of the photoelectric property and photocatalytic activity by facilitating the separation and restricting the recombination of photogenerated carriers.

Among them, the BB02 sample

exhibited optimum visible-light-induced photocatalytic activity, and besides, it could be easily recycled without an obvious decrease of photocatalytic activity. In a word, the as-obtained

BiFeO3/Bi2Fe4O9

heterojunction

nanofibers

could

be

used

as

a

high-performance visible-light-driven photocatalyst for potential applications in environmental protection and solar fuel production.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51272185, 11474224 & 61575145) and the National Program on Key Basic Research Project (973 Grant No. 2012CB821404).

ASSOCIATED CONTENT Supporting information Available The prepared process, characterization, photoelectrochemical properties measurement and XPS spectra can be found in supporting information. This information is available free of charge via the internet at http://pubs.acs.org.

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TiO2

Nanofiber

Photocatalyst

with

Enhanced

H2-Production Activity. Int. J. Hydrogen Energ. 2014, 39, 15394-15402.

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Photocatalytic


Figures captions: Figure 1 XRD patterns of BiFeO3, Bi2Fe4O9 and BiFeO3/Bi2Fe4O9 with different content of Bi2Fe4O9 (named as BB01, BB02, BB03 and BB04, respectively).

Figure 2 (a) SEM image of BB02 nanofiber precursor. (b) TEM image of BB02 nanofiber (annealed at 600 oC). (c) TEM image of a single BB02 nanofiber. (d) HRTEM image of BB02 nanofiber.

Figure 3 (a) Ultraviolet–visible diffuse reflectance spectra of as-synthesized BiFeO3, Bi2Fe4O9, and BiFeO3/Bi2Fe4O9 heterojunction samples. (b) (αhv)1/2 as a function of photon energy, where α is the absorption coefficient, and the intercepts of extrapolated straight line give the corresponding direct band gaps of different samples.

Figure 4 (a)Visible light-driven photocatalytic degradation of RhB using BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers. (b) Stable hydrogen evolution from water using BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers under visible light irradiation (λ> 420 nm) (A typical time courses of H2 production from water containing 10 vol.% triethanolamine as an electron donor). (c) Recyclability of the BB02 for the degradation of RhB under visible light irradiation. (d) Recyclability of the BB02 for the H2 evolution from water under visible light irradiation.

Figure 5 (a) Transient photocurrent responses of as-synthesized samples under visible light irradiation in 1 M Na2SO4 aqueous solution; [Na2SO4] = 0.5 M; λ > 420 nm, continuously N2 purged.

(b) Fluorescence spectral changes in 5 x10-4M NaOH solution

of terephthalic acid for different samples.

Figure 6 (a) Photocatalytic activities of BiFeO3/Bi2Fe4O9 heterojunction with different 20


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scavengers for the degradation of RhB under visible light irradiation. (b) Time resolved photoluminescence spectra for BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers.

Scheme 1 Proposed schematic diagram for the electron-hole transport at the interface of the BiFeO3/Bi2Fe4O9 heterojunction nanofibers.

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TABLE OF CONTENTS GRAPHIC

Facial Synthesis and Photoreaction Mechanism of BiFeO3/Bi2Fe4O9 Heterojunction Nanofibers Ting Zhang, Yang Shen, Yunhang Qiu,Yong Liu, Rui Xiong, Jing Shi, Jianhong Wei*

A Z-scheme photoreaction mechanism toward a green, sustainable and high performance photocatalyst-BiFeO3/Bi2Fe4O9 heterojunction nanofibers was proposed in this work.

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Figure 1 XRD patterns of BiFeO3, Bi2Fe4O9 and BiFeO3/Bi2Fe4O9 with different content of Bi2Fe4O9 (named as BB01, BB02, BB03 and BB04, respectively). 297x233mm (300 x 300 DPI)



Figure 2 (a) SEM image of BB02 nanofiber precursor. (b) TEM image of BB02 nanofiber (annealed at 600 oC). (c) TEM image of a single BB02 nanofiber. (d) HRTEM image of BB02 nanofiber. 207x155mm (150 x 150 DPI)


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Figure 3 (a) Ultraviolet–visible diffuse reflectance spectra of as-synthesized BiFeO3, Bi2Fe4O9, and BiFeO3/Bi2Fe4O9 heterojunction samples. 928x644mm (96 x 96 DPI)



Figure 3(b) (αhv)1/2 as a function of photon energy, where α is the absorption coefficient, and the intercepts of extrapolated straight line give the corresponding direct band gaps of different samples. 297x207mm (300 x 300 DPI)


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Figure 4 (a)Visible light-driven photocatalytic degradation of RhB using BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers. 850x651mm (96 x 96 DPI)



Figure 4(b) Stable hydrogen evolution from water using BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers under visible light irradiation (λ> 420 nm) (A typical time courses of H2 production from water containing 10 vol.% triethanolamine as an electron donor). 928x644mm (96 x 96 DPI)


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Figure 4(c) Recyclability of the BB02 for the degradation of RhB under visible light irradiation. 297x206mm (300 x 300 DPI)



Figure 4(d) Recyclability of the BB02 for the H2 evolution from water under visible light irradiation. 297x205mm (300 x 300 DPI)


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Figure 5 (a) Transient photocurrent responses of as-synthesized samples under visible light irradiation in 1 M Na2SO4 aqueous solution; [Na2SO4] = 0.5 M; λ > 420 nm, continuously N2 purged. 928x731mm (96 x 96 DPI)



Figure 5 (b) Fluorescence spectral changes in 5 x10-4M NaOH solution of terephthalic acid for different samples. 793x624mm (96 x 96 DPI)


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Figure 6 (a) Photocatalytic activities of BiFeO3/Bi2Fe4O9 heterojunction with different scavengers for the degradation of RhB under visible light irradiation. 850x651mm (96 x 96 DPI)



Figure 6 (b) Time resolved photoluminescence spectra for BiFeO3, Bi2Fe4O9 and BiFeO3 / Bi2Fe4O9 heterojunction nanofibers. 928x731mm (96 x 96 DPI)


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Scheme 1 Proposed schematic diagram for the electron-hole transport at the interface of the BiFeO3/Bi2Fe4O9 heterojunction nanofibers.

218x134mm (96 x 96 DPI)


Bi2Fe4O9

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