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Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity
Author’s Accepted Manuscript Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity Gongjuan Wu, Yan Zhao, Yawen Li, Bouasavanh Souvanhthong, Hongmei Ma, Jingzhe Zhao www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)32876-6 https://doi.org/10.1016/j.ceramint.2017.12.168 CERI17048
To appear in: Ceramics International Received date: 30 October 2017 Revised date: 22 December 2017 Accepted date: 23 December 2017 Cite this article as: Gongjuan Wu, Yan Zhao, Yawen Li, Bouasavanh Souvanhthong, Hongmei Ma and Jingzhe Zhao, Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.168 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity Gongjuan Wu, Yan Zhao*, Yawen Li, Bouasavanh Souvanhthong, Hongmei Ma, Jingzhe Zhao* College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China [email protected] [email protected] Corresponding authors. Tel: +86-731-82548686;
ABSTRACT Bi4O5Br2 nanosheets have been fast synthesized via a facile aqueous method. The morphology, crystal structure, optical property, and electronic structure of the obtained samples were discussed. During the reaction process, it was found that the reaction parameters including the amount of NaBr and the amount of NaOH significantly influenced the composition and structure of the samples. The photocatalytic performance of the prepared samples was explored by degrading BPA under visible-light irradiation. Bi4O5Br2 nanosheets exhibited enhanced photocatalytic activity in comparison to BiOBr or Bi5O7Br, which can be attributed to its higher visible-light absorption and lower interface charge transport resistance. Additionally, the photocatalytic degradation mechanism over Bi4O5Br2 nanosheets was also investigated. h+ and •O2− were the main photoactive species during the visible-light photocatalytic process by quencher experiments and EPR results. This study is expected to provide new insights into the development on facile fabrication and 1
potential application of Bi-deficient bismuth oxybromide photocatalysts. Keywords: Bi4O5Br2 nanosheet; high visible-light absorption; low interface charge transport resistance; photocatalytic activity.
1. Introduction Bismuth oxyhalide (BiOX, X= Cl, Br, I) layered materials have received considerable interests in recent years, because they have great potential applications in the field of photocatalysis for energy conversion and environment remediation [1–7]. Bismuth oxyhalide belongs to a novel class of promising layered materials interleaved with [Bi2O2]2+ slabs and double X− slabs [1, 3, 5, 7–9]. The strong intralayer covalent bonding of [Bi2O2]2+ slabs and the weak interlayer van der Waals interaction of X− slabs lead to the nonuniform charge distribution in the crystal structure, which would polarize the related atoms and orbitals to form internal electric fields perpendicular to each layer [2, 5, 7]. The internal electric field induced by the unique layered structure is believed to promote the separation of photogenerated hole-electron pairs and the migration to catalyst surface, which are favorable for the subsequent redox reactions on the surface of materials [2, 7]. Br-deficient Bismuth oxybromide (BixOyBrz), as a new type of Bismuth oxyhalide, was reported to exhibited excellent photocatalytic activity due to high optical absorption and efficient separation of photoinduced hole-electron pairs compared to that of BiOBr [10–18]. Extensive investigations were devoted to the fabrication of BixOyBrz materials, including solvothermal method [11, 18], hydrothermal method 2
[15–17, 19], and microwave-assisted method [20, 21]. For example, Xia group declared that Bi4O5Br2 nanosheets with good photocatalytic activity for ciprofloxacin photo-degradation were prepared via an ionic liquid-assisted solvothermal method at 140 °C for 24 h [11]. Zhang group accomplished the synthesis of Bi3O4Br by a hydrothermal
strategy,
its
application
on
the
decomposition
of
sodium
pentachlorophenate exhibited good photoactivity [17]. Nan group reported a microwave heating route to synthesize Bi24O31Br10 nanoflakes at 400 W for 3 min, good photocatalytic activities were achieved in tetracycline hydrochloride degradation [20]. However, it still remains a challenge to develop simple, energy-efficient and low-cost strategies for the synthesis of Br-deficient bismuth oxybromides. Herein, we present a facile aqueous method to synthesize Bi4O5Br2 nanosheets at 70 °C for 10 mins. During the prepared process, the amount of NaBr and NaOH involved in the reaction has vital effects on the composition and structure of samples. The morphology, crystal structure, optical property and electronic structure of Bi4O5Br2
nanosheets
were
systematically
investigated.
The
visible-light
photocatalytic activity of Bi4O5Br2 nanosheets was studied on colorless bisphenol-A (BPA) degradation, and the degradation mechanism was proposed accordingly. 2. Experimental section Sample preparation. All reagents were analytical grade and used without further purification. In a typical experiment for the preparation of Bi4O5Br2 nanosheets, 10 mL of 1 mol L−1 Bi(NO3)3 solution (with 0.02 mol of HNO3 in the solution) was first added into a three-neck flask and stirred for 2 min at 70 °C, and then 0.07 g of 3
cetyltrimethylammonium bromide (CTAB) dissolved in 20 mL of distilled water was put into the reaction system. Two minutes later, 30 mL of mixed solutions containing 3.6 g of NaOH and 6.2 g of NaBr (6 mmol) were simultaneously dumped into the reaction vessel, and then the reactions were performed for 10 min under continuous stirring to obtain the resulting products. Finally, the precipitates were collected and washed several times, and then dried in an air oven at 60 °C for 6 h to get sample powders. For comparison, BiOBr crystals were also synthesized under parallel conditions without NaOH introduction. Sample characterization. X-ray diffraction (XRD) measurements were detected by using a shimadzu XRD-6000 instrument. Morphologies of samples were examined by a Hitachi S-4800 field emission scanning electron microscope (FESEM). A Tecnai G2 F20 equipment was used to characterize high resolution transmission electron microscopy (HRTEM) images. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher Scientific K-Alpha 1063. UV-vis absorption spectra were analyzed by a Shimadzu UV-1800 spectrophotometer, and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) were used to determine the optical properties of the samples on a spectrophotometer (Shimadzu UV-2600, Japan). Electrochemical measurements were performed on an electrochemical workstation (CHI-660E, China). Photocatalytic experiments. Photocatalytic activities of the as-obtained samples were evaluated by degrading colorless bisphenol-A (BPA) solution under visible-light illumination. The photocatalytic experiments were carried out in a photochemical instrument cooled with circulating water, and a 500 W metal halide lamp with a 4
420-nm cutoff filter was selected as visible-light source. For the degradation of BPA, 100 mg of photocatalyst was dispersed into 100 mL of BPA solution (20 mg L−1). Before illumination, the suspensions were stirred for 1 hour in dark to ensure the establishment of adsorption-desorption equilibrium. At given time intervals, 2 mL of the suspension was taken out and subsequently centrifuged to eliminate the sample powders. Then, the powder-free solutions were measured by recording variations of the absorption band maximum (275 nm) of BPA by using UV-vis spectroscope. 3. Results and discussion 3.1 Morphology and structure of the as-synthesized samples The morphology and crystalline structure of the as-obtained samples were characterized by SEM and XRD methods, the result of a representative Bi4O5Br2 sample is shown in Figure 1a and b (reaction temperature: 70 °C; NaOH: 3.6 g; R(Br/Bi) ratio = 6:1; CTAB: 3 wt%, and the sample was assigned as S1). The diffraction peaks of the XRD pattern in Figure 1b were indexed to the pure Bi4O5Br2 (the standard card calculated from ICSD #412591), which matched well with the reported studies [10, 12]. When NaOH was not introduced into the reaction system, the XRD pattern in Figure 1d revealed that the synthesized sample was BiOBr (PDF #73-2061). SEM images in Figure 1a and c showed that both Bi4O5Br2 and BiOBr belonged to nanosheet structures, whereas Bi4O5Br2 appeared as thinner bent nanosheets compared to BiOBr straight nanosheets.
5
Figure 1. SEM images and XRD patterns of the as-synthesized Bi4O5Br2 (a, b) and BiOBr (c, d).
TEM strategy was further carried out to investigate the detailed structures of the as-obtained Bi4O5Br2. The TEM image of Bi4O5Br2 in Figure 2a displayed a sheet-like morphology with thickness of about 8 nm. Figure 2b of HRTEM image presents distinct lattice spacings of 0.283 nm, corresponding to (020) crystallographic plane of Bi4O5Br2 crystallites [22]. The simulated crystal structure in Figure 2c reveals that Bi4O5Br2 is a layer structure interleaved with [Bi4O5]2+ slabs and double slabs of Br− ions, which is related to the formation of nanosheet structures.
6
Figure 2. (a) TEM and (b) HRTEM images of Bi4O5Br2 nanosheets; (c) simulated crystal structure of Bi4O5Br2 X-ray photoelectron spectroscopy (XPS) technique was used to study the chemical composition and surface chemical states of Bi4O5Br2 nanosheets. The XPS survey spectrum in Figure 3a showed that Bi, O, Br and C elements are consistent with their chemical compositions. The high-resolution spectra of Bi 4f in Figure 3b can be fitted into four peaks, indicating two coordination environments of Bi in Bi4O5Br2 nanosheets [15, 17, 23, 24]. Two main peaks at 165.28 and 159.98 eV arising from 4f5/2 and 4f7/2 demonstrates +3 oxidation state of Bi. The additional peaks with lower 7
binding energies at 163.88 and 158.68 eV could be ascribed to lower valence of oxygen vacancy-related Bi atoms, which were generated by X-ray irradiation during examination process. The symmetric O 1s peak at 531.18 eV was assigned to lattice oxygen in Bi4O5Br2 (Figure 3c). Figure 3d showed that the binding energy of 69.27 and 67.13 eV were ascribed to Br 3d3/2 and Br 3d5/2 for the Bi4O5Br2 sample.
Figure 3. XPS spectras of the Bi4O5Br2 Sample. (a) Survey of the Sample; (b) Bi 4f; (c) O 1s; and (d) Br 3d. For the synthesis of Bi4O5Br2 crystals, simultaneous addition of NaBr and NaOH to the reaction system led to competition reaction of Br− and OH− with [Bi2O2]2+, resulting in the evolution of Br-poor bismuth oxybromides. Accordingly, the introducing amount of NaBr in the reaction would have vital effects on the 8
composition and structure of samples. The molar ratio of NaBr to Bi(NO3)3 was assigned as R(Br/Bi), and R(Br/Bi) of 6:1 was chosen for typical experiments. Figure 4 gives SEM and XRD results of two samples prepared with R(Br/Bi) =3:1 and 1:1, and other reaction parameters were controlled to be constant parallel to sample S1. When R(Br/Bi) decreased from 6:1 to 3:1, the sample was the mixture of Bi4O5Br2 and Bi5O7Br in crystallinity (Figure 4b), and microrods appeared in the image except for the nanosheets (Figure 4a). When R(Br/Bi) further reduced to 1:1, the sample was mainly composed of Bi5O7Br micro/nanorods (Figure 4c and d, assigned as sample S2). The results demonstrated that the substitution reaction of Br with O atoms speeded up under a little NaBr introduction, and thus the crystalline structure changed from Bi4O5Br2 to Bi5O7Br.
Figure 4. SEM images and XRD patterns of samples with different molar ratios of NaBr to Bi(NO3)3: (a, b) R(Br/Bi) =3:1 and (c, d) R(Br/Bi) =1:1. 9
NaOH amount also played crucial effects on regulating the morphology and structure of the obtained samples, as shown in Figure 5. When the amount of NaOH increased to 3.8 g, the crystalline structure was still Bi4O5Br2, and the nanosheet became thinner compared to sample S1. While the amount of NaOH increased to 4.0 g, Bi5O7Br microrods appeared (Figure 5c and d). Therefore, with the increase of NaOH amount, Br− was gradually substituted by OH− and the final product changed from Bi4O5Br2 to Bi5O7Br. Furthermore, we extended the reaction time to 30 minutes at different NaOH amount, the results were consistent with the samples reacted for 10 minutes (Figure 6). The sample in Figure 6g and h was assigned as S3.
Figure 5. SEM images and XRD patterns of samples synthesized in 10 minutes with different amount of NaOH: (a, b) 3.8 g; (c, d) 4 g.
10
Figure 6. SEM images and XRD patterns of samples synthesized in 30 minutes with different amount of NaOH: (a, b) 3.6 g; (c, d) 3.8 g; (e, f) 4.0 g; (g, h) 4.5 g. Temperature-dependent experiments were also carried out to examine the construction of materials. Figure 7 gives SEM and XRD results of four samples 11
prepared at different temperature of 40–90 °C, other reaction parameters were parallel to that of sample S1, and the samples were assigned as S4, S5, S6, and S7 for clear explanation. SEM and XRD patterns of samples S4-S7 (temperature: 90 °C, 80 °C, 60 °C and 40 °C) in Figure 7a-f reveal that pure Bi4O5Br2 nanosheets can be successfully synthesized in a facile method in the temperature range of 60–90 °C. When the reaction temperature reduced to 40 °C, we can detect the impurity of BiOBr and its morphology became messy (Figure 7h).
12
Figure 7. SEM images and XRD patterns of samples at different temperatures: (a, b) Sample S4, 90 °C; (c, d) Sample S5, 80 °C; (e, f) Sample S6, 60 °C; (g, h) Sample S7, 40 °C.
13
CTAB were found to be a reaction parameter to control the morphology rather than the crystalline structures of the samples. CTAB-dependent experiments were done on the basis of sample S1 (3 wt% of CTAB). XRD patterns in Figure 8b and d reveal Bi4O5Br2 crystals of the samples with 0 and 6 wt% of CTAB, which demonstrates less influences of CTAB amount on the crystalline structure of samples. As for the morphology, no CTAB introduction in the experiment led to the sample of messy and clustered nanosheets (Figure 8a), while the Bi4O5Br2 sample with 6 wt% of CTAB introduction have clustered nanosheet morphology (Figure 8c).
Figure 8. SEM images and XRD patterns with different amount of CTAB: (a) no CTAB; (b) 6 wt% of CTAB. 3.2. Optical properties and energy band structures UV-vis DRS was employed to characterize the optical absorption property of samples S1-S3 and S7, and the results are displayed in Figure 9a. The absorption edge 14
of Bi4O5Br2 (sample S1) extends to 465 nm, and those of Bi5O7Br (sample S2), two impure Bi4O5Br2 samples (S3 and S7) and BiOBr (Figure 1c and d) are at about 378, 445, 450 and 430 nm, respectively. It reveals pronounced visible-light absorptions for samples S1, S3, S7 and BiOBr except for S2 of Bi5O7Br. As is known, for a semiconductor material, the absorption character is determined by the band gap. The approximate band gaps of the samples were estimated by Tauc formula [25, 26]. A plot of (αhv)1/2 versus photo energy yields a band gap of 2.68, 2.80, 2.82, 2.88 and 3.28 eV for Bi4O5Br2 (S1), Bi4O5Br2/BiOBr (S7), Bi4O5Br2/Bi5O7Br (S3), BiOBr and Bi5O7Br (S2), respectively (Figure 9b). Accordingly, sample S1 of Bi4O5Br2 has the smallest band gap, and sample S2 of Bi5O7Br has the largest one. Furthermore, the band structures of Bi4O5Br2 and BiOBr were investigated using Mott-Schottky plot measurement. As shown in Figure 9c, the flat band potentials of Bi4O5Br2 and BiOBr are −0.68 and −0.71 eV (vs. SCE). The conduction band (CB) positions of Bi4O5Br2 and BiOBr were estimated to be −0.54 and −0.57 eV (vs. NHE) [17, 23, 27]. Combined with the band gap data in Figure 9b, the valence band (VB) potentials of Bi4O5Br2 and BiOBr can be calculated according to the equation of EVB−ECB=Eg [25, 28], and the resultant values of EVB are 2.14 and 2.31 eV. A schematic band structures of Bi4O5Br2 and BiOBr are shown in Figure 9d.
15
Figure 9. (a) UV-vis DRS of samples S1-S3, S7 and BiOBr; (b) The plots of (αhv)1/2 vs. photon energy (hv); (c) Mott-Schottky plots (vs. SCE) for samples S1 and BiOBr; (d) the estimated band potentials (vs. NHE) of samples S1 and BiOBr.
To better reveal the carrier transport and separation processes in the as-obtained samples, EIS measurements were performed. The smaller diameter of the semicircle means a lower resistance [23, 29–32]. Sample S1 of Bi4O5Br2 displayed strikingly fast carrier transport compared to BiOBr, Bi5O7Br (S2), and other impure Bi4O5Br2 samples (S3 and S7), which was verified from Figure 10 by the much lower interfacial charge transport resistance. The rapid migration of electrons over Bi4O5Br2 nanosheets apparently inhibited the recombination of e-h+ pairs, which would enhance the photocatalytic performance.
16
Figure 10. Electrochemical impedance spectroscopy (EIS) spectra of samples S1-S3, S7 and BiOBr.
3.3. Photocatalytic activities of the as-synthesized samples To examine the activity of the as-prepared materials, four samples (S1-S3 and S7) with different compositions and BiOBr were chosen as photocatalysts for colorless BPA degradation. Since the colorless characteristic of BPA could eliminate the indirect photosensitization process, thus highlight the direct photoexcitation efficiency of our samples. Figure 11 shows detailed degradation profiles of BPA over S1-S3, S7 and BiOBr. The degradation system was kept in the dark for 60 minutes to reach absorption-desorption equilibrium. After that, photocatalytic degradation reactions occurred by exposing to simulated visible-light. The degradation efficiencies of BPA over samples S1, S2, S3, S7 and BiOBr reached 90%, 39%, 68%, 54% and 23% after 6 hours, respectively. It is obvious that Bi4O5Br2 involved samples (S1, S3 and S7) showed superior photodegradation activities than pure Bi5O7Br (S2) and BiOBr samples, and sample S1 of pure Bi4O5Br2 exhibited the best performance, which 17
would be attributed to the combined effects of stronger visible-light absorption and lower electrochemical impedance. Even though sample S3 is Bi5O7Br controlled sample with minor amount of Bi4O5Br2 (Figure 6g and h), and shows similar bond vibrations as sample S2 of pure Bi5O7Br (FT-IR patterns in Figure S1, Supporting Information), less introduction of Bi4O5Br2 into Bi5O7Br matrix largely enhanced the visible-light absorption and photocatalytic activity of the sample on comparison to pure Bi5O7Br. The valid active species during photodegradation process over pure Bi4O5Br2 (sample S1) were investigated through trapping experiments, and the results are shown in Figure 11b. The typical •O2− quencher of 4-benzoquinone (BQ) and h+ quencher of glucose had the key influences on the photocatalytic activity. No obvious change in the removal rate was detected when typical quencher of •OH, Isopropyl alcohol (IPA), was introduced into the degradation reaction. Meanwhile, EPR spin-trap with DMPO technique was implemented to further clarify the active species over Bi4O5Br2 nanosheets. The ESR spectra in Figure 12 were obtained in a methanol suspension for DMPO/•O2− adducts and an aqueous suspension for DMPO/•OH adducts. No detectable signal before irradiation demonstrated no active radicals in the dark. Under visible-light (> 420 nm) irradiation, signals of DMPO-•O2− adducts were distinctly observed as a 4-fold characteristic peaks with an intensity ratio of 1:1:1:1. No obvious DMPO/•OH signals revealed no or less generation of •OH radical in the photocatalytic process. On the combination of trapping experiments and EPR results, a conclusion can be made that the photodegradation of BPA over Bi4O5Br2 nanosheets 18
under visible-light irradiation was mainly dominated by •O2− and h+. The proposed degradation mechanism over Bi4O5Br2 nanosheets are illustrated in Figure 13. As a colorless molecule, BPA exhibited a direct semiconductor photoexcitation mechanism on degradation, eliminating the indirect dye-sensitization process [33-35]. Electron-hole (e−- h+) pairs were first produced under visible-light irradiation, with electrons in conduction band (CB) of catalyst and holes left in valence band (VB). In our work, the neutral condition (pH=7) of photocatalytic system provided no additional H+ ions, the active e− reacted with O2 to form •O2− radicals, and further formation of •OH was not available. According to the estimated band structures of Bi4O5Br2 nanosheets (Figure 9d), its valence band potential of 2.14 eV is lower than that for H2O/•OH couple (Eº = +2.80 V vs NHE) [36], therefore water oxidation to •OH by h+ on Bi4O5Br2 was thermodynamically impossible, h+ directly decomposed BPA to CO2 and H2O. The sheet-like structure and unique band composition of Bi4O5Br2 facilitated the separation of electron-hole pairs, resulting in superior photocatalytic performance.
Figure 11. (a) Photocatalytic degradation of BPA over samples S1-S3, S7 and BiOBr. (b) Removal rate of BPA over sample S1 with glucose, 1, 4-benzoquinone and 19
isopropyl alcohol for quenching h+, •O2−, and •OH, respectively. Data for no quencher are also given in the image.
Figure 12. DMPO spin-trapping EPR spectra for DMPO-•O2− and DMPO-•OH over Bi4O5Br2 nanosheets under visible-light irradiation and in dark.
Figure 13. Schematic photodegradation mechanism of BPA over Bi4O5Br2 nanosheets. 4. Conclusion In summary, we successfully prepared Bi4O5Br2 nanosheets via a facile and fast aqueous method. The amount of NaBr and NaOH had crucial influences on the composition and structure of sample powders. The morphology, crystal structure, 20
optical property and band structures of the obtained samples were investigated. In comparison to BiOBr nanosheets and Bi5O7Br micro/nanorods, Bi4O5Br2 nanosheets displayed superior photoactivity for BPA degradation under visible-light irradiation, which could be attributed to its higher visible-light absorption and lower interfacial charge transport resistance. Trapping experiments and EPR results indicated that •O2−and h+ are the main active species during the visible-light photocatalytic process.
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PII: DOI: Reference:
S0272-8842(17)32876-6 https://doi.org/10.1016/j.ceramint.2017.12.168 CERI17048
To appear in: Ceramics International Received date: 30 October 2017 Revised date: 22 December 2017 Accepted date: 23 December 2017 Cite this article as: Gongjuan Wu, Yan Zhao, Yawen Li, Bouasavanh Souvanhthong, Hongmei Ma and Jingzhe Zhao, Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.168 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile aqueous synthesis of Bi4O5Br2 nanosheets for improved visible-light photocatalytic activity Gongjuan Wu, Yan Zhao*, Yawen Li, Bouasavanh Souvanhthong, Hongmei Ma, Jingzhe Zhao* College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China [email protected] [email protected] Corresponding authors. Tel: +86-731-82548686;
ABSTRACT Bi4O5Br2 nanosheets have been fast synthesized via a facile aqueous method. The morphology, crystal structure, optical property, and electronic structure of the obtained samples were discussed. During the reaction process, it was found that the reaction parameters including the amount of NaBr and the amount of NaOH significantly influenced the composition and structure of the samples. The photocatalytic performance of the prepared samples was explored by degrading BPA under visible-light irradiation. Bi4O5Br2 nanosheets exhibited enhanced photocatalytic activity in comparison to BiOBr or Bi5O7Br, which can be attributed to its higher visible-light absorption and lower interface charge transport resistance. Additionally, the photocatalytic degradation mechanism over Bi4O5Br2 nanosheets was also investigated. h+ and •O2− were the main photoactive species during the visible-light photocatalytic process by quencher experiments and EPR results. This study is expected to provide new insights into the development on facile fabrication and 1
potential application of Bi-deficient bismuth oxybromide photocatalysts. Keywords: Bi4O5Br2 nanosheet; high visible-light absorption; low interface charge transport resistance; photocatalytic activity.
1. Introduction Bismuth oxyhalide (BiOX, X= Cl, Br, I) layered materials have received considerable interests in recent years, because they have great potential applications in the field of photocatalysis for energy conversion and environment remediation [1–7]. Bismuth oxyhalide belongs to a novel class of promising layered materials interleaved with [Bi2O2]2+ slabs and double X− slabs [1, 3, 5, 7–9]. The strong intralayer covalent bonding of [Bi2O2]2+ slabs and the weak interlayer van der Waals interaction of X− slabs lead to the nonuniform charge distribution in the crystal structure, which would polarize the related atoms and orbitals to form internal electric fields perpendicular to each layer [2, 5, 7]. The internal electric field induced by the unique layered structure is believed to promote the separation of photogenerated hole-electron pairs and the migration to catalyst surface, which are favorable for the subsequent redox reactions on the surface of materials [2, 7]. Br-deficient Bismuth oxybromide (BixOyBrz), as a new type of Bismuth oxyhalide, was reported to exhibited excellent photocatalytic activity due to high optical absorption and efficient separation of photoinduced hole-electron pairs compared to that of BiOBr [10–18]. Extensive investigations were devoted to the fabrication of BixOyBrz materials, including solvothermal method [11, 18], hydrothermal method 2
[15–17, 19], and microwave-assisted method [20, 21]. For example, Xia group declared that Bi4O5Br2 nanosheets with good photocatalytic activity for ciprofloxacin photo-degradation were prepared via an ionic liquid-assisted solvothermal method at 140 °C for 24 h [11]. Zhang group accomplished the synthesis of Bi3O4Br by a hydrothermal
strategy,
its
application
on
the
decomposition
of
sodium
pentachlorophenate exhibited good photoactivity [17]. Nan group reported a microwave heating route to synthesize Bi24O31Br10 nanoflakes at 400 W for 3 min, good photocatalytic activities were achieved in tetracycline hydrochloride degradation [20]. However, it still remains a challenge to develop simple, energy-efficient and low-cost strategies for the synthesis of Br-deficient bismuth oxybromides. Herein, we present a facile aqueous method to synthesize Bi4O5Br2 nanosheets at 70 °C for 10 mins. During the prepared process, the amount of NaBr and NaOH involved in the reaction has vital effects on the composition and structure of samples. The morphology, crystal structure, optical property and electronic structure of Bi4O5Br2
nanosheets
were
systematically
investigated.
The
visible-light
photocatalytic activity of Bi4O5Br2 nanosheets was studied on colorless bisphenol-A (BPA) degradation, and the degradation mechanism was proposed accordingly. 2. Experimental section Sample preparation. All reagents were analytical grade and used without further purification. In a typical experiment for the preparation of Bi4O5Br2 nanosheets, 10 mL of 1 mol L−1 Bi(NO3)3 solution (with 0.02 mol of HNO3 in the solution) was first added into a three-neck flask and stirred for 2 min at 70 °C, and then 0.07 g of 3
cetyltrimethylammonium bromide (CTAB) dissolved in 20 mL of distilled water was put into the reaction system. Two minutes later, 30 mL of mixed solutions containing 3.6 g of NaOH and 6.2 g of NaBr (6 mmol) were simultaneously dumped into the reaction vessel, and then the reactions were performed for 10 min under continuous stirring to obtain the resulting products. Finally, the precipitates were collected and washed several times, and then dried in an air oven at 60 °C for 6 h to get sample powders. For comparison, BiOBr crystals were also synthesized under parallel conditions without NaOH introduction. Sample characterization. X-ray diffraction (XRD) measurements were detected by using a shimadzu XRD-6000 instrument. Morphologies of samples were examined by a Hitachi S-4800 field emission scanning electron microscope (FESEM). A Tecnai G2 F20 equipment was used to characterize high resolution transmission electron microscopy (HRTEM) images. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher Scientific K-Alpha 1063. UV-vis absorption spectra were analyzed by a Shimadzu UV-1800 spectrophotometer, and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) were used to determine the optical properties of the samples on a spectrophotometer (Shimadzu UV-2600, Japan). Electrochemical measurements were performed on an electrochemical workstation (CHI-660E, China). Photocatalytic experiments. Photocatalytic activities of the as-obtained samples were evaluated by degrading colorless bisphenol-A (BPA) solution under visible-light illumination. The photocatalytic experiments were carried out in a photochemical instrument cooled with circulating water, and a 500 W metal halide lamp with a 4
420-nm cutoff filter was selected as visible-light source. For the degradation of BPA, 100 mg of photocatalyst was dispersed into 100 mL of BPA solution (20 mg L−1). Before illumination, the suspensions were stirred for 1 hour in dark to ensure the establishment of adsorption-desorption equilibrium. At given time intervals, 2 mL of the suspension was taken out and subsequently centrifuged to eliminate the sample powders. Then, the powder-free solutions were measured by recording variations of the absorption band maximum (275 nm) of BPA by using UV-vis spectroscope. 3. Results and discussion 3.1 Morphology and structure of the as-synthesized samples The morphology and crystalline structure of the as-obtained samples were characterized by SEM and XRD methods, the result of a representative Bi4O5Br2 sample is shown in Figure 1a and b (reaction temperature: 70 °C; NaOH: 3.6 g; R(Br/Bi) ratio = 6:1; CTAB: 3 wt%, and the sample was assigned as S1). The diffraction peaks of the XRD pattern in Figure 1b were indexed to the pure Bi4O5Br2 (the standard card calculated from ICSD #412591), which matched well with the reported studies [10, 12]. When NaOH was not introduced into the reaction system, the XRD pattern in Figure 1d revealed that the synthesized sample was BiOBr (PDF #73-2061). SEM images in Figure 1a and c showed that both Bi4O5Br2 and BiOBr belonged to nanosheet structures, whereas Bi4O5Br2 appeared as thinner bent nanosheets compared to BiOBr straight nanosheets.
5
Figure 1. SEM images and XRD patterns of the as-synthesized Bi4O5Br2 (a, b) and BiOBr (c, d).
TEM strategy was further carried out to investigate the detailed structures of the as-obtained Bi4O5Br2. The TEM image of Bi4O5Br2 in Figure 2a displayed a sheet-like morphology with thickness of about 8 nm. Figure 2b of HRTEM image presents distinct lattice spacings of 0.283 nm, corresponding to (020) crystallographic plane of Bi4O5Br2 crystallites [22]. The simulated crystal structure in Figure 2c reveals that Bi4O5Br2 is a layer structure interleaved with [Bi4O5]2+ slabs and double slabs of Br− ions, which is related to the formation of nanosheet structures.
6
Figure 2. (a) TEM and (b) HRTEM images of Bi4O5Br2 nanosheets; (c) simulated crystal structure of Bi4O5Br2 X-ray photoelectron spectroscopy (XPS) technique was used to study the chemical composition and surface chemical states of Bi4O5Br2 nanosheets. The XPS survey spectrum in Figure 3a showed that Bi, O, Br and C elements are consistent with their chemical compositions. The high-resolution spectra of Bi 4f in Figure 3b can be fitted into four peaks, indicating two coordination environments of Bi in Bi4O5Br2 nanosheets [15, 17, 23, 24]. Two main peaks at 165.28 and 159.98 eV arising from 4f5/2 and 4f7/2 demonstrates +3 oxidation state of Bi. The additional peaks with lower 7
binding energies at 163.88 and 158.68 eV could be ascribed to lower valence of oxygen vacancy-related Bi atoms, which were generated by X-ray irradiation during examination process. The symmetric O 1s peak at 531.18 eV was assigned to lattice oxygen in Bi4O5Br2 (Figure 3c). Figure 3d showed that the binding energy of 69.27 and 67.13 eV were ascribed to Br 3d3/2 and Br 3d5/2 for the Bi4O5Br2 sample.
Figure 3. XPS spectras of the Bi4O5Br2 Sample. (a) Survey of the Sample; (b) Bi 4f; (c) O 1s; and (d) Br 3d. For the synthesis of Bi4O5Br2 crystals, simultaneous addition of NaBr and NaOH to the reaction system led to competition reaction of Br− and OH− with [Bi2O2]2+, resulting in the evolution of Br-poor bismuth oxybromides. Accordingly, the introducing amount of NaBr in the reaction would have vital effects on the 8
composition and structure of samples. The molar ratio of NaBr to Bi(NO3)3 was assigned as R(Br/Bi), and R(Br/Bi) of 6:1 was chosen for typical experiments. Figure 4 gives SEM and XRD results of two samples prepared with R(Br/Bi) =3:1 and 1:1, and other reaction parameters were controlled to be constant parallel to sample S1. When R(Br/Bi) decreased from 6:1 to 3:1, the sample was the mixture of Bi4O5Br2 and Bi5O7Br in crystallinity (Figure 4b), and microrods appeared in the image except for the nanosheets (Figure 4a). When R(Br/Bi) further reduced to 1:1, the sample was mainly composed of Bi5O7Br micro/nanorods (Figure 4c and d, assigned as sample S2). The results demonstrated that the substitution reaction of Br with O atoms speeded up under a little NaBr introduction, and thus the crystalline structure changed from Bi4O5Br2 to Bi5O7Br.
Figure 4. SEM images and XRD patterns of samples with different molar ratios of NaBr to Bi(NO3)3: (a, b) R(Br/Bi) =3:1 and (c, d) R(Br/Bi) =1:1. 9
NaOH amount also played crucial effects on regulating the morphology and structure of the obtained samples, as shown in Figure 5. When the amount of NaOH increased to 3.8 g, the crystalline structure was still Bi4O5Br2, and the nanosheet became thinner compared to sample S1. While the amount of NaOH increased to 4.0 g, Bi5O7Br microrods appeared (Figure 5c and d). Therefore, with the increase of NaOH amount, Br− was gradually substituted by OH− and the final product changed from Bi4O5Br2 to Bi5O7Br. Furthermore, we extended the reaction time to 30 minutes at different NaOH amount, the results were consistent with the samples reacted for 10 minutes (Figure 6). The sample in Figure 6g and h was assigned as S3.
Figure 5. SEM images and XRD patterns of samples synthesized in 10 minutes with different amount of NaOH: (a, b) 3.8 g; (c, d) 4 g.
10
Figure 6. SEM images and XRD patterns of samples synthesized in 30 minutes with different amount of NaOH: (a, b) 3.6 g; (c, d) 3.8 g; (e, f) 4.0 g; (g, h) 4.5 g. Temperature-dependent experiments were also carried out to examine the construction of materials. Figure 7 gives SEM and XRD results of four samples 11
prepared at different temperature of 40–90 °C, other reaction parameters were parallel to that of sample S1, and the samples were assigned as S4, S5, S6, and S7 for clear explanation. SEM and XRD patterns of samples S4-S7 (temperature: 90 °C, 80 °C, 60 °C and 40 °C) in Figure 7a-f reveal that pure Bi4O5Br2 nanosheets can be successfully synthesized in a facile method in the temperature range of 60–90 °C. When the reaction temperature reduced to 40 °C, we can detect the impurity of BiOBr and its morphology became messy (Figure 7h).
12
Figure 7. SEM images and XRD patterns of samples at different temperatures: (a, b) Sample S4, 90 °C; (c, d) Sample S5, 80 °C; (e, f) Sample S6, 60 °C; (g, h) Sample S7, 40 °C.
13
CTAB were found to be a reaction parameter to control the morphology rather than the crystalline structures of the samples. CTAB-dependent experiments were done on the basis of sample S1 (3 wt% of CTAB). XRD patterns in Figure 8b and d reveal Bi4O5Br2 crystals of the samples with 0 and 6 wt% of CTAB, which demonstrates less influences of CTAB amount on the crystalline structure of samples. As for the morphology, no CTAB introduction in the experiment led to the sample of messy and clustered nanosheets (Figure 8a), while the Bi4O5Br2 sample with 6 wt% of CTAB introduction have clustered nanosheet morphology (Figure 8c).
Figure 8. SEM images and XRD patterns with different amount of CTAB: (a) no CTAB; (b) 6 wt% of CTAB. 3.2. Optical properties and energy band structures UV-vis DRS was employed to characterize the optical absorption property of samples S1-S3 and S7, and the results are displayed in Figure 9a. The absorption edge 14
of Bi4O5Br2 (sample S1) extends to 465 nm, and those of Bi5O7Br (sample S2), two impure Bi4O5Br2 samples (S3 and S7) and BiOBr (Figure 1c and d) are at about 378, 445, 450 and 430 nm, respectively. It reveals pronounced visible-light absorptions for samples S1, S3, S7 and BiOBr except for S2 of Bi5O7Br. As is known, for a semiconductor material, the absorption character is determined by the band gap. The approximate band gaps of the samples were estimated by Tauc formula [25, 26]. A plot of (αhv)1/2 versus photo energy yields a band gap of 2.68, 2.80, 2.82, 2.88 and 3.28 eV for Bi4O5Br2 (S1), Bi4O5Br2/BiOBr (S7), Bi4O5Br2/Bi5O7Br (S3), BiOBr and Bi5O7Br (S2), respectively (Figure 9b). Accordingly, sample S1 of Bi4O5Br2 has the smallest band gap, and sample S2 of Bi5O7Br has the largest one. Furthermore, the band structures of Bi4O5Br2 and BiOBr were investigated using Mott-Schottky plot measurement. As shown in Figure 9c, the flat band potentials of Bi4O5Br2 and BiOBr are −0.68 and −0.71 eV (vs. SCE). The conduction band (CB) positions of Bi4O5Br2 and BiOBr were estimated to be −0.54 and −0.57 eV (vs. NHE) [17, 23, 27]. Combined with the band gap data in Figure 9b, the valence band (VB) potentials of Bi4O5Br2 and BiOBr can be calculated according to the equation of EVB−ECB=Eg [25, 28], and the resultant values of EVB are 2.14 and 2.31 eV. A schematic band structures of Bi4O5Br2 and BiOBr are shown in Figure 9d.
15
Figure 9. (a) UV-vis DRS of samples S1-S3, S7 and BiOBr; (b) The plots of (αhv)1/2 vs. photon energy (hv); (c) Mott-Schottky plots (vs. SCE) for samples S1 and BiOBr; (d) the estimated band potentials (vs. NHE) of samples S1 and BiOBr.
To better reveal the carrier transport and separation processes in the as-obtained samples, EIS measurements were performed. The smaller diameter of the semicircle means a lower resistance [23, 29–32]. Sample S1 of Bi4O5Br2 displayed strikingly fast carrier transport compared to BiOBr, Bi5O7Br (S2), and other impure Bi4O5Br2 samples (S3 and S7), which was verified from Figure 10 by the much lower interfacial charge transport resistance. The rapid migration of electrons over Bi4O5Br2 nanosheets apparently inhibited the recombination of e-h+ pairs, which would enhance the photocatalytic performance.
16
Figure 10. Electrochemical impedance spectroscopy (EIS) spectra of samples S1-S3, S7 and BiOBr.
3.3. Photocatalytic activities of the as-synthesized samples To examine the activity of the as-prepared materials, four samples (S1-S3 and S7) with different compositions and BiOBr were chosen as photocatalysts for colorless BPA degradation. Since the colorless characteristic of BPA could eliminate the indirect photosensitization process, thus highlight the direct photoexcitation efficiency of our samples. Figure 11 shows detailed degradation profiles of BPA over S1-S3, S7 and BiOBr. The degradation system was kept in the dark for 60 minutes to reach absorption-desorption equilibrium. After that, photocatalytic degradation reactions occurred by exposing to simulated visible-light. The degradation efficiencies of BPA over samples S1, S2, S3, S7 and BiOBr reached 90%, 39%, 68%, 54% and 23% after 6 hours, respectively. It is obvious that Bi4O5Br2 involved samples (S1, S3 and S7) showed superior photodegradation activities than pure Bi5O7Br (S2) and BiOBr samples, and sample S1 of pure Bi4O5Br2 exhibited the best performance, which 17
would be attributed to the combined effects of stronger visible-light absorption and lower electrochemical impedance. Even though sample S3 is Bi5O7Br controlled sample with minor amount of Bi4O5Br2 (Figure 6g and h), and shows similar bond vibrations as sample S2 of pure Bi5O7Br (FT-IR patterns in Figure S1, Supporting Information), less introduction of Bi4O5Br2 into Bi5O7Br matrix largely enhanced the visible-light absorption and photocatalytic activity of the sample on comparison to pure Bi5O7Br. The valid active species during photodegradation process over pure Bi4O5Br2 (sample S1) were investigated through trapping experiments, and the results are shown in Figure 11b. The typical •O2− quencher of 4-benzoquinone (BQ) and h+ quencher of glucose had the key influences on the photocatalytic activity. No obvious change in the removal rate was detected when typical quencher of •OH, Isopropyl alcohol (IPA), was introduced into the degradation reaction. Meanwhile, EPR spin-trap with DMPO technique was implemented to further clarify the active species over Bi4O5Br2 nanosheets. The ESR spectra in Figure 12 were obtained in a methanol suspension for DMPO/•O2− adducts and an aqueous suspension for DMPO/•OH adducts. No detectable signal before irradiation demonstrated no active radicals in the dark. Under visible-light (> 420 nm) irradiation, signals of DMPO-•O2− adducts were distinctly observed as a 4-fold characteristic peaks with an intensity ratio of 1:1:1:1. No obvious DMPO/•OH signals revealed no or less generation of •OH radical in the photocatalytic process. On the combination of trapping experiments and EPR results, a conclusion can be made that the photodegradation of BPA over Bi4O5Br2 nanosheets 18
under visible-light irradiation was mainly dominated by •O2− and h+. The proposed degradation mechanism over Bi4O5Br2 nanosheets are illustrated in Figure 13. As a colorless molecule, BPA exhibited a direct semiconductor photoexcitation mechanism on degradation, eliminating the indirect dye-sensitization process [33-35]. Electron-hole (e−- h+) pairs were first produced under visible-light irradiation, with electrons in conduction band (CB) of catalyst and holes left in valence band (VB). In our work, the neutral condition (pH=7) of photocatalytic system provided no additional H+ ions, the active e− reacted with O2 to form •O2− radicals, and further formation of •OH was not available. According to the estimated band structures of Bi4O5Br2 nanosheets (Figure 9d), its valence band potential of 2.14 eV is lower than that for H2O/•OH couple (Eº = +2.80 V vs NHE) [36], therefore water oxidation to •OH by h+ on Bi4O5Br2 was thermodynamically impossible, h+ directly decomposed BPA to CO2 and H2O. The sheet-like structure and unique band composition of Bi4O5Br2 facilitated the separation of electron-hole pairs, resulting in superior photocatalytic performance.
Figure 11. (a) Photocatalytic degradation of BPA over samples S1-S3, S7 and BiOBr. (b) Removal rate of BPA over sample S1 with glucose, 1, 4-benzoquinone and 19
isopropyl alcohol for quenching h+, •O2−, and •OH, respectively. Data for no quencher are also given in the image.
Figure 12. DMPO spin-trapping EPR spectra for DMPO-•O2− and DMPO-•OH over Bi4O5Br2 nanosheets under visible-light irradiation and in dark.
Figure 13. Schematic photodegradation mechanism of BPA over Bi4O5Br2 nanosheets. 4. Conclusion In summary, we successfully prepared Bi4O5Br2 nanosheets via a facile and fast aqueous method. The amount of NaBr and NaOH had crucial influences on the composition and structure of sample powders. The morphology, crystal structure, 20
optical property and band structures of the obtained samples were investigated. In comparison to BiOBr nanosheets and Bi5O7Br micro/nanorods, Bi4O5Br2 nanosheets displayed superior photoactivity for BPA degradation under visible-light irradiation, which could be attributed to its higher visible-light absorption and lower interfacial charge transport resistance. Trapping experiments and EPR results indicated that •O2−and h+ are the main active species during the visible-light photocatalytic process.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21571057, 21701042), and the Fundamental Research Funds for the Central Universities. References [1] H. Cheng, B. Huang, Y. Dai, Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications, Nanoscale 6 (2014) 2009–2026. [2] J. Li, H. Li, G. Zhan, L. Zhang, Solar Water Splitting and Nitrogen Fixation with Layered Bismuth Oxyhalides, Accounts of chemical research 50 (2017) 112–121. [3] H. Li, J. Shang, Z. Ai, L. Zhang, Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets, Journal of the American Chemical Society 137 (2015) 6393–6399. [4] M. Pan, H. Zhang, G. Gao, L. Liu, W. Chen, Facet-dependent catalytic activity of nanosheet-assembled bismuth oxyiodide microspheres in degradation of bisphenol A, 21
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