English

• 0.   Introduction

  Semiconductor photocatalysis is considered to be one of the most promising green and environmentally - friendly approaches for solving the issues of environ-mental pollution and increasing energy demand^[1-5]. In recent years, bismuth-based oxides with Aurivillius-layered structure such as BiOX, Bi₂WO₆, Bi₂MoO₆, BiVO₄, BiFeO₃ and Bi₂O ₂CO ₃ have been considered to be highly promising visible - light - activated photocata-lysts due to their unique crystal and electronic structure^[6-14]. In particular, bismuth-based complex oxides Bi_n+1Fe_n-3Ti₃O_3n+3 formed by perovskite-type (Bi_n-1Fe_n-3Ti₃O_3n+1)^2- blocks sandwiched between fluorite -type (Bi₂O₂)²⁺ slabs have received increasing attention due to their photocatalytic activity in the degradation of organic pollutants. By adjusting the layer number (n=3, 4, 5, 6, etc.) or doping ions (Co³⁺, La³⁺, Ni²⁺, Mn²⁺, etc.), the Bi_n+1Fe_n-3Ti₃O_3n+3 displayed prominent visible -light-driven photocatalytic and other unique proper-ties. In 2008, Sun et al. prepared an excellent visible -light driven photocatalyst Bi₅FeTi₃O₁₅ with 4 layer by a facile hydrothermal method^[15]. In 2013, Hou et al. syn-thesized the Bi₄Ti₃O₁₂ (n=3) nanofibers with enhanced visible-light photocatalytic activity^[16]. In 2014, Li et al. prepared the visible light responsive Bi ₇Fe₃Ti₃O₂₁ (n=6) nanoshelf photocatalysts^[17]. In 2015, by Co ions doping, Ge et al. synthesized the Bi₆Fe_1.9Co_0.1Ti₃O₁₈ (n=5) nano-crystal with different morphologys exhibiting visi-ble-light photocatalytic activity^[18]. However, the perfor-mance of the pristine Bi_n+1Fe_n-3Ti₃O_3n+3 materials is lim-ited by their low photocatalytic activity. Therefore, the decoration of noble metals on semiconductor photocata-lysts is a promising approach for enhancing their photo-catalytic activity.

  The high activity of the decorated semiconductor photocatalysts is related to the generation and migra-tion of the photogenerated hot electrons arising due to the Schottky barrier formed between the metal and the semiconductor. Moreover, noble metals nanoparticles can improve visible light absorption due to the surface plasmon resonance (SPR) effect^[19-22]. In 2017, Liu et al. synthesized the Au nanoparticles loaded on Fe-doped Bi₄Ti₃O₁₂ nanosheets with exposed {001} facets. The Au-2%Fe/Bi₄Ti₃O₁₂ sample exhibited the highest photo-catalytic activity compared with other samples^[23]. In 2018, Li et al. prepared a high efficiency photocatalyst Bi₄Ti₃O₁₂/Au, in which Au nanorods selectively an-chored on the active (001) facets of Bi₄Ti₃O₁₂ nanosheets^[24]. In 2019, Zhao et al. synthesized the Au-Ag@Bi₄Ti₃O₁₂ composite with the aim of synergisti-cally enhancing the photocatalytic performance^[25]. Gu et al. prepared the Au@Bi₆Fe₂Ti₃O₁₈ nanofibers by elec-trospinning method with improved the photocatalysis performance^[26].

  Therefore, in this work, we synthesized visible -light-driven BFCTO/Au nanocomposite photocatalysts by a facile assembly method. The photocatalytic activi-ty of the obtained nanocomposites was enhanced rela-tive to that of pristine BFCTO by introducing Au nanoparticles with different sizes into the BFCTO nano-plates. The BFCTO/Au-1 sample exhibited the stron-gest photocatalytic activity under visible light irradia-tion.

  1.   Experimental

  1.1   Materials

  All the chemicals, namely Ti(OC₄H₉)₄ (≥99.7%), Bi(NO₃)₃·5H₂O (≥99%), Fe(NO₃)₃·9H₂O (≥98.5%), Co(NO₃)₂·6H₂O (≥98.5%), HNO₃ (65.0~68.0%), NaOH (≥96.0%), ethanol (99.7%), 3-aminopropyltriethoxysi-lane (APTES, > 98%), hydrogen tetrachloroaurate (47.8%), trisodium citrate dehydrate (≥99.0%), were purchased from Sinopharm Chemical Reagent Co., Ltd. without further purification.

  1.2   Synthesis of BFCTO nanoparticles

  The BFCTO nanoparticles were synthesized by the hydrothermal method according to our previous re-port^[18]. The concentration of NaOH added to the above solutions was adjusted to 0.75 mol·L^-1.

  1.3   Synthesis of Au nanoparticles

  Au nanoparticles were prepared using the method introduced by Frens^[27]. Au nanoparticles with various diameters of ~23 nm, ~36 nm, ~55 nm and ~80 nm were prepared by rapidly injecting a 38.8 mmol·L^-1 sodium citrate solution with different volumes of 2.0, 1.5, 1.0 and 0.5 mL into a boiling HAuCl₄ aqueous so-lution under vigorous stirring, respectively.

  1.4   Synthesis of BFCTO/Au nanoparticels

  The prepared 0.2 g BFCTO nanoparticles were added into the APTES solution, and stirred for 6 h at room temperature. Then the prepared Au nanoparticles solution was added. The obtained nanoparticles were collected by centrifugation and washed for several times with deionized water, and dried at 60 ℃ in vacu-um for 12 h. In this work, Au nanoparticles with vari-ous diameters (~23 nm, ~36 nm, ~55 nm and ~80 nm) were loaded on the surface of BFCTO nanoplates. The obtained products were denoted as BFCTO/Au-1, BFC-TO/Au-2, BFCTO/Au-3 and BFCTO/Au-4, respectively.

  1.5   Characterization

  Powder X-ray diffraction (XRD) patterns were characterized by Philips X'pert diffractometer operat-ing at 40 kV and 30 mA with a scan step width of 0.02° in a 2θ range of from 10° to 70° employing Cu Kα radi-ation (λ=0.154 05 nm). Sizes and morphologies of the samples were observed by scanning electron microsco-py (SEM, JEOL, JSM-6700F, 10 kV) and transmission electron microscopy (TEM, Tecnai G2 TF30, 200 kV). The X -ray photoelectron spectroscopy (XPS) measure-ments were carried out on an ESCALAB 250 system with a monochromatic Al Kα X-ray source (Thermo-VG Scientific). Ultraviolet-visible- near infrared (UV-Visible - NIR) diffuse reflectance spectra were recorded with a Shimadzu SolidSpec-3700 equipped with an integrating sphere, and BaSO₄ was used as the refer-ence. The specific surface area was measured by BET (Brunauer-Emmett-Teller) method using the N₂ adsorp-tion isotherm (Quantachrome QuadraWin). The photo-luminescence (PL) spectra (l_ex=325 nm) were recorded by a fluorescence spectrophotometer (FLS 980).

  1.6   Photocatalytic activity test

  The photocatalytic performances of the as -prepared samples were evaluated by the decomposition of rhoda-mine B (RhB)/methyl orange (MO) aqueous solution under visible-light irradiation. The light source was a 20 W fluorescent lamp with wavelength of 400~720 nm. The irradiation distance between the light source and the liqud level of RhB aqueous solution was 15 cm. Prior to irradiation, the RhB solution (50 mL, 5 mg·L^-1) with 50 mg photocatalyst was stirred for 30 min in the dark to ensure the establishment of an adsorption-desorption equilibrium. At a certain time interval, 3 mL of the reaction solution was taken and centrifuged. The absorbances of filtrates were mea-sured on a UV-Visible spectrometer at a maximum absorption wavelength of 554 or 463 nm.

  2.   Results and discussion

  The synthetic route used to obtain the BFCTO/Au samples is illustrated in Fig. 1a. The BFCTO nano-plates' surfaces were positively charged after their modification with APTES. This enabled the uniform assembly of the negatively charged Au nanoparticles on the BFCTO nanoplate surfaces^[28]. As show in Fig. 1b, the XRD patterns of the BFCTO nanoplates can be indexed by a single-phase orthorhombic lattice (B2cb space group, PDF No. 21- 0101) with no detect-able secondary phases^[14]. All of the BFCTO/Au sam-ples showed XRD patterns similar to that of BFCTO, suggesting that the decoration with Au nanoparticles has no obvious influence on the BFCTO crystal struc-ture. Additionally, a weak diffraction peak appeared at 38.2° corresponding to the (111) reflection plane of Au (PDF No.04-0784). The intensity of this peak increased with increasing Au nanoparticle size^[29].

  Figure 1

  

  Figure 1.  (a) Schematic diagram of synthesis procedure used to obtain BFCTO/Au nanocomposites; (b) XRD patterns of the samples

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  According to Fig. 2a and 2g, the BFCTO displayed well-defined shape of nanoplate. The edge length ranged from 80 to 350 nm with an average thickness of 25 nm. Moreover, the elemental mapping images of Bi, Fe, Co, Ti and O are shown in Fig. 2b~2f from SEM im-age in Fig. 2a. The results indicate that all the elements have quite uniform distribution over the whole imaging area. As seen from the HRTEM image in Fig. 2h, the lattice fringes with spacings of 0.273, 0.273 and 0.382 nm can be attributed to (200), (020) and (110) facets of BFCTO orthorhombic phase, respectively^{[12, 14]}. Besides, the HRTEM image of laterally-viewed of nanosheet in Fig. 2i displayed five pseudo-perovskite layers between two bismuth oxide layers. And the inserted FFT patterns confirm the formation of well-developed single-crystalline of BFCTO.

  Figure 2

  

  Figure 2.  (a) SEM image for BFCTO; (b~f) EDS mapping images of Bi, Fe, Co, Ti, O elements, respectively; (g) TEM image for BFCTO and (h, i) HRTEM images taken from (g) indicated by red and green rectangle, respectively

  Inset: corresponding FFT images

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  As showed in Fig. 3, the morphology of all BFCTO/ Au samples was investigated by SEM, TEM and HR-TEM characterizations. Fig. 3a and 3b display the Au nanoparticles with the size of~23 nm assembled on the BFCTO nanoplate surface. In Fig. 3a, the Au nanoparti-cles are marked by green rectangles, and the corre-sponding EDS spectrum that detected the Au element content is shown in the inset of Fig. 3a. An examination of the TEM image presented in Fig. 3b further proved that the Au nanoparticles with the size of ~23 nm were deposited on the BFCTO nanoplates. The EDS spec-trum presented in Fig. 3c provided the information about the elemental compositions of the BFCTO/Au (~23 nm) nanocomposites marked by the yellow rectan-gle in Fig. 3b, demonstrating the presence of Au in the nanocomposites. The HRTEM image presented in the inset in Fig. 3c shows the lattice spacings of 0.23 nm for the (111) plane of Au nanoparticles^[30-31]. Additional-ly, the obtained TEM images (Fig. 3d~3f) revealed the morphology of other Au nanoparticles with various sizes (~36 nm, ~55 nm and ~80 nm) deposited on the BFCTO nanoplates. The SEM, TEM and HRTEM results presented in Fig. 3 confirm that Au nanoparti-cles with various sizes were deposited on the surfaces of the BFCTO nanoplates.

  Figure 3

  

  Figure 3.  (a) SEM and (b) TEM images of BFCTO/Au-1 nanocomposites; (c) EDS spectra of Au nanoparticle marked by the yellow rectangle in (b) and corresponding HRTEM image of Au nanoparticle (Inset); (d) TEM images of BFCTO/Au-2 (e) BFCTO/Au-3 and (f) BFCTO/Au-4 nanocomposites

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  To determine the chemical state of elements and the surface defects, XPS analysis was carried out on all samples and the results are shown in Fig. 4. The obtained binding energies in XPS analysis were cor-rected by specimen charging which was executed by referencing the C1s line to 284.5 eV. Comparison of the XPS spectra presented in Fig. 4a to the full survey spectrum of BFCTO shows that Au, Bi, Fe, Co, Ti and O are present in the BFCTO/Au samples. For the Au XPS spectrum presented in Fig. 4b, two significant binding energy peaks at 83.7 and 87.2eV were observed corresponding to the Au4f_7/2 and Au4f_5/2 electronic states, respectively, indicating that the Au NPs exist in the metallic state^[32]. Additionally, it was observed in XPS spectrum of O (Fig. 4g) that the O1s peak can be divided into three different peaks at about 533.2, 531.0 and 529.4 eV, indicating the co-existence of three dif-ferent chemical states of the oxygen atoms on this sur-face. The main peak at 531.0 eV is attributed to the Ti-O bonds, the small peak at 529.4 eV is assigned to the Bi -O bonds, and the peak at 533.2 eV may be due to the oxygen adsorbed on the surface^[33-35]. Besides, as shown in (Fig. 4c~4f), the peaks of Bi4f_5/2 and Bi4f_7/2, Fe2p_1/2 and Fe2p_3/2, Co2p_1/2, and Co2p_3/2 and Ti2p_1/2 and Ti2p_3/2 suggest that Bi, Fe, Co and Ti are +3, +3, +3 and +4 valence states, respectively^[36-39].

  Figure 4

  

  Figure 4.  (a) Full scan XPS spectra of all samples; (b) Au4f, (c) Bi4f, (d) Fe2p, (e) Co2p, (f) Ti2p and (g) O1s XPS spectra of BFCTO/Au-4 nanocomposite

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  The optical properties of all samples were investi-gated using UV-Vis-NIR absorption spectra. As shown in Fig. 5a, the BFCTO sample absorbed light in the visi-ble range due to the Fe and Co dopant ions. Additional-ly, the color photos of all samples were shown in the inset in Fig. 5a. The BFCTO sample was yellow. As the Au nanoparticle size decreased, the color of BFCTO/ Au samples gradually became darker. More important-ly, compared to BFCTO, all of the BFCTO/Au samples showed an enhanced, red-shifted and broadened ab-sorption in the visible light region that generally can be ascribed to the following reasons: as the Au nanoparticles size is increased, the SPR peak has a red-shift^[40-42]; the strong plasmon coupling effect among the neighboring Au nanoparticles can result in broaden SPR peak^[43-45]; the strong interaction between BFCTO and Au nanoparticels may alter the SPR fea-ture due to the change in the dielectric property of the surrounding microenvironment^[46]. Meanwhile, after the decoration with Au nanoparticles, the band gap (E_g) energies of all BFCTO/Au samples decreased com-pared with BFCTO nanoparticles (E_g=2.410 eV). Inter-estingly, with increase of Au nanoparticles size, the E_g values gradually increased. The obtained E_g were 2.316, 2.333, 2.357 and 2.369 eV for BFCTO/Au-1, BFCTO/Au-2, BFCTO/Au-3 and BFCTO/Au-4, respec-tively (Fig. 5b).

  Figure 5

  

  Figure 5.  (a) UV-Vis diffuse reflectance spectra of the samples; (b) Relationships between (αhν)² and photon energy (hν) for the samples

  Inset: optical photos of the samples

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  The N₂ adsorption-desorption isotherms and BET specific surface areas (S_BET) of different samples are showed in Fig. 6. The isotherms of samples showed that the samples had stronger interaction with N₂ at the low-er pressure region. And the hysteresis loops were pre-sented at higher pressure region for all catalysts. Fur-thermore, the calculated S_BET were 13, 12, 12, 11 and 10 m²·g^-1 for BFCTO/Au-1, BFCTO/Au-2, BFCTO/ Au-3, BFCTO/Au-4 and BFCTO samples, respectively (Fig. 6f). Apparently, after combining the Au nanoparti-cles with BFCTO, the S_BET of BFCTO/Au system increased. However, with increase of Au nanoparticles size, the S_BET of BFCTO/Au samples decreased gradual-ly, which was related to its morphological characters as depicted in TEM image (Fig. 3).

  Figure 6

  

  Figure 6.  (a~e) N₂ adsorption-desorption isotherms of various samples; (f) BET specific surface areas for various samples

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  The photocatalytic activities of BFCTO and all BFCTO/Au samples were investigated by the photodeg-radation of RhB under visible light, which was a typi-cal organic azo-dye pollutant in the textile industry. Fig. 7a shows the RhB degradation efficiency curves as a function of the irradiation time for all samples. It is clear that the photocatalytic activity increases with the increasing amount of the Au nanoparticles introduced into the BFCTO nanoplates; thus, for all samples, the photocatalytic activity was enhanced relative to pris-tine BFCTO. The BFCTO/Au-1 sample exhibited the strongest photocatalytic activity under visible light irra-diation for 150 min, and the photocatalytic activities of all of the samples followed the order of BFCTO/Au-1 (80.1%) > BFCTO/Au-3(71.3%) > BFCTO/Au-4 (67.8%) > BFCTO/Au-2 (56.5%) > BFCTO (38.9%). Meanwhile, the calculated values of the apparent rate constants (k_app) for all of the samples are displayed in Fig. 7b. The order of the calculated k_app values for all samples was: BFCTO/Au-1 (0.012 18 min^-1) > BFCTO/ Au-3 (0.008 55 min^-1) > BFCTO/Au-4 (0.007 34 min^-1) > BFCTO/Au-2 (0.005 21 min^-1) > BFCTO (0.003 09 min^-1). It is clear that the BFCTO/Au -1 nanocomposite shows the highest photodegradation rate. This phenomenon may be attributed to the following reasons. Firstly, as shown in Fig. 6, the BFCTO/Au-1 sample has the larg-est specific surface area. This provides sufficient inter-facial area for the adsorption of RhB molecules. More-over, the large specific surface area increases the num-ber of active sites and promotes the separation efficien-cy of the electron-hole pairs in photocatalytic reac-tions. Secondly, the SPR effect of Au nanoparticles enhances visible light absorption, improving the photo-catalytic performance. Thirdly, the decorated Au nanoparticles promote the generation and migration of the photogenerated hot electrons due to the formation of a Schottky barrier between the Au nanoparticles and BFCTO^{[2, 5, 23, 47-50]}. As depicted in Fig. 7c, no obvious de-cay for photocatalytic decomposition of RhB after five cycles can be found, indicating the good durability and stability of BFCTO/Au-1. Besides, as shown in Fig. 7d, the photocatalytic activities of all samples were also tested by the photodegradation of MO under visible light. Obviously, the photodegradation efficiency of MO was not as good as RhB.

  Figure 7

  

  Figure 7.  (a) Photocatalytic RhB degradation curves in the presence of BFCTO and all of the BFCTO/Au samples under visible light irradiation; (b) Corresponding kinetic linear simulation curves of RhB under visible irradiation for the samples; (c) Cyclic photocatalytic degradation experiments of RhB by BFCTO/Au-1 under the visible light irradiation; (d) Photocatalytic methyl orange (MO) degradation curves for the samples

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  Based on the above characterization and analysis results, a possible mechanism was proposed for the enhancement in photocatalytic activity of BFCTO/Au (Fig. 8). Because of SPR effect, the visible light absorp-tion of BFCTO/Au samples enhanced. Therefore, under visible light illumination, both of BFCTO and Au nanoparticles can be photoexcited to generate photoin-duced carriers^[19-22]. Besides, the PL spectrum showed in Fig. S1 (Supporting information) reveals the separa-tion and recombination of photo-generated elec-tron-hole pairs of photocatalysts. Clearly, when Au nanoparticles are on the surface of the BFCTO, the PL intensity decreases significantly, indicating lower recombination rate of photo-electrons and holes pairs^[23-24]. By overcoming Schottky barrier, plasmonic excited hot electron of Au nanoparticle is injected to the conduction band (CB) of BFCTO. Thus, the more electrons in the CB of BFCTO can react with O ₂ to pro-duce ·O₂^-. Simultaneously, the photogenerated holes in the valence band (VB) of BFCTO can directly oxidize H₂O to yield ·OH. As a result, the photocatalytic per-formance can be effectively enhanced by loading Au nanoparticles on BFCTO^[51-52].

  Figure 8

  

  Figure 8.  Schematic illustration of proposed mechanism for photogenerated charge carrier transfer in BFCTO/ Au nanocomposite under visible light irradiation

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  3.   Conclusions

  In summary, we have successfully fabricated visible-light-absorbing BFCTO/Au nanocomposites for the first time. The introduction of Au nanoparticles with different sizes into BFCTO nanoplates enhances the photocatalytic activity with the BFCTO/Au-1 sam-ple exhibiting the highest photocatalytic activity.

  Acknowledgements: This research was funded by the Na-tional Natural Science Foundation of China (Grants No.21701140, 51262032), the Guiding Program of Scientific Re-search Fund of Yunnan Education Department (Grant No. 2017ZDX049), the Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province and the Doctor Start-up Foundation of Yunnan Normal Universi-ty (No.2016zb001).

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  (a) Schematic diagram of synthesis procedure used to obtain BFCTO/Au nanocomposites; (b) XRD patterns of the samples

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  Figure 2  (a) SEM image for BFCTO; (b~f) EDS mapping images of Bi, Fe, Co, Ti, O elements, respectively; (g) TEM image for BFCTO and (h, i) HRTEM images taken from (g) indicated by red and green rectangle, respectively

  Inset: corresponding FFT images

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  Figure 3  (a) SEM and (b) TEM images of BFCTO/Au-1 nanocomposites; (c) EDS spectra of Au nanoparticle marked by the yellow rectangle in (b) and corresponding HRTEM image of Au nanoparticle (Inset); (d) TEM images of BFCTO/Au-2 (e) BFCTO/Au-3 and (f) BFCTO/Au-4 nanocomposites

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  Figure 4  (a) Full scan XPS spectra of all samples; (b) Au4f, (c) Bi4f, (d) Fe2p, (e) Co2p, (f) Ti2p and (g) O1s XPS spectra of BFCTO/Au-4 nanocomposite

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  Figure 5  (a) UV-Vis diffuse reflectance spectra of the samples; (b) Relationships between (αhν)² and photon energy (hν) for the samples

  Inset: optical photos of the samples

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  Figure 6  (a~e) N₂ adsorption-desorption isotherms of various samples; (f) BET specific surface areas for various samples

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  Figure 7  (a) Photocatalytic RhB degradation curves in the presence of BFCTO and all of the BFCTO/Au samples under visible light irradiation; (b) Corresponding kinetic linear simulation curves of RhB under visible irradiation for the samples; (c) Cyclic photocatalytic degradation experiments of RhB by BFCTO/Au-1 under the visible light irradiation; (d) Photocatalytic methyl orange (MO) degradation curves for the samples

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  Figure 8  Schematic illustration of proposed mechanism for photogenerated charge carrier transfer in BFCTO/ Au nanocomposite under visible light irradiation

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