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BixMyOz (M = W, Mo) heterojunctions with enhanced photocatalytic activities
Applied Surface Science 475 (2019) 785–792
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A generic strategy for preparation of TiO2/BixMyOz (M = W, Mo) heterojunctions with enhanced photocatalytic activities Guoli Fanga,b, , Jia Liua, Jiandong Wua, Mengyue Lia, Xianghui Yana, Danyang Wangb, ⁎
a b
T ⁎
School of Materials Science and Engineering, North Minzu University, Yinchuan 750021, China School of Materials Science and Engineering, University of New South Wales, NSW 2052, Australia
ARTICLE INFO
ABSTRACT
Keywords: TiO2/Bi2WO6 heterojunctions TiO2/Bi3.64Mo0.36O6.55 heterojunctions TiO2/Bi2MoO6 heterojunctions Defects induction Photocatalytic activities
Use of nanoscale TiO2/BixMyOz (M = W, Mo) heterojunctions is one of the most promising strategies for improving the photocatalytic efficiency. However, the controllable synthesis and morphology modification of these heterojunctions are still highly challenging. In this work, we developed a generic approach to hydrothermally synthesize TiO2/BixMyOz heterojunctions and tailor their morphologies. The key to this strategy is to intentionally utilize the surface defects of TiO2 as highly active sites to adsorb the intermediate hydrolysis-products, which is in marked contrast to the conventional direct precipitation methods. In the subsequent hydrothermal reactions, MO42− replaced the hydroxyl and nitrate radicals to form stable TiO2/BixMyOz heterojunctions, in which the second phase BixMyOz occupied the defect sites of TiO2 nanobelts. Under visible light irradiation, the photocatalytic reaction rate constant of TiO2/Bi2WO6 heterojunctions was four times higher than that of single phase Bi2WO6 nanosheets, while the photocatalytic reaction rate constant of TiO2/ Bi3.64Mo0.36O6.55 heterojunctions exhibited a seven-fold increase compared with Bi3.64Mo0.36O6.55 nanopaticles. The substantial enhancement of photocatalytic activity is primarily ascribed to the matching energy band structure in the TiO2/BixMyOz heterojunctions, which is able to improve the separation efficiency of photogenerated electron-hole pairs and prolong the lifetime of charge carriers in the heterojunctions.
1. Introduction Semiconductor photocatalysts are capable of directly absorbing sunlight to generate highly reactive electrons/holes without causing any unfavorable changes to the environment. These photo-generated charge carriers can be used to produce clean chemical fuels [1], degrade harmful pollutants [2,3] and induce organic synthesis [4]. In this regard, photocatalysis is deemed to be one of the most promising technologies for the utilization of solar energy [5]. In a typical process of semiconductor photocatalysis, the electrons are excited to the conduction band by a given wavelength with the reciprocal generation of holes in the valence band. The photo-generated free electrons/holes can migrate to the surface and subsequently be consumed by surface redox reactions. On the other hand, the surface reaction often compete against the recombination of free electrons/holes [6]. For instance, recombination of electrons/holes in TiO2 takes picoseconds to nanoseconds, whereas the electrons/holes need several hundred nanoseconds to allow the completion of reaction with surface reducing or oxidizing agents [7]. Obviously, the photocatalytic performance of semiconductor materials is determined by the number of photo-generated
⁎
electrons/holes that can be transferred into the surface redox reactions. Heterojunction is broadly defined as the interface between two semiconductors with dissimilar crystalline structures, band gap energy and physicochemical properties [8], which can be engineered to spatially separate photo-generated electron-hole pairs, prolong the lifetime of photo-generated carriers [9] and expand the responsive spectrum of photocatalysts to visible light [10]. TiO2/BixMyOz (M = W, Mo) heterojunctions are one of the most prominent photocatalysts for efficient utilization of solar energy and show great potential and versatility in disinfection of bacteria [11], degradation of organic pollutants [12,13] and splitting water [14]. However, it is quite challenging to synthesize high-performance and stable TiO2/BixMyOz heterojunctions, because of the complex heterostructures and large difference in physiochemical properties between TiO2 and BixMyOz. In the traditional synthesis routes, either BixMyOz directly precipitated from saturated solution and deposited on the surface of TiO2 at random [15–17], or TiO2 deposited on the surface of BixMyOz as a secondary phase [18]. Despite the great advances in controllable synthesis of TiO2/ BixMyOz heterojunctions [19,20], it is still very difficult to tailor the growth site and size of BixMyOz on the surface of TiO2.
Corresponding authors. E-mail addresses: [email protected] (G. Fang), [email protected] (D. Wang).
https://doi.org/10.1016/j.apsusc.2018.12.297 Received 6 September 2018; Received in revised form 2 December 2018; Accepted 31 December 2018 Available online 02 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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We were inspired by the fact that the surface energy of a crystal will proportionally increase with the increase of the number of broken bonds [21]. The surface atoms in the vicinity of a defect, which are weakly bonded to the adjacent atoms, have higher free energies than those in a defect-free lattice. Moreover, the more weakly the surface atom is bonded to surrounding atoms, the stronger its ability to attract small adsorbates [22]. Taking these into consideration, we propose to utilize the surface defects of TiO2 nanobelts to adsorb hydrolysis-produced [Bi6O6(OH)3−x](NO3)3+x, which is commonly used as bismuth precursor to synthesize bismuth-containing oxides such as Bi2WO6 [23], BiOX [24] and Bi2MoO6 [25]. Subsequently, OH− and NO3− groups of the precursors are replaced by MO42− in the process of hydrothermal reaction to allow in situ heterogeneous nucleation of BixMyOz at surface defect sites of TiO2 nanobelts. The BixMyOz crystal nuclei will grow to form stable TiO2/BixMyOz heterojunctions. Both Bi2WO6 and Bi2MoO6 are representatives of Aurivillius semiconductive oxides with visiblelight-driven photocatalytic activity [26,27]. Cubic phase Bi3.64Mo0.36O6.55, which is a visible-light responsive photocatalyst, is often present in the hydrothermal preparation process of Bi2MoO6 as a by-product [28]. In view of that, we employed WO42− and MoO42− to synthesize TiO2/Bi2MoO6, TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions in a controllable way for crosschecking our strategy. Our results show that the defects can be deliberately used to induce the hetero-growth of Bi2WO6, Bi2MoO6 and Bi3.64Mo0.36O6.55 on the surface of TiO2 nanobelts, and tailor the size of Bi2WO6 nanosheets in TiO2/ Bi2WO6 heterojunctions. The photocatalytic properties of our TiO2/ BixMyOz (M = W, Mo) heterojunctions were investigated under visible light irradiation. The outstanding visible-light-driven photocatalytic activities are attributed to the higher carrier separation efficiency and longer lifetime of the photo-generated carriers.
Fig. 1. Flowchat of the synthesis of TiO2/Bi2WO6, TiO2/Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions via hydrothermal method.
was employed to perform transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images at an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images were taken by field emission scanning electron microscope (Zeiss SUPRA TM-40, German) at 10 kV. XPS measurements were carried out on a Thermo Scientific ESCALAB 250XI system (Thermo Scientific, USA) using a 150 W monochromatized Al Kα (hν = 1486.7 eV) excitation source with a spot size of 500 µm. The narrow-scan spectra which were collected at pass energy of 20 eV, have been charge corrected to obtain the adventitious C 1s spectral binding energy of 284.8 eV. Diffuse reflectance spectra (DRS) were recorded on a UV–Vis spectrometer (UV-2550, Shimadzu) using BaSO4 as a reference standard. The Brunauer-EmmettTeller (BET) surface area was calculated by N2 sorption isotherms measured on a Micromeritics ASAP 2020 analyzer. The photocatalytic activities of the samples were evaluated by the degradation of Rhodamine B (Rh B) with an initial concentration of 10 mg/L. The as-synthesized heterojunction (30 mg) was dispersed into 30 ml of the Rh B aqueous solution in the quartz tube of an XPA-photochemical reactor. The mixture suspension was first stirred in the dark for 30 min to establish an adsorption-desorption equilibrium state on the photocatalyst surface. A 300 W Xe lamp equipped with a 400 nm cut-off filter was then used as simulated visible light source, and the distance between quartz tube and light source was 60 mm. At a given illumination time interval, 3 ml of suspension was taken out and centrifuged to remove photocatalyst. The residual concentrations of Rh B in the supernatant were analyzed by a UV-2550 spectrometer. Electrochemical measurements were measured on a CHI 660D electrochemical workstation (Shanghai Chenhua, China). A three-electrode system consisting of a working electrode, a counter electrode (Pt wire) and Ag/AgCl reference electrode was employed with 0.1 mol/L Na2SO4 solution as an supporting electrolyte. A 300 W Xe lamp was employed to irradiate the working electrode, and a 400 nm cut-off filter was used to block UV light.
2. Experimental Analytical grade reagents were used for the hydrothermal synthesis without further purification. TiO2 nanobelts with intentional defects were prepared by a two-step method. The detailed preparation process of TiO2 nanobelts was given in the Supporting Information. In a typical synthesis of TiO2/[Bi6O6(OH)3−x](NO3)3+x precursor, 0.25 mmol of Bi(NO3)3·5H2O was first dispersed in 15 ml of deionized water, and the intrinsic pH value of the suspension was 1.1. After being stirred for 45 min, 0.08 g of TiO2 nanobelts were added into the Bi (NO3)3 suspension as the growth template of the heterojunctions. The pH value of the suspension was then adjusted to 5.5 by adding sodium hydroxide solution. White TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precipitated from the suspension during the continuously stirring process [29]. The obtained TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor mixture will be used to react with Na2WO4 or Na2MoO4 solution in the subsequent hydrothermal process. The synthesis routes of TiO2/Bi2WO6, TiO2/Bi2MoO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions are shown in Fig. 1. Precursors were introduced in the synthesis process according to the stoichiometry of Bi2WO6, Bi2MoO6 and Bi3.64Mo0.36O6.55. The TiO2/BixMyOz (M = W, Mo) heterojunctions were hydrothermally synthesized in a Teflon-lined autoclave with a stainless steel tank. The hydrothermal temperature, reaction time and pH value for the preparation of each heterojunction were given in Fig. 1. It was noteworthy that pH value of the starting mixture for hydrothermal reaction played an important role in the formation of heterojunctions with desired compositions. The resulting heterojunctions were washed with deionized water and ethanol to remove any ionic residuals and dried at 60 °C. Bi2WO6 nanosheets, Bi2MoO6 and Bi3.64Mo0.36O6.55 nanoparticles were also synthesized in the absence of TiO2 nanobelts under the similar conditions to the preparation of corresponding heterojunctions for comparison purpose. Powder X-ray diffraction analyses (XRD) were conducted on a SHIMADZU XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5406 Å). A JEOL JEM-2100 transmission electron microscope
3. Results and discussion 3.1. Characterization of TiO2/Bi2WO6 heterojunctions. Fig. 2 shows the XRD patterns of TiO2/Bi2WO6 heterojunctions and TiO2 nanobelts (growth template). The XRD pattern of TiO2 nanobelts shows an anatase phase having tetragonal structure (JCPDS card No. 65-5714) without any detectable peaks from impurities or secondary 786
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Ti 2p, Bi 4f and W 4f narrow-scan spectra of TiO2/Bi2WO6 heterojunctions are shown in Fig. 3. The peaks at 464.7 and 458.6 eV having a separation of 6.1 eV in the Ti 2p spectrum can be assigned to Ti 2p1/2 and Ti 2p3/2 of Ti4+, respectively [2]. Comparing with the Ti 2p spectrum of TiO2 nanobelts, a shift (0.5–0.6 eV) toward higher binding energy is observed after the hetero-growth of Bi2WO6 on the surface of TiO2 nanobelts. As shown in Bi 4f spectrum, the intensive peaks centered at 164.2 eV and 158.9 eV are attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which correspond to Bi3+ in TiO2/Bi2WO6 heterojunctions. The W 4f spectrum exhibits the W 4f5/2 peak at 37.2 eV and the W 4f7/2 peak at 35.1 eV, reflecting a +6 oxide state of W in the prepared TiO2/Bi2WO6 heterojunctions [30]. The results of XPS spectra confirm that the TiO2/Bi2WO6 heterojuncions have been successfully synthesized by two-step hydrothermal method. The TEM images of TiO2 nanobelts, TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor and TiO2/Bi2WO6 heterojunctions are shown in Figs. 4 and S1, respectively. It is interesting that TiO2 nanobelts exhibit simultaneous regular defects (marked by red rectangles) and fine crystalline structures, as shown in Fig. 4(a) and (b). The average length of defects is ∼5 nm, and the lattice spacing is determined to be 0.351 nm, which coincides with thermodynamically stable (1 0 1) crystal plane of anatase TiO2 [30]. Figs. 4(c) and S1(b) show the HRTEM and TEM images of TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor, respectively. The defects of TiO2 nanobelts almost vanished in the precursor, primarily because [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) adsorbates occupy the defect sites on the surface of TiO2 nanobelts. This can be understood by the low coordination number of atoms residing at
Fig. 2. The XRD patterns of the as-synthesized TiO2/Bi2WO6 heterojunctions and TiO2 nanobelts.
phases. In the XRD pattern of TiO2/Bi2WO6 heterojunctions, the characteristic diffraction peaks of anatase TiO2 phase i.e. (1 0 1), (0 0 4), (2 0 0) and (1 0 5) are still visible, while the other diffraction peaks can be indexed to orthorhombic Bi2WO6 with layered Aurivillius structure (JCPDS card No. 73-1126). The surface chemical composition and elements valence state of TiO2/Bi2WO6 heterojunctions were derived from the XPS spectra. The
Fig. 3. The XPS narrow-scan spectra of Ti 2p, W 4f and Bi 4f for the as-synthesized TiO2/Bi2WO6 heterojunctions. The Ti 2p spectrum of TiO2 nanobelts was also given for comparison purpose.
Fig. 4. The (a) TEM and (b) HRTEM images of TiO2 nanobelts; (c) HRTEM image of TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor; (d) TEM and (e) HRTEM images of TiO2/Bi2WO6 heterojunctions, and (f) HRTEM image of the typical interfacial region of TiO2/Bi2WO6 heterojunctions. 787
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Fig. 5. Front views of the atomic arrangements of (a) orthorhombic Bi2WO6 around [1¯ 2¯ 1] zone axis and (b) anatase TiO2 around [1¯ 0 1] zone axis; (c) Schematic diagrams illustrating the formation mechanism of Bi2WO6 (1 1 3) plane.
the edge of defects, resulting in high chemical potential that can promote the bonding with other molecules [31]. In other words, the atoms near defects are prone to adsorb small adsorbates so that defect sites are highly active for hetero-adsorbing and growth. The prepared TiO2/ [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor then reacts with Na2WO4 to form TiO2/Bi2WO6 heterojunctions in the hydrothermal process. Figs. S1(c), 4(d) and S2(b) show the low, high magnification TEM and SEM images of TiO2/Bi2WO6 heterojunctions, respectively. It is found that wing-like Bi2WO6 nanosheets grow side by side with the TiO2 nanobelts, and the thickness of the Bi2WO6 nanosheets is less than 15 nm. Notably, there is a disorder layer with ∼6 nm thickness at the interface of TiO2/Bi2WO6 heterojunctions, and the angle between the (1¯ 1 1) planes of Bi2WO6 and the (1 0 1) planes of TiO2 is ∼27 degrees as shown in Fig. 4(e) and (f). In Fig. 4(e), the lattice fringe distances marked in yellow are 0.372, 0.314 and 0.351 nm, which are corresponding to the interplanar spacing of Bi2WO6 (1¯ 1 1), (1 1 3) and TiO2 (1 0 1), respectively. The progressive rotation of Bi2WO6 (1¯ 1 1) planes relative to TiO2 (1 0 1) improves the lattice matching between Bi2WO6 and TiO2, and facilitates the preferred heterogeneous growth of Bi2WO6 on TiO2 template. According to the interfacial structure of TiO2/Bi2WO6 heterojuncions shown in HRTEM images, schematic diagrams illustrating the formation mechanism of this heterojunctions were constructed. Fig. 5(a) and (b) show the front views of the atomic arrangements of orthorhombic Bi2WO6 around [1¯ 2¯ 1] zone axis and anatase TiO2 around [1¯ 0 1] zone axis, respectively. The lattice of Bi2WO6 is built up by alternating layers of [Bi2O2]2+ and [WO4]2−, in which the [WO4]2− layers refer to corner-sharing [WO6] octahedra [32]. Given the angled growth and lattice distortion, the lattice fringe distance of Bi2WO6 (1¯ 1 1) plane (∼0.372 nm), which is equal to the thickness of two [WO6] layers as shown in Fig. 5(a), is close to that of the TiO2 (1 0 1) plane (d(1 0 1) = 0.351 nm), resulting in the lattice-matched TiO2/ Bi2WO6 heterojuncions. Moreover, anatase TiO2 can be regarded as the spatial arrangement of [TiO6] octahedra that share corner to form the (1 0 1) planes [33]. These facts underpin that the growth of
heterointerface between Bi2WO6 and TiO2 i.e. the growth relationship is Bi2WO6 (1¯ 1 1) ‖ TiO2 (1 0 1). 3.2. Formation mechanisms of TiO2/BixMyOz heterojunctions. To understand the formation mechanisms of TiO2/Bi2WO6 heterojunctions, the chemical reactions involved in this process were presented as follows: (a) Hydrolysis, pH = 1.1, Bi(NO3)3 + H2O → [Bi6O4(OH)4](NO3)6
(1)
−
pH = 5.5, [Bi6O4(OH)4](NO3)6 + OH + TiO2 → TiO2/[Bi6O6(OH)3−x] (NO3)3+x (x = 0–2) + H2O + NO3− (2) (b) Hydrothermal reactions, TiO2/[Bi6O6(OH)3−x](NO3)3+x + OH− → TiO2/Bi2O2(OH) (NO3) + NO3− TiO2/Bi2O2(OH)(NO3) + WO4
2−
−
→ TiO2/Bi2WO6 + OH +
(3) NO3−
(4)
The hydrolysis-produced [Bi6O4(OH)4](NO3)6, which is prepared at room temperature with pH = 1.1, can be converted into white precipitate [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) when tuning the pH value to 5.5 by adding NaOH into the suspension [29]. The precipitate [Bi6O6(OH)3−x](NO3)3+x adsorbs at the highly active defect sites of TiO2 nanobelts to form TiO2/[Bi6O6(OH)3−x](NO3)3+x precursor. During the subsequently hydrothermal process, the [Bi6O6(OH)3−x] (NO3)3+x adsorbed on the stable (1 0 1) surface of TiO2 nanobelts undergoes two-step reactions (3) and (4) [34]. In particular, WO42− replaces OH− and NO3− in TiO2/Bi2O2(OH)(NO3) to enable in situ heterogeneous nucleation of Bi2WO6 on the (1 0 1) surface of TiO2. Both Bi2O2(OH)(NO3) and Bi2WO6 have a similar sandwich-like layered 788
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Fig. 6. Schematic illustration of the generic route for synthesizing TiO2/BixMyOz (M = W, Mo) heterojunctions.
Fig. 7. (a) XRD pattern, (b) TEM and (c) HRTEM images of TiO2/Bi3.64Mo0.36O6.55 heterojunctions; and (d) XRD pattern, (e) TEM and (f) HRTEM images of TiO2/ Bi2MoO6 heterojunctions.
structure with weak interlayer bonding (van der Waals force) as illustrated in Fig. 5(c) [35]. Another weak hydrogen bond between [OH] and [NO3] also dwells in the [(OH)(NO3)]2− layers [36]. This weak hydrogen bond can be easily ruptured. As a result, OH− and NO3− could be substituted by [WO4]2− which provide stronger intralayer chemical bonding. To further clarify the role of surface defects in the nucleation and growth of heterointerfaces, TiO2 nanobelts were corroded by sulfuric acid (0.2 mol/l) at 100 °C for 12 h to intentionally increase the density of surface defects. These TiO2 nanobelts were used as growth template to tune the morphology of TiO2/Bi2WO6 heterojunctions. Fig. S3 exhibits the TEM and HRTEM images of the pruned-TiO2/Bi2WO6 heterojunctions. The Bi2WO6 nanosheets in pruned-TiO2/Bi2WO6 heterojunctions are evidently smaller than those shown in Fig. 4(d). It is also noted that the cambered edge of Bi2WO6 nanosheets are formed in pruned-TiO2/Bi2WO6 heterojunctions. In the classical thermodynamics, the higher nucleation rate in a given saturated solution will result in a smaller average crystal size [37]. In this sense, the increased density of surface defects facilitates massive hetero-nucleation at the expense of the suppression of the growth of initial nuclei. Therefore, the morphology of Bi2WO6 nanosheets can be tailored by controlling the density of the surface defects on TiO2 nanobelts. Based on the results above, a generic route of synthesizing TiO2/ BixMyOz heterojunctions is established and schematically shown in Fig. 6. To further confirm the viability of this synthesis route, MoO42− was employed as another model system to fabrication heterojunctions
on uncorroded TiO2 nanobelts, because of the similar structure between Bi2WO6 and Bi2MoO6. TiO2/Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions were successfully synthesized when the pH values of the hydrothermal suspension were adjusted to 8 and 4, respectively. The XRD patterns, TEM and SEM images of the as-synthesized TiO2/ Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions are shown in Figs. 7 and S2. The TiO2/Bi3.64Mo0.36O6.55 consists of anatase TiO2 and cubic Bi3.64Mo0.36O6.55 (JCPDS No. 43-0446) as evidenced by the XRD pattern in Fig. 7(a). The TEM image (Fig. 7(b)) and SEM image (Fig. S2(c)) reveal Bi3.64Mo0.36O6.55 nanoparticles with an average size of ∼70 nm in diameter are embedded on the surface of TiO2 nanobelts. The lattice spacing of Bi3.64Mo0.36O6.55 nanoparticles was found to be 0.326 nm, which is indexed to (1 1 1) crystal plane. This well-matched lattice with TiO2 (1 0 1) plane (d(1 0 1) = 0.351 nm) enables the growth of hetero-interface Bi3.64Mo0.36O6.55 (1 1 1)‖TiO2 (1 0 1) as shown in Fig. 7(c). The XPS narrow-scan spectra of Ti, Mo and Bi for TiO2/ Bi3.64Mo0.36O6.55 heterojunctions are shown in Fig. S4. Comparing with TiO2 nanobelts, the Ti 2P spectrum of TiO2/Bi3.64Mo0.36O6.55 heterojunctions displays a slight shift to higher binding energy, similar to that of TiO2/Bi2WO6 heterojunctions. It is evident that TiO2/ Bi3.64Mo0.36O6.55 heterojunctions can also be synthesized by the generic route in Fig. 6. Fig. 7(d) shows the XRD pattern of TiO2/Bi2MoO6 heterojunctions containing anatase TiO2 and orthorhombic Bi2MoO6 with Aurivillius layered structure (JCPDS No. 07-0401) [38]. The SEM image of TiO2/ Bi2MoO6 heterojunctions (Fig. S2(d)) exhibits that Bi2MoO6 789
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Fig. 8. (a) Efficiencies of photo-degradation of Rh B using various photocatalysts as a function of time under visible light irradiation conditions; (b) the corresponding first-order kinetic linear fitting curves for photo-degradation of Rh B, and (c) photocatalytic efficiencies of degrading Rh B using TiO2/Bi2WO6 heterojunctions in five consecutive tests.
Fig. 9. Transient photocurrent densities under visible light irradiation of (a) Bi2WO6 nanosheets, and TiO2/Bi2WO6 heterojunctions; (b) Bi3.64Mo0.36O6.55 nanoparticles and TiO2/Bi3.64Mo0.36O6.55 heterojunctions; (c) Bi2MoO6 nanoparticles and TiO2/Bi2MoO6 heterojunctions. The time-dependent photocurrent density of TiO2 nanobelts was also included for comparison purpose.
nanoparticles hetero-grow on the surface of TiO2 nanobelts. Fig. 7(f) confirmed the angled growth of (1¯ 1 1¯) planes of Bi2MoO6 on the (1 0 1) planes of TiO2 [39], which is similar to the TiO2/Bi2WO6 heterojunctions. These referred results demonstrate that the TiO2/BixMyOz heterojunctions can be obtained and tailored by a generic two-step hydrothermal synthesis route consisting of an inducing adsorption and an in situ anion-exchange.
heterojuntions are greater than those of bare TiO2 nanobelts (32.3 m2/ g), Bi2WO6 nanosheets (26.3 m2/g), Bi3.64Mo0.36O6.55 (17.9 m2/g) and Bi2MoO6 (14.2 m2/g) nanopartiles (Fig. S5). As shown in Fig. S6, the absorption edges of TiO2/Bi2WO6, TiO2/Bi3.64Mo0.36O6.55 and TiO2/ Bi2MoO6 heterojuntions exhibit an evidently red shift compared with TiO2 nanobelts, indicating that the prepared TiO2/BixMyOz (M = Mo, W) heterojunctions can be responsive to visible light. Fig. 8 shows the Rh B degradation efficiencies using different heterojunctions photocatalysts under visible light irradiation, and their corresponding first-order kinetic linear fitting curves. After visible light irradiating for 60 min, the removal rate of Rh B with TiO2/Bi2WO6
3.3. Photocatalytic activities Additionally, the specific surface areas of TiO2/Bi2WO6 (34.1 m2/g), TiO2/Bi3.64Mo0.36O6.55 (32.4 m2/g) and TiO2/Bi2MoO6 (32.6 m2/g) 790
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Fig. 10. Energy band diagrams and photocatalytic mechanism of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions under visible light irradiation.
heterojunctions is nearly 98%, which is much higher than those using bare Bi2WO6 (40.1%) and TiO2 (5.6%) as photocatalysts. Incidentally, the reported carbon fiber loaded TiO2/Bi2WO6 nanostructured heterojunctions with higher cost could almost completely photo-degrade Rh B after 80 min under visible light irradiation [40]. Moreover, the as-prepared TiO2/Bi2WO6 heterojunctions can retain a high photo-degradation efficiency, i.e. ∼97% photo-degradation rate of Rh B within 60 min time frame, even after five consecutive photocatalytic tests. This stability is much better than that of the TiO2/Bi2WO6 hollow microsphere [20]. The photocatalytic reaction rate constant of TiO2/Bi2WO6 heterojunctions is four times higher than that of bare Bi2WO6. Similarly, the photocatalytic reaction rate constant of TiO2/ Bi3.64Mo0.36O6.55 heterojunctions renders a 7-fold increase compared with the case of bare Bi3.64Mo0.36O6.55. However, the photocatalytic performance of TiO2/Bi2MoO6 heterojunctions is only slightly better than that of single phase Bi2MoO6. The photocurrent responses of different photocatalysts under visible light irradiation, which were measured by cyclically switching on and off of a 300 W Xe lamp with a time interval of 50 s, are shown in Fig. 9. It is observed that, except for the Bi2MoO6 nanopaticles, the photocurrent densities of all the photocatalysts dropped to ∼zero almost instantaneously when the light source is turned off. The as-prepared Bi2MoO6 nanoparticles exhibits significant relaxation behavior in photocurrent densities under the same illumination conditions, owing to the long relaxation time for recombination of photo-generated electronhole pairs relative to switching speed as well as the slow recombination process in deep trap sites [41]. The slow and incomplete discharge of trapped photo-generated carriers can prolong the lifetime of photogenerated carriers. The effective separation of photo-generated electron-hole pairs on the interface of TiO2/Bi2MoO6 heterojuntions led to an increase in photocurrent stability as shown in Fig. 9(c). However, when light was off, the recombination taking place between the holes transferred from Bi2MoO6 to TiO2 and the electrons left in Bi2MoO6 made the photocurrent rapidly quenched [42]. This also explains the insignificant enhancement of photocatalytic activity in TiO2/Bi2MoO6 heterojunctions [43]. Both TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions display much higher photocurrent densities than the single phase TiO2, Bi2WO6 and Bi3.64Mo0.36O6.55 as shown in Fig. 9(a) and (b). The results are attributed to the fact that photo-generated holes can be rapidly transferred from Bi2WO6 or Bi3.64Mo0.36O6.55 to TiO2 and suppress the recombination of photo-generated electron-hole pairs, i.e. the blocked electron transfer formed at the interfaces improves the effective separation of photo-generated electron-hole pairs in TiO2/
BixMyOz heterojunctions [19]. Therefore, the free charge carrier concentrations in TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions are greater than those in single phase TiO2, Bi2WO6 and Bi3.64Mo0.36O6.55, which echoes their better photocatalytic activities. Fig. 10 illustrates the band structures of TiO2/Bi2WO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions, which are used to understand their photocatalytic mechanisms under visible light irradiation. The conduction band bottom (ECB) and the valance band top potentials (EVB) were estimated by the empirical equations [44]:
ECB = X
EC
0.5Eg
EVB = ECB + Eg
(1) (2)
where X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms evaluated via electron affinity and first ionization potential [45]. EC is ∼4.5 eV. The Eg of TiO2 nanobelts, Bi2WO6 nanosheets and Bi3.64Mo0.36O6.55 nanoparticles were obtained from the fitting of UV–Vis diffuse reflectance spectra (Fig. S6). Under visible light irradiation, the electrons residing in the energy levels closer to the top of valence band of Bi2WO6 and Bi3.64Mo0.36O6.55 can be readily excited to their conduction band due to narrow bandgap with the reciprocal generation of holes in their valance band. Because the EVB of TiO2 (2.98 eV) is more negative than those of Bi2WO6 and Bi3.64Mo0.36O6.55, the photo-generated holes can migrate to the valance band of TiO2. At the same time, the photo-generated electrons still remain in the conduction band of Bi2WO6 and Bi3.64Mo0.36O6.55, because of their more positive ECB compared with TiO2. When photo-generated holes migrated into EVB of TiO2, the recombination of photo-generated electronhole pairs can be suppressed in TiO2/Bi2WO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions [8]. In such a way, photo-generated electrons and holes are effectively separated in TiO2/BixMyOz heterojunctions [12,13], and the lifetime of charge carriers are prolonged [14]. The free charge carriers will subsequently react with redox mediators, i.e. O2, H2O and OH−, to yield active species, or else directly mineralize organic dye [46]. Consequently, the photocatalytic activities of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions can be notably improved under visible light. 4. Conclusions A generic two-step hydrothermal synthesis method, consisting of an inducing adsorption and an in situ anion exchange, is reported to 791
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selectively prepare TiO2/Bi2WO6, TiO2/Bi2MoO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions. The defects on the surface of TiO2 play a key role in promoting the adsorption and growth of secondary phase. The morphology of TiO2/BixMyOz heterojunctions can be tailored by controlling the density of TiO2 surface defects. The photocatalytic activities of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions are considerably improved under visible light irradiation. This is primarily attributed to the band structure matching of TiO2/ BixMyOz heterojunctions, which can effectively promote the separation of photo-generated electron-hole pairs, and prolong the lifetime of charge carriers.
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Acknowledgment G. Fang and J. Liu are contributed equally to this work and should be considered co-first authors. Financial supports from the Natural Science Foundation of NingXia Province (No. NZ17108), National Natural Science Foundation of China (No. 51262001), and China Scholarship Council are acknowledged. This work was also partially supported by the Australian Research Council Discovery Project (Grant No. DP170103514). We also thank Prof. Guozhong Cao, University of Washington, for his valuable suggestions and help. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.12.297. References [1] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J.K. Nørskov, Materials for solar fuels and chemicals, Nat. Mater. 16 (2017) 70–81. [2] M. Reli, P. Huo, M. Šihor, N. Ambrožová, I. Troppová, L. Matějová, J. Lang, L. Svoboda, P. Kuśtrowski, M. Ritz, P. Praus, K. Kočí, Novel TiO2/C3N4 photocatalysts for photocatalytic reduction of CO2 and for photocatalytic decomposition of N2O, J. Phys. Chem. A 120 (2016) 8564–8573. [3] S. Liu, Q. Hu, J. Qiu, F. Wang, W. Lin, F. Zhu, C. Wei, N. Zhou, G. Ouyang, Enhanced photocatalytic degradation of environmental pollutants under visible irradiation by a composite coating, Environ. Sci. Technol. 51 (2017) 5137–5145. [4] T.P. Yoon, M.A. Ischay, J. Du, Visible light photocatalysis as a greener approach to photochemical synthesis, Nat. Chem. 2 (2010) 527–532. [5] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [6] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986. [7] J.J.M. Vequizo, H. Matsunaga, T. Ishiku, S. Kamimura, T. Ohno, A. Yamakata, Trapping-induced enhancement of photocatalytic activity on brookite TiO2 powders: comparison with anatase and rutile TiO2 powders, ACS Catal. 7 (2017) 2644–2651. [8] J. Low, J. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [9] M. Zhu, Q. Liu, W. Chen, Y. Yin, L. Ge, H. Li, K. Wang, Boosting the visible-light photoactivity of BiOCl/BiVO4/N-GQD ternary heterojunctions based on internal Zscheme charge transfer of N-GQDs: simultaneous band gap narrowing and carrier lifetime prolonging, ACS Appl. Mater. Interfaces 9 (2017) 38832–38841. [10] J. Jian, Y. Sang, G. Yu, H. Jiang, X. Mu, H. Liu, A Bi2WO6-based hybrid photocatalyst with broad spectrum photocatalytic properties under UV, visible, and nearinfrared irradiation, Adv. Mater. 25 (2013) 5075–5080. [11] Y. Jia, S. Zhan, S. Ma, Q. Zhou, Fabrication of TiO2-Bi2WO6 binanosheet for enhanced solar photocatalytic disinfection of E. coli: insights on the mechanism, ACS Appl. Mater. Interfaces 8 (2016) 6841–6851. [12] G. Tian, Y. Chen, R. Zhai, J. Zhou, W. Zhou, R. Wang, K. Pan, C. Tian, H. Fu, Hierarchical flake-like Bi2MoO6/TiO2 bilayer films for visible-light-induced selfcleaning applications, J. Mater. Chem. 1 (2013) 6961–6968. [13] J. Xu, W. Wang, S. Sun, L. Wang, Enhancing visible-light-induced photocatalytic activity by coupling with wide-band-gap semiconductor: a case study on Bi2WO6/ TiO2, Appl. Catal. B: Environ. 111 (2012) 126–132. [14] M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Long-lived, visible-light-excited charge carriers of TiO2/BiVO4 nanocomposites and their unexpected photoactivity for water splitting, Adv. Energy Mater. 4 (2014) 1300995. [15] J. Li, X. Liu, Z. Sun, L. Pan, Novel Bi2MoO6/TiO2 heterostructure microspheres for degradation of benzene series compound under visible light irradiation, J. Colloid Interface Sci. 463 (2016) 145–153.
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Full Length Article
A generic strategy for preparation of TiO2/BixMyOz (M = W, Mo) heterojunctions with enhanced photocatalytic activities Guoli Fanga,b, , Jia Liua, Jiandong Wua, Mengyue Lia, Xianghui Yana, Danyang Wangb, ⁎
a b
T ⁎
School of Materials Science and Engineering, North Minzu University, Yinchuan 750021, China School of Materials Science and Engineering, University of New South Wales, NSW 2052, Australia
ARTICLE INFO
ABSTRACT
Keywords: TiO2/Bi2WO6 heterojunctions TiO2/Bi3.64Mo0.36O6.55 heterojunctions TiO2/Bi2MoO6 heterojunctions Defects induction Photocatalytic activities
Use of nanoscale TiO2/BixMyOz (M = W, Mo) heterojunctions is one of the most promising strategies for improving the photocatalytic efficiency. However, the controllable synthesis and morphology modification of these heterojunctions are still highly challenging. In this work, we developed a generic approach to hydrothermally synthesize TiO2/BixMyOz heterojunctions and tailor their morphologies. The key to this strategy is to intentionally utilize the surface defects of TiO2 as highly active sites to adsorb the intermediate hydrolysis-products, which is in marked contrast to the conventional direct precipitation methods. In the subsequent hydrothermal reactions, MO42− replaced the hydroxyl and nitrate radicals to form stable TiO2/BixMyOz heterojunctions, in which the second phase BixMyOz occupied the defect sites of TiO2 nanobelts. Under visible light irradiation, the photocatalytic reaction rate constant of TiO2/Bi2WO6 heterojunctions was four times higher than that of single phase Bi2WO6 nanosheets, while the photocatalytic reaction rate constant of TiO2/ Bi3.64Mo0.36O6.55 heterojunctions exhibited a seven-fold increase compared with Bi3.64Mo0.36O6.55 nanopaticles. The substantial enhancement of photocatalytic activity is primarily ascribed to the matching energy band structure in the TiO2/BixMyOz heterojunctions, which is able to improve the separation efficiency of photogenerated electron-hole pairs and prolong the lifetime of charge carriers in the heterojunctions.
1. Introduction Semiconductor photocatalysts are capable of directly absorbing sunlight to generate highly reactive electrons/holes without causing any unfavorable changes to the environment. These photo-generated charge carriers can be used to produce clean chemical fuels [1], degrade harmful pollutants [2,3] and induce organic synthesis [4]. In this regard, photocatalysis is deemed to be one of the most promising technologies for the utilization of solar energy [5]. In a typical process of semiconductor photocatalysis, the electrons are excited to the conduction band by a given wavelength with the reciprocal generation of holes in the valence band. The photo-generated free electrons/holes can migrate to the surface and subsequently be consumed by surface redox reactions. On the other hand, the surface reaction often compete against the recombination of free electrons/holes [6]. For instance, recombination of electrons/holes in TiO2 takes picoseconds to nanoseconds, whereas the electrons/holes need several hundred nanoseconds to allow the completion of reaction with surface reducing or oxidizing agents [7]. Obviously, the photocatalytic performance of semiconductor materials is determined by the number of photo-generated
⁎
electrons/holes that can be transferred into the surface redox reactions. Heterojunction is broadly defined as the interface between two semiconductors with dissimilar crystalline structures, band gap energy and physicochemical properties [8], which can be engineered to spatially separate photo-generated electron-hole pairs, prolong the lifetime of photo-generated carriers [9] and expand the responsive spectrum of photocatalysts to visible light [10]. TiO2/BixMyOz (M = W, Mo) heterojunctions are one of the most prominent photocatalysts for efficient utilization of solar energy and show great potential and versatility in disinfection of bacteria [11], degradation of organic pollutants [12,13] and splitting water [14]. However, it is quite challenging to synthesize high-performance and stable TiO2/BixMyOz heterojunctions, because of the complex heterostructures and large difference in physiochemical properties between TiO2 and BixMyOz. In the traditional synthesis routes, either BixMyOz directly precipitated from saturated solution and deposited on the surface of TiO2 at random [15–17], or TiO2 deposited on the surface of BixMyOz as a secondary phase [18]. Despite the great advances in controllable synthesis of TiO2/ BixMyOz heterojunctions [19,20], it is still very difficult to tailor the growth site and size of BixMyOz on the surface of TiO2.
Corresponding authors. E-mail addresses: [email protected] (G. Fang), [email protected] (D. Wang).
https://doi.org/10.1016/j.apsusc.2018.12.297 Received 6 September 2018; Received in revised form 2 December 2018; Accepted 31 December 2018 Available online 02 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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We were inspired by the fact that the surface energy of a crystal will proportionally increase with the increase of the number of broken bonds [21]. The surface atoms in the vicinity of a defect, which are weakly bonded to the adjacent atoms, have higher free energies than those in a defect-free lattice. Moreover, the more weakly the surface atom is bonded to surrounding atoms, the stronger its ability to attract small adsorbates [22]. Taking these into consideration, we propose to utilize the surface defects of TiO2 nanobelts to adsorb hydrolysis-produced [Bi6O6(OH)3−x](NO3)3+x, which is commonly used as bismuth precursor to synthesize bismuth-containing oxides such as Bi2WO6 [23], BiOX [24] and Bi2MoO6 [25]. Subsequently, OH− and NO3− groups of the precursors are replaced by MO42− in the process of hydrothermal reaction to allow in situ heterogeneous nucleation of BixMyOz at surface defect sites of TiO2 nanobelts. The BixMyOz crystal nuclei will grow to form stable TiO2/BixMyOz heterojunctions. Both Bi2WO6 and Bi2MoO6 are representatives of Aurivillius semiconductive oxides with visiblelight-driven photocatalytic activity [26,27]. Cubic phase Bi3.64Mo0.36O6.55, which is a visible-light responsive photocatalyst, is often present in the hydrothermal preparation process of Bi2MoO6 as a by-product [28]. In view of that, we employed WO42− and MoO42− to synthesize TiO2/Bi2MoO6, TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions in a controllable way for crosschecking our strategy. Our results show that the defects can be deliberately used to induce the hetero-growth of Bi2WO6, Bi2MoO6 and Bi3.64Mo0.36O6.55 on the surface of TiO2 nanobelts, and tailor the size of Bi2WO6 nanosheets in TiO2/ Bi2WO6 heterojunctions. The photocatalytic properties of our TiO2/ BixMyOz (M = W, Mo) heterojunctions were investigated under visible light irradiation. The outstanding visible-light-driven photocatalytic activities are attributed to the higher carrier separation efficiency and longer lifetime of the photo-generated carriers.
Fig. 1. Flowchat of the synthesis of TiO2/Bi2WO6, TiO2/Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions via hydrothermal method.
was employed to perform transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images at an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) images were taken by field emission scanning electron microscope (Zeiss SUPRA TM-40, German) at 10 kV. XPS measurements were carried out on a Thermo Scientific ESCALAB 250XI system (Thermo Scientific, USA) using a 150 W monochromatized Al Kα (hν = 1486.7 eV) excitation source with a spot size of 500 µm. The narrow-scan spectra which were collected at pass energy of 20 eV, have been charge corrected to obtain the adventitious C 1s spectral binding energy of 284.8 eV. Diffuse reflectance spectra (DRS) were recorded on a UV–Vis spectrometer (UV-2550, Shimadzu) using BaSO4 as a reference standard. The Brunauer-EmmettTeller (BET) surface area was calculated by N2 sorption isotherms measured on a Micromeritics ASAP 2020 analyzer. The photocatalytic activities of the samples were evaluated by the degradation of Rhodamine B (Rh B) with an initial concentration of 10 mg/L. The as-synthesized heterojunction (30 mg) was dispersed into 30 ml of the Rh B aqueous solution in the quartz tube of an XPA-photochemical reactor. The mixture suspension was first stirred in the dark for 30 min to establish an adsorption-desorption equilibrium state on the photocatalyst surface. A 300 W Xe lamp equipped with a 400 nm cut-off filter was then used as simulated visible light source, and the distance between quartz tube and light source was 60 mm. At a given illumination time interval, 3 ml of suspension was taken out and centrifuged to remove photocatalyst. The residual concentrations of Rh B in the supernatant were analyzed by a UV-2550 spectrometer. Electrochemical measurements were measured on a CHI 660D electrochemical workstation (Shanghai Chenhua, China). A three-electrode system consisting of a working electrode, a counter electrode (Pt wire) and Ag/AgCl reference electrode was employed with 0.1 mol/L Na2SO4 solution as an supporting electrolyte. A 300 W Xe lamp was employed to irradiate the working electrode, and a 400 nm cut-off filter was used to block UV light.
2. Experimental Analytical grade reagents were used for the hydrothermal synthesis without further purification. TiO2 nanobelts with intentional defects were prepared by a two-step method. The detailed preparation process of TiO2 nanobelts was given in the Supporting Information. In a typical synthesis of TiO2/[Bi6O6(OH)3−x](NO3)3+x precursor, 0.25 mmol of Bi(NO3)3·5H2O was first dispersed in 15 ml of deionized water, and the intrinsic pH value of the suspension was 1.1. After being stirred for 45 min, 0.08 g of TiO2 nanobelts were added into the Bi (NO3)3 suspension as the growth template of the heterojunctions. The pH value of the suspension was then adjusted to 5.5 by adding sodium hydroxide solution. White TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precipitated from the suspension during the continuously stirring process [29]. The obtained TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor mixture will be used to react with Na2WO4 or Na2MoO4 solution in the subsequent hydrothermal process. The synthesis routes of TiO2/Bi2WO6, TiO2/Bi2MoO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions are shown in Fig. 1. Precursors were introduced in the synthesis process according to the stoichiometry of Bi2WO6, Bi2MoO6 and Bi3.64Mo0.36O6.55. The TiO2/BixMyOz (M = W, Mo) heterojunctions were hydrothermally synthesized in a Teflon-lined autoclave with a stainless steel tank. The hydrothermal temperature, reaction time and pH value for the preparation of each heterojunction were given in Fig. 1. It was noteworthy that pH value of the starting mixture for hydrothermal reaction played an important role in the formation of heterojunctions with desired compositions. The resulting heterojunctions were washed with deionized water and ethanol to remove any ionic residuals and dried at 60 °C. Bi2WO6 nanosheets, Bi2MoO6 and Bi3.64Mo0.36O6.55 nanoparticles were also synthesized in the absence of TiO2 nanobelts under the similar conditions to the preparation of corresponding heterojunctions for comparison purpose. Powder X-ray diffraction analyses (XRD) were conducted on a SHIMADZU XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5406 Å). A JEOL JEM-2100 transmission electron microscope
3. Results and discussion 3.1. Characterization of TiO2/Bi2WO6 heterojunctions. Fig. 2 shows the XRD patterns of TiO2/Bi2WO6 heterojunctions and TiO2 nanobelts (growth template). The XRD pattern of TiO2 nanobelts shows an anatase phase having tetragonal structure (JCPDS card No. 65-5714) without any detectable peaks from impurities or secondary 786
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Ti 2p, Bi 4f and W 4f narrow-scan spectra of TiO2/Bi2WO6 heterojunctions are shown in Fig. 3. The peaks at 464.7 and 458.6 eV having a separation of 6.1 eV in the Ti 2p spectrum can be assigned to Ti 2p1/2 and Ti 2p3/2 of Ti4+, respectively [2]. Comparing with the Ti 2p spectrum of TiO2 nanobelts, a shift (0.5–0.6 eV) toward higher binding energy is observed after the hetero-growth of Bi2WO6 on the surface of TiO2 nanobelts. As shown in Bi 4f spectrum, the intensive peaks centered at 164.2 eV and 158.9 eV are attributed to Bi 4f5/2 and Bi 4f7/2, respectively, which correspond to Bi3+ in TiO2/Bi2WO6 heterojunctions. The W 4f spectrum exhibits the W 4f5/2 peak at 37.2 eV and the W 4f7/2 peak at 35.1 eV, reflecting a +6 oxide state of W in the prepared TiO2/Bi2WO6 heterojunctions [30]. The results of XPS spectra confirm that the TiO2/Bi2WO6 heterojuncions have been successfully synthesized by two-step hydrothermal method. The TEM images of TiO2 nanobelts, TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor and TiO2/Bi2WO6 heterojunctions are shown in Figs. 4 and S1, respectively. It is interesting that TiO2 nanobelts exhibit simultaneous regular defects (marked by red rectangles) and fine crystalline structures, as shown in Fig. 4(a) and (b). The average length of defects is ∼5 nm, and the lattice spacing is determined to be 0.351 nm, which coincides with thermodynamically stable (1 0 1) crystal plane of anatase TiO2 [30]. Figs. 4(c) and S1(b) show the HRTEM and TEM images of TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor, respectively. The defects of TiO2 nanobelts almost vanished in the precursor, primarily because [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) adsorbates occupy the defect sites on the surface of TiO2 nanobelts. This can be understood by the low coordination number of atoms residing at
Fig. 2. The XRD patterns of the as-synthesized TiO2/Bi2WO6 heterojunctions and TiO2 nanobelts.
phases. In the XRD pattern of TiO2/Bi2WO6 heterojunctions, the characteristic diffraction peaks of anatase TiO2 phase i.e. (1 0 1), (0 0 4), (2 0 0) and (1 0 5) are still visible, while the other diffraction peaks can be indexed to orthorhombic Bi2WO6 with layered Aurivillius structure (JCPDS card No. 73-1126). The surface chemical composition and elements valence state of TiO2/Bi2WO6 heterojunctions were derived from the XPS spectra. The
Fig. 3. The XPS narrow-scan spectra of Ti 2p, W 4f and Bi 4f for the as-synthesized TiO2/Bi2WO6 heterojunctions. The Ti 2p spectrum of TiO2 nanobelts was also given for comparison purpose.
Fig. 4. The (a) TEM and (b) HRTEM images of TiO2 nanobelts; (c) HRTEM image of TiO2/[Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor; (d) TEM and (e) HRTEM images of TiO2/Bi2WO6 heterojunctions, and (f) HRTEM image of the typical interfacial region of TiO2/Bi2WO6 heterojunctions. 787
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Fig. 5. Front views of the atomic arrangements of (a) orthorhombic Bi2WO6 around [1¯ 2¯ 1] zone axis and (b) anatase TiO2 around [1¯ 0 1] zone axis; (c) Schematic diagrams illustrating the formation mechanism of Bi2WO6 (1 1 3) plane.
the edge of defects, resulting in high chemical potential that can promote the bonding with other molecules [31]. In other words, the atoms near defects are prone to adsorb small adsorbates so that defect sites are highly active for hetero-adsorbing and growth. The prepared TiO2/ [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) precursor then reacts with Na2WO4 to form TiO2/Bi2WO6 heterojunctions in the hydrothermal process. Figs. S1(c), 4(d) and S2(b) show the low, high magnification TEM and SEM images of TiO2/Bi2WO6 heterojunctions, respectively. It is found that wing-like Bi2WO6 nanosheets grow side by side with the TiO2 nanobelts, and the thickness of the Bi2WO6 nanosheets is less than 15 nm. Notably, there is a disorder layer with ∼6 nm thickness at the interface of TiO2/Bi2WO6 heterojunctions, and the angle between the (1¯ 1 1) planes of Bi2WO6 and the (1 0 1) planes of TiO2 is ∼27 degrees as shown in Fig. 4(e) and (f). In Fig. 4(e), the lattice fringe distances marked in yellow are 0.372, 0.314 and 0.351 nm, which are corresponding to the interplanar spacing of Bi2WO6 (1¯ 1 1), (1 1 3) and TiO2 (1 0 1), respectively. The progressive rotation of Bi2WO6 (1¯ 1 1) planes relative to TiO2 (1 0 1) improves the lattice matching between Bi2WO6 and TiO2, and facilitates the preferred heterogeneous growth of Bi2WO6 on TiO2 template. According to the interfacial structure of TiO2/Bi2WO6 heterojuncions shown in HRTEM images, schematic diagrams illustrating the formation mechanism of this heterojunctions were constructed. Fig. 5(a) and (b) show the front views of the atomic arrangements of orthorhombic Bi2WO6 around [1¯ 2¯ 1] zone axis and anatase TiO2 around [1¯ 0 1] zone axis, respectively. The lattice of Bi2WO6 is built up by alternating layers of [Bi2O2]2+ and [WO4]2−, in which the [WO4]2− layers refer to corner-sharing [WO6] octahedra [32]. Given the angled growth and lattice distortion, the lattice fringe distance of Bi2WO6 (1¯ 1 1) plane (∼0.372 nm), which is equal to the thickness of two [WO6] layers as shown in Fig. 5(a), is close to that of the TiO2 (1 0 1) plane (d(1 0 1) = 0.351 nm), resulting in the lattice-matched TiO2/ Bi2WO6 heterojuncions. Moreover, anatase TiO2 can be regarded as the spatial arrangement of [TiO6] octahedra that share corner to form the (1 0 1) planes [33]. These facts underpin that the growth of
heterointerface between Bi2WO6 and TiO2 i.e. the growth relationship is Bi2WO6 (1¯ 1 1) ‖ TiO2 (1 0 1). 3.2. Formation mechanisms of TiO2/BixMyOz heterojunctions. To understand the formation mechanisms of TiO2/Bi2WO6 heterojunctions, the chemical reactions involved in this process were presented as follows: (a) Hydrolysis, pH = 1.1, Bi(NO3)3 + H2O → [Bi6O4(OH)4](NO3)6
(1)
−
pH = 5.5, [Bi6O4(OH)4](NO3)6 + OH + TiO2 → TiO2/[Bi6O6(OH)3−x] (NO3)3+x (x = 0–2) + H2O + NO3− (2) (b) Hydrothermal reactions, TiO2/[Bi6O6(OH)3−x](NO3)3+x + OH− → TiO2/Bi2O2(OH) (NO3) + NO3− TiO2/Bi2O2(OH)(NO3) + WO4
2−
−
→ TiO2/Bi2WO6 + OH +
(3) NO3−
(4)
The hydrolysis-produced [Bi6O4(OH)4](NO3)6, which is prepared at room temperature with pH = 1.1, can be converted into white precipitate [Bi6O6(OH)3−x](NO3)3+x (x = 0–2) when tuning the pH value to 5.5 by adding NaOH into the suspension [29]. The precipitate [Bi6O6(OH)3−x](NO3)3+x adsorbs at the highly active defect sites of TiO2 nanobelts to form TiO2/[Bi6O6(OH)3−x](NO3)3+x precursor. During the subsequently hydrothermal process, the [Bi6O6(OH)3−x] (NO3)3+x adsorbed on the stable (1 0 1) surface of TiO2 nanobelts undergoes two-step reactions (3) and (4) [34]. In particular, WO42− replaces OH− and NO3− in TiO2/Bi2O2(OH)(NO3) to enable in situ heterogeneous nucleation of Bi2WO6 on the (1 0 1) surface of TiO2. Both Bi2O2(OH)(NO3) and Bi2WO6 have a similar sandwich-like layered 788
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Fig. 6. Schematic illustration of the generic route for synthesizing TiO2/BixMyOz (M = W, Mo) heterojunctions.
Fig. 7. (a) XRD pattern, (b) TEM and (c) HRTEM images of TiO2/Bi3.64Mo0.36O6.55 heterojunctions; and (d) XRD pattern, (e) TEM and (f) HRTEM images of TiO2/ Bi2MoO6 heterojunctions.
structure with weak interlayer bonding (van der Waals force) as illustrated in Fig. 5(c) [35]. Another weak hydrogen bond between [OH] and [NO3] also dwells in the [(OH)(NO3)]2− layers [36]. This weak hydrogen bond can be easily ruptured. As a result, OH− and NO3− could be substituted by [WO4]2− which provide stronger intralayer chemical bonding. To further clarify the role of surface defects in the nucleation and growth of heterointerfaces, TiO2 nanobelts were corroded by sulfuric acid (0.2 mol/l) at 100 °C for 12 h to intentionally increase the density of surface defects. These TiO2 nanobelts were used as growth template to tune the morphology of TiO2/Bi2WO6 heterojunctions. Fig. S3 exhibits the TEM and HRTEM images of the pruned-TiO2/Bi2WO6 heterojunctions. The Bi2WO6 nanosheets in pruned-TiO2/Bi2WO6 heterojunctions are evidently smaller than those shown in Fig. 4(d). It is also noted that the cambered edge of Bi2WO6 nanosheets are formed in pruned-TiO2/Bi2WO6 heterojunctions. In the classical thermodynamics, the higher nucleation rate in a given saturated solution will result in a smaller average crystal size [37]. In this sense, the increased density of surface defects facilitates massive hetero-nucleation at the expense of the suppression of the growth of initial nuclei. Therefore, the morphology of Bi2WO6 nanosheets can be tailored by controlling the density of the surface defects on TiO2 nanobelts. Based on the results above, a generic route of synthesizing TiO2/ BixMyOz heterojunctions is established and schematically shown in Fig. 6. To further confirm the viability of this synthesis route, MoO42− was employed as another model system to fabrication heterojunctions
on uncorroded TiO2 nanobelts, because of the similar structure between Bi2WO6 and Bi2MoO6. TiO2/Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions were successfully synthesized when the pH values of the hydrothermal suspension were adjusted to 8 and 4, respectively. The XRD patterns, TEM and SEM images of the as-synthesized TiO2/ Bi3.64Mo0.36O6.55 and TiO2/Bi2MoO6 heterojunctions are shown in Figs. 7 and S2. The TiO2/Bi3.64Mo0.36O6.55 consists of anatase TiO2 and cubic Bi3.64Mo0.36O6.55 (JCPDS No. 43-0446) as evidenced by the XRD pattern in Fig. 7(a). The TEM image (Fig. 7(b)) and SEM image (Fig. S2(c)) reveal Bi3.64Mo0.36O6.55 nanoparticles with an average size of ∼70 nm in diameter are embedded on the surface of TiO2 nanobelts. The lattice spacing of Bi3.64Mo0.36O6.55 nanoparticles was found to be 0.326 nm, which is indexed to (1 1 1) crystal plane. This well-matched lattice with TiO2 (1 0 1) plane (d(1 0 1) = 0.351 nm) enables the growth of hetero-interface Bi3.64Mo0.36O6.55 (1 1 1)‖TiO2 (1 0 1) as shown in Fig. 7(c). The XPS narrow-scan spectra of Ti, Mo and Bi for TiO2/ Bi3.64Mo0.36O6.55 heterojunctions are shown in Fig. S4. Comparing with TiO2 nanobelts, the Ti 2P spectrum of TiO2/Bi3.64Mo0.36O6.55 heterojunctions displays a slight shift to higher binding energy, similar to that of TiO2/Bi2WO6 heterojunctions. It is evident that TiO2/ Bi3.64Mo0.36O6.55 heterojunctions can also be synthesized by the generic route in Fig. 6. Fig. 7(d) shows the XRD pattern of TiO2/Bi2MoO6 heterojunctions containing anatase TiO2 and orthorhombic Bi2MoO6 with Aurivillius layered structure (JCPDS No. 07-0401) [38]. The SEM image of TiO2/ Bi2MoO6 heterojunctions (Fig. S2(d)) exhibits that Bi2MoO6 789
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Fig. 8. (a) Efficiencies of photo-degradation of Rh B using various photocatalysts as a function of time under visible light irradiation conditions; (b) the corresponding first-order kinetic linear fitting curves for photo-degradation of Rh B, and (c) photocatalytic efficiencies of degrading Rh B using TiO2/Bi2WO6 heterojunctions in five consecutive tests.
Fig. 9. Transient photocurrent densities under visible light irradiation of (a) Bi2WO6 nanosheets, and TiO2/Bi2WO6 heterojunctions; (b) Bi3.64Mo0.36O6.55 nanoparticles and TiO2/Bi3.64Mo0.36O6.55 heterojunctions; (c) Bi2MoO6 nanoparticles and TiO2/Bi2MoO6 heterojunctions. The time-dependent photocurrent density of TiO2 nanobelts was also included for comparison purpose.
nanoparticles hetero-grow on the surface of TiO2 nanobelts. Fig. 7(f) confirmed the angled growth of (1¯ 1 1¯) planes of Bi2MoO6 on the (1 0 1) planes of TiO2 [39], which is similar to the TiO2/Bi2WO6 heterojunctions. These referred results demonstrate that the TiO2/BixMyOz heterojunctions can be obtained and tailored by a generic two-step hydrothermal synthesis route consisting of an inducing adsorption and an in situ anion-exchange.
heterojuntions are greater than those of bare TiO2 nanobelts (32.3 m2/ g), Bi2WO6 nanosheets (26.3 m2/g), Bi3.64Mo0.36O6.55 (17.9 m2/g) and Bi2MoO6 (14.2 m2/g) nanopartiles (Fig. S5). As shown in Fig. S6, the absorption edges of TiO2/Bi2WO6, TiO2/Bi3.64Mo0.36O6.55 and TiO2/ Bi2MoO6 heterojuntions exhibit an evidently red shift compared with TiO2 nanobelts, indicating that the prepared TiO2/BixMyOz (M = Mo, W) heterojunctions can be responsive to visible light. Fig. 8 shows the Rh B degradation efficiencies using different heterojunctions photocatalysts under visible light irradiation, and their corresponding first-order kinetic linear fitting curves. After visible light irradiating for 60 min, the removal rate of Rh B with TiO2/Bi2WO6
3.3. Photocatalytic activities Additionally, the specific surface areas of TiO2/Bi2WO6 (34.1 m2/g), TiO2/Bi3.64Mo0.36O6.55 (32.4 m2/g) and TiO2/Bi2MoO6 (32.6 m2/g) 790
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Fig. 10. Energy band diagrams and photocatalytic mechanism of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions under visible light irradiation.
heterojunctions is nearly 98%, which is much higher than those using bare Bi2WO6 (40.1%) and TiO2 (5.6%) as photocatalysts. Incidentally, the reported carbon fiber loaded TiO2/Bi2WO6 nanostructured heterojunctions with higher cost could almost completely photo-degrade Rh B after 80 min under visible light irradiation [40]. Moreover, the as-prepared TiO2/Bi2WO6 heterojunctions can retain a high photo-degradation efficiency, i.e. ∼97% photo-degradation rate of Rh B within 60 min time frame, even after five consecutive photocatalytic tests. This stability is much better than that of the TiO2/Bi2WO6 hollow microsphere [20]. The photocatalytic reaction rate constant of TiO2/Bi2WO6 heterojunctions is four times higher than that of bare Bi2WO6. Similarly, the photocatalytic reaction rate constant of TiO2/ Bi3.64Mo0.36O6.55 heterojunctions renders a 7-fold increase compared with the case of bare Bi3.64Mo0.36O6.55. However, the photocatalytic performance of TiO2/Bi2MoO6 heterojunctions is only slightly better than that of single phase Bi2MoO6. The photocurrent responses of different photocatalysts under visible light irradiation, which were measured by cyclically switching on and off of a 300 W Xe lamp with a time interval of 50 s, are shown in Fig. 9. It is observed that, except for the Bi2MoO6 nanopaticles, the photocurrent densities of all the photocatalysts dropped to ∼zero almost instantaneously when the light source is turned off. The as-prepared Bi2MoO6 nanoparticles exhibits significant relaxation behavior in photocurrent densities under the same illumination conditions, owing to the long relaxation time for recombination of photo-generated electronhole pairs relative to switching speed as well as the slow recombination process in deep trap sites [41]. The slow and incomplete discharge of trapped photo-generated carriers can prolong the lifetime of photogenerated carriers. The effective separation of photo-generated electron-hole pairs on the interface of TiO2/Bi2MoO6 heterojuntions led to an increase in photocurrent stability as shown in Fig. 9(c). However, when light was off, the recombination taking place between the holes transferred from Bi2MoO6 to TiO2 and the electrons left in Bi2MoO6 made the photocurrent rapidly quenched [42]. This also explains the insignificant enhancement of photocatalytic activity in TiO2/Bi2MoO6 heterojunctions [43]. Both TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions display much higher photocurrent densities than the single phase TiO2, Bi2WO6 and Bi3.64Mo0.36O6.55 as shown in Fig. 9(a) and (b). The results are attributed to the fact that photo-generated holes can be rapidly transferred from Bi2WO6 or Bi3.64Mo0.36O6.55 to TiO2 and suppress the recombination of photo-generated electron-hole pairs, i.e. the blocked electron transfer formed at the interfaces improves the effective separation of photo-generated electron-hole pairs in TiO2/
BixMyOz heterojunctions [19]. Therefore, the free charge carrier concentrations in TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions are greater than those in single phase TiO2, Bi2WO6 and Bi3.64Mo0.36O6.55, which echoes their better photocatalytic activities. Fig. 10 illustrates the band structures of TiO2/Bi2WO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions, which are used to understand their photocatalytic mechanisms under visible light irradiation. The conduction band bottom (ECB) and the valance band top potentials (EVB) were estimated by the empirical equations [44]:
ECB = X
EC
0.5Eg
EVB = ECB + Eg
(1) (2)
where X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms evaluated via electron affinity and first ionization potential [45]. EC is ∼4.5 eV. The Eg of TiO2 nanobelts, Bi2WO6 nanosheets and Bi3.64Mo0.36O6.55 nanoparticles were obtained from the fitting of UV–Vis diffuse reflectance spectra (Fig. S6). Under visible light irradiation, the electrons residing in the energy levels closer to the top of valence band of Bi2WO6 and Bi3.64Mo0.36O6.55 can be readily excited to their conduction band due to narrow bandgap with the reciprocal generation of holes in their valance band. Because the EVB of TiO2 (2.98 eV) is more negative than those of Bi2WO6 and Bi3.64Mo0.36O6.55, the photo-generated holes can migrate to the valance band of TiO2. At the same time, the photo-generated electrons still remain in the conduction band of Bi2WO6 and Bi3.64Mo0.36O6.55, because of their more positive ECB compared with TiO2. When photo-generated holes migrated into EVB of TiO2, the recombination of photo-generated electronhole pairs can be suppressed in TiO2/Bi2WO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions [8]. In such a way, photo-generated electrons and holes are effectively separated in TiO2/BixMyOz heterojunctions [12,13], and the lifetime of charge carriers are prolonged [14]. The free charge carriers will subsequently react with redox mediators, i.e. O2, H2O and OH−, to yield active species, or else directly mineralize organic dye [46]. Consequently, the photocatalytic activities of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions can be notably improved under visible light. 4. Conclusions A generic two-step hydrothermal synthesis method, consisting of an inducing adsorption and an in situ anion exchange, is reported to 791
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selectively prepare TiO2/Bi2WO6, TiO2/Bi2MoO6 and TiO2/ Bi3.64Mo0.36O6.55 heterojunctions. The defects on the surface of TiO2 play a key role in promoting the adsorption and growth of secondary phase. The morphology of TiO2/BixMyOz heterojunctions can be tailored by controlling the density of TiO2 surface defects. The photocatalytic activities of TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions are considerably improved under visible light irradiation. This is primarily attributed to the band structure matching of TiO2/ BixMyOz heterojunctions, which can effectively promote the separation of photo-generated electron-hole pairs, and prolong the lifetime of charge carriers.
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Acknowledgment G. Fang and J. Liu are contributed equally to this work and should be considered co-first authors. Financial supports from the Natural Science Foundation of NingXia Province (No. NZ17108), National Natural Science Foundation of China (No. 51262001), and China Scholarship Council are acknowledged. This work was also partially supported by the Australian Research Council Discovery Project (Grant No. DP170103514). We also thank Prof. Guozhong Cao, University of Washington, for his valuable suggestions and help. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.12.297. References [1] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, J.K. Nørskov, Materials for solar fuels and chemicals, Nat. Mater. 16 (2017) 70–81. [2] M. Reli, P. Huo, M. Šihor, N. Ambrožová, I. Troppová, L. Matějová, J. Lang, L. Svoboda, P. Kuśtrowski, M. Ritz, P. Praus, K. Kočí, Novel TiO2/C3N4 photocatalysts for photocatalytic reduction of CO2 and for photocatalytic decomposition of N2O, J. Phys. Chem. A 120 (2016) 8564–8573. [3] S. Liu, Q. Hu, J. Qiu, F. Wang, W. Lin, F. Zhu, C. Wei, N. Zhou, G. Ouyang, Enhanced photocatalytic degradation of environmental pollutants under visible irradiation by a composite coating, Environ. Sci. Technol. 51 (2017) 5137–5145. [4] T.P. Yoon, M.A. Ischay, J. Du, Visible light photocatalysis as a greener approach to photochemical synthesis, Nat. Chem. 2 (2010) 527–532. [5] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. [6] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986. [7] J.J.M. Vequizo, H. Matsunaga, T. Ishiku, S. Kamimura, T. Ohno, A. Yamakata, Trapping-induced enhancement of photocatalytic activity on brookite TiO2 powders: comparison with anatase and rutile TiO2 powders, ACS Catal. 7 (2017) 2644–2651. [8] J. Low, J. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [9] M. Zhu, Q. Liu, W. Chen, Y. Yin, L. Ge, H. Li, K. Wang, Boosting the visible-light photoactivity of BiOCl/BiVO4/N-GQD ternary heterojunctions based on internal Zscheme charge transfer of N-GQDs: simultaneous band gap narrowing and carrier lifetime prolonging, ACS Appl. Mater. Interfaces 9 (2017) 38832–38841. [10] J. Jian, Y. Sang, G. Yu, H. Jiang, X. Mu, H. Liu, A Bi2WO6-based hybrid photocatalyst with broad spectrum photocatalytic properties under UV, visible, and nearinfrared irradiation, Adv. Mater. 25 (2013) 5075–5080. [11] Y. Jia, S. Zhan, S. Ma, Q. Zhou, Fabrication of TiO2-Bi2WO6 binanosheet for enhanced solar photocatalytic disinfection of E. coli: insights on the mechanism, ACS Appl. Mater. Interfaces 8 (2016) 6841–6851. [12] G. Tian, Y. Chen, R. Zhai, J. Zhou, W. Zhou, R. Wang, K. Pan, C. Tian, H. Fu, Hierarchical flake-like Bi2MoO6/TiO2 bilayer films for visible-light-induced selfcleaning applications, J. Mater. Chem. 1 (2013) 6961–6968. [13] J. Xu, W. Wang, S. Sun, L. Wang, Enhancing visible-light-induced photocatalytic activity by coupling with wide-band-gap semiconductor: a case study on Bi2WO6/ TiO2, Appl. Catal. B: Environ. 111 (2012) 126–132. [14] M. Xie, X. Fu, L. Jing, P. Luan, Y. Feng, H. Fu, Long-lived, visible-light-excited charge carriers of TiO2/BiVO4 nanocomposites and their unexpected photoactivity for water splitting, Adv. Energy Mater. 4 (2014) 1300995. [15] J. Li, X. Liu, Z. Sun, L. Pan, Novel Bi2MoO6/TiO2 heterostructure microspheres for degradation of benzene series compound under visible light irradiation, J. Colloid Interface Sci. 463 (2016) 145–153.
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