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Materials Research Bulletin 98 (2018) 103–110
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Facile synthesis and photocatalytic activity of La-doped BiOCl hierarchical, flower-like nano-/micro-structures ⁎
Keke Xua,b, Xiuli Fua, , Zhijian Pengb,
MARK
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a State Key Laboratory of Information Photonics and Optical Communications, and School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China b School of Science, China University of Geosciences, Beijing 100083, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: BiOCl La doping Hierarchical structure Ginkgo-leaf-like structure Photocatalysis
A series of La-doped BiOCl hierarchical, flower-like nano-/micro-structures (NMFs) with ginkgo-leaf-like “petals” were prepared by etching La-doped Bi2O3 films in HCl solution at room temperature. For the La-doped Bi2O3 films, they were fabricated through hot-dipping Bi2O3 thin films in La2O3 powder at 650 °C. Microstructure examination revealed that the ginkgo-leaf-like La-doped BiOCl petals have a multilayered structure consisting of several ultra-thin nanosheets with different radii due to the external-to-internal etching process. The investigation on their formation mechanism revealed that the La-doped BiOCl hierarchical NMFs grew anisotropically through Ostwald ripening mechanism. Besides, the photodegradation of rhodamine B under simulated sunlight irradiation revealed that the La-doped BiOCl hierarchical NMFs possessed higher performance than pure BiOCl sample. And the active species trapping tests indicated that the photogenerated holes were the main active radicals for the degradation of rhodamine B.
1. Introduction As a member of V-VI-VIIA groups compound semiconductors, bismuth oxychloride (BiOCl) protrudes outstandingly among its congeners in optical, catalytic, luminescent, electrical and gas sensitive properties. Consequently, it has various promising applications in industries, like paints in cosmetic industry [1], photocatalysts for degrading many organic dyes such as rhodamine B (RhB), ciprofloxacin and eosin Y under visible-light irradiation [2,3], high-performance electrocatalysts for air electrode of Al-air batteries [4], and gas sensors to CO, CO2 and O2 [5,6]. To date, many approaches have been proposed to synthesize various forms of BiOCl materials, including electrospinning, solvothermal synthesis, hydrolysis, hydrothermal synthesis, sonochemical route, refluxing method, solution oxidation process, and electrochemical route [7–14]. And a variety of BiOCl materials with different nano-/microstructures, like one dimensional nano-rods/wires, two dimensional nano-plates/sheets and three dimensional hierarchical architectures, have been fabricated. However, most of their aggregates are powders or particles, which have some inevitable problems [15,16]. (i) They are not easily applied to continuous flow system. (ii) The suspended catalyst powders or particles tend to aggregate, especially when they are of high concentration, weakening their photocatalytic activity. And (iii)
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Corresponding authors. E-mail addresses: [email protected] (X. Fu), [email protected] (Z. Peng).
http://dx.doi.org/10.1016/j.materresbull.2017.10.013 Received 4 August 2017; Received in revised form 7 October 2017; Accepted 9 October 2017 Available online 10 October 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
the separation of the photocatalysts with organic pollutants is very difficult and expensive; so they cannot be recycled and reused, preventing some special applications such as battery electrode or photocatalysis reaction. Therefore, many scientists have still striven to synthesize various BiOCl materials with specific nano-/micro-structures. On the other hand, although BiOCl has attracted considerable attention for potential photocatalytic application, its large band-gap (about 3.46 eV [17]) makes it useful only under ultraviolet irradiation, hindering its applications under visible light, the main part (about 96%) of the natural solar light. In order to improve the absorption and utilization efficiency of BiOCl for visible light, the modification of BiOCl has been a hot topic in this field. And it has been reported that the visible light driven photocatalytic activity of BiOCl can be improved to some extent by changing the crystal structure, band gap or surface microstructure. For example, Xie et al. [18] synthesized Sn-doped BiOCl photocatalysts at room temperature via an oxidation-reduction method, indicating that Sn(10%)-doped BiOCl sample exhibited higher photocatalytic degradation on benzoic acid and RhB than the other Sn-doped BiOCl and pure BiOCl ones due to its narrowed band gap (2.91 eV). Ding et al. [19] synthesized Er3+ doped BiOCl hierarchical microspheres by a solvothermal method. The presence of Er3+ did not affect the microstructure and morphology of BiOCl, but the Er3+ doped BiOCl microspheres exhibited an enhanced photocatalytic activity for the
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λ=1.5418 Å) in a 2θ range of 10° to 90° under a continuous scanning mode with a scanning rate of 6°/min at an incidence angle of 1° of the X-ray. The morphology of the samples was observed by field emission scanning electron microscope (SEM, S4800). The samples were also examined by transmission electron microscopy (TEM, Tecnai G2 F20 UTWIN, America) and high-resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectroscopy (XPS, non-monochromated Mg Kα radiation, photon energy 1253.6 eV) was performed to characterize the chemical state and composition of the samples, during which the spectrometer was calibrated by the binding energy of C1s line (285 eV). The UV–vis absorption spectra were recorded on a Cary 5000 UV–vis spectrometer (Varian) equipped with a DRA-CA-30I integrating sphere for solid-phase characterization.
degradation of RhB under visible light (λ > 420 nm) irradiation. The enhanced activity was attributed to the up-conversion resulted in by the dopant that could transform visible into ultraviolet light. Besides, La doping is also an effective way to improve the catalytic activity of some oxide semiconductors photocatalysts. Not only can La present as stable La3+ ion because there existed no variable valence state for it, but also improve the quantum yield of the doped oxides by prolonging the life of the photo-generated carries because of the shallow potential trap formed by its full electronic configuration [20]. For instance, Yang et al. [21] prepared La doped Bi2O3 by an impregnation method, indicating that an appropriate amount of La doping can effectively prevent the transformation of Bi2O3 from tetragonal to monoclinic phase. Their UV–vis absorption spectra revealed an extension of light absorption into the visible region (higher than 550 nm). The doped La would partially substitute bismuth in the lattice of Bi2O3, existing in a form of Bi-O-La chemical bond, and a new complex metal oxide compound (La0.176Bi0.824O1.5) appeared in the catalysts, which could inhibit the recombination between photoelectrons and holes, leading to enhanced photocatalytic quantum efficiency. Sun et al. [20] synthesized La-doped TiO2 film by a sol-gel method. La3+ dispersed into TiO2 in the form of mainly La2O3 and partly Ti-O-La bond. And La doping can inhibit the phase transformation of TiO2 by enhancing its phase transformation temperature, and is also beneficial for the formation of TiO2 in smaller particle size and thus with large surface area. However, the application of La doping into BiOCl to improve its photocatalytic properties has not been reported. Therefore, in this work, we present a simple approach to prepare Ladoped BiOCl films, which are composed of hierarchical, flower-like nano-/micro-structures (NMFs) with ginkgo-leaf-like “petals”, by etching La-doped Bi2O3 films in HCl acid at room temperature. For the applied La-doped Bi2O3 films, they were fabricated through hot-dipping Bi2O3 thin films in La2O3 powder at 650 °C. Special emphasis was paid on the effects of the hot-dipping time in La2O3 and etching time in HCl acid on the composition, structure and photocatalytic properties of the as-prepared La-doped BiOCl films. The formation mechanism was proposed. And the mechanism for their improved photocatalytic performances was also suggested.
2.3. Photocatalytic activity The photodegradation of model dye RhB under simulated sunlight irradiation (SSI) was carried out to evaluate the photocatalytic activity of the obtained La-doped BiOCl hierarchical NMFs. The samples were set on the bottom of a glass tube with the substrate surface covered with the La-doped BiOCl hierarchical NMFs toward the light (provide by an 8 W halogen lamp, which emits light with a wavelength approximately in the range of 400–790 nm). Then the glass tube was filled with 5 mmol·L−1 RhB solution, keeping the ratio of the mass of the La-doped BiOCl catalyst (mg) to the volume of the RhB solution (mL) equal to 1:1. After the light irradiation for each 10 min, 3 mL of the reactive solution was withdrawn from the reaction system for the measurement of the remnant RhB content in it by an UV–vis spectroscope (SP-752). After the measurement, the reactive solution was immediately poured back into the glass tube to keep the reaction in almost the same state. For comparison, the photocatalytic activity of the pure BiOCl sample was also examined under the same conditions. 2.4. Active species trapping experiments In order to examine the active species generated in the photocatalytic process, 1 mM benzoquinone (BQ, a quencher of superoxide radicals %O2− [22]), 1 mM triethanolamine (TEOA, a scavenger of holes h+ [22]), and 1 mM isopropanol (IPA, a capturer of hydroxyl radicals % OH [23]) were added into the reaction system, respectively, together with the prepared catalyst before light irradiation. And all the other experimental parameters during the tests were kept as those of the above RhB photodegradation experiments.
2. Experimental section 2.1. Samples preparation P-type conducting silicon wafers purchased from GRINM Advanced Materials Co. Ltd (China) were used as the substrates. Before use, the substrates were ultrasonically cleaned in absolute alcohol and de-ionized water for 20 min, respectively, and then completely dried in an open oven. All the applied chemicals were of analytical grade and used as received without further purification. Among them, HCl acid was bought from Beijing Chemical Works (China), and the others were from Xilong Chemical Co. Ltd (China). In a typical process, Bi2O3 films were first prepared onto silicon substrates by covering the substrates with Bi2O3 powder and then heating at 800 °C for 1 h in a muffle furnace (KSL-1100, Hefei Kejing Materials Technology Co. Ltd, China). After heating, extra Bi2O3 powder became a chunk, which could be peeled off from the substrate, and thus thin Bi2O3 film was left on it. Then, the Bi2O3 film was covered with La2O3 powder and heated at 650 °C for 5, 10, 20, 30 and 40 min, respectively. After that, the samples were further etched in HCl aqueous solution (HCl:H2O = 1:100) for 120 s. Finally, the etched samples were rinsed by de-ionized water and dried by N2 flow. For comparison, a non-doped, pure BiOCl film was also prepared by the same process.
3. Results and discussion 3.1. Composition and microstructure Typical XRD patterns of the La-doped BiOCl films produced from Bi2O3 films via hot-dipping in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s are shown in Fig. 1a. For comparison, that of the sample without La2O3 hot-dipping is also presented. As is seen from this figure, all the diffraction peaks of the samples, whether they were hot-dipped in La2O3 powder or not, can be assigned to those of the tetragonal phase of BiOCl with a space group of P4/nmm (JCPDS card no. 85-0861). Because there are no other diffraction peaks detected, it can be concluded that the applied Bi2O3 films were converted into BiOCl during the present etching processes. But due to the detection limit of XRD, no new phase in correlation with the doped La3+ ions could be identified. And the strong and sharp diffraction peaks indicated that the samples prepared by this approach are well crystallized. However, when Bi2O3 was hot-dipped in La2O3 powder for increasing time up to 20 min, a peak shift to large angle of 2θ could be easily observed. From the XRD patterns as shown in Fig. 1a, the lattice constants of a and c of the BiOCl phase were calculated, and the results are displayed in Fig. 1b as a function of the La2O3 hot-
2.2. Materials characterization The phase composition of the samples were identified by grazing incidence X-ray diffraction (GI-XRD, D/max-RB, Cu Kα radiation, 104
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Fig. 1. Typical results of XRD analyses: (a) XRD patterns of the La-doped BiOCl films prepared by hot-dipping Bi2O3 films in La2O3 powder at 650 °C for different times followed with etching in HCl acid for 120 s; and (b) evolution of the lattice parameters a and c of the samples. For comparison, the results of the samples without La2O3 hot-dipping are also presented.
peak at 530.42 eV should be attributed to the lattice oxygen, and the one at 532.35 eV can be ascribed to the absorbed oxygen species, such as surface physically adsorbed oxygen species (CO2, etc.), surface bridging oxygen, hydroxyl and so on [21]. The recorded Cl 2p spectrum presents two peaks at 198.38 eV (Cl 2p3/2) and 199.83 eV (Cl 2p1/2) as shown in Fig. 3d. The high-resolution XPS spectrum of La 3d shown in Fig. 3e could be fitted into a peak corresponding to the binding energy of 834.06 eV, which is close to the reported value for La 3d5/2 [24,25]. It should be noted that, the binding energies of Bi 4f, lattice oxygen, and Cl 2p3/2 in the La-doped BiOCl films have a slight increase than the ones of pure BiOCl (see its microstructure in Fig. S1 in Supplementary materials). The binding energy increase could be ascribed to the decrease of the electron cloud density around the Bi, O and Cl atoms after La doping [18,21]. Meanwhile, it could be also found that the peak intensity of the adsorbed oxygen species decreased obviously after La-doping. The above results indicate that the Bi-O bonds and the surface state of BiOCl have been influenced by La doping.
dipping time. It is seen from Fig. 1b that the lattice constants of a and c increased with the prolonged La2O3 hot-dipping time, because the position of Bi3+ ions (ionic radius: 0.103 nm) in BiOCl lattice were partially occupied by larger La3+ ions (ionic radius: 0.106 nm), which is correspondent with the Vegard's law. The microstructure of the obtained La-doped BiOCl films was examined by SEM and TEM. Typical results on the samples prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl solution for 120 s are shown in Fig. 2. The SEM images as shown in Fig. 2a and 2b revealed that the films consisted of a large quantity of hierarchical, flower-like structures with average diameter of 4 ± 0.5 μm. Such structures were composed of densely packed ginkgo-leaf-like plates as the “petals” of a flower, with a thickness of 30–50 nm, which were assembled together. All the plates in a single flowery structure seem to grow from a centre. TEM examination was further carried out to investigate the detailed crystal structure of the La-doped BiOCl film, and typical images are presented in Fig. 2c–f. As is also seen from Fig. 2c, the individual La-doped BiOCl nanopetal looks like a ginkgo-leaf as shown in the inset of Fig. 2b, which is a novel morphology never reported in literature. Interestingly, the high magnification image as shown in Fig. 2d reveals that each petal has a multilayered structure consisting of several ultra-thin nanosheets with different radii. The contrast variation along the radius direction as shown in Fig. 2c indicates that the ginkgo-leaf-like nanopetal is not homogeneous in thickness, due to the different radius of each ultra-thin nanosheet. Generally, the root of the ginkgo-leaf-like nanopetal is thicker than the brim. Moreover, the HRTEM images of a single nanosheet as shown in Fig. 2e–f present clearly lattice fringes with a spacing of 0.275 nm, which is completely corresponding with the dspacing of the [110] reflection of BiOCl phase. XPS measurements were employed to confirm the chemical composition and surface chemical state of the La-doped BiOCl films, as depicted in Fig. 3. Typical XPS survey spectrum is displayed in Fig. 3a, which undoubtedly indicates the coexistence of Bi, Cl, O and La in the present sample. But, the peak of La is much weaker than that of the others, which might be assigned to the small content of La in the sample. The as-observed carbon may come from the carbon contamination in the equipment for the measurement. Moreover, the corresponding high resolution XPS spectra of Bi 4f, O 1s, Cl 2p and La 3d of the La-doped BiOCl sample are shown in Fig. 3b-e, respectively. For comparison, the XPS spectra of Bi 4f, O 1 s and Cl 2p of the non-doped, pure BiOCl are also presented in Fig. 3b–d. From Fig. 3b, two strong peaks centered at about 159.69 and 165.00 eV corresponding to Bi 4f7/ 2 and Bi 4f5/2 could be observed, indicating that Bi element in the Ladoped BiOCl film mainly existed as Bi3+ [18]. The recorded asymmetric O 1 s XPS spectrum (Fig. 3c) of the La-doped BiOCl films could be fitted into two peaks centered at 530.42 and 532.35 eV, respectively. The
3.2. Formation mechanism In general, flower-like hierarchical structures could be usually obtained via a hierarchical assembly process or the localized Ostwald ripening process. In the assembly process, tiny nanoplates should be formed at the early stage, followed by the oriented attachment of these building blocks into 2D nanosheets. Then, the 2D nanosheets were arranged into 3D hierarchical structures [26–28]. But in the Ostwald ripening process, tiny crystalline nuclei were generated first in a supersaturated solution and grown into nanoparticles, and these newly formed nanoparticles were spontaneously aggregated to minimize their surface energy. Then these crystallized nanoparticles grew anisotropically along the 2D direction, resulting in the formation of the flakes. Finally, flower-like superstructures were formed [29–31]. In present work, all plates in a single flowery structure were obviously grown from a centre as shown in Fig. 2b and 2c. Thus, the formation of the flower-like La-doped BiOCl hierarchical structures was proposed to follow the Ostwald ripening mechanism. The time-dependent evolution experiments were further conducted to reveal the formation process of the present La-doped BiOCl hierarchical NMFs. The products collected from the reactions with different etching times in HCl solution were observed by SEM (see Fig. 4). Fig. 4a is a typical SEM image of Bi2O3 after hot-dipping in La2O3 powder at 650 °C for 20 min, showing the morphology feature of the resultant thin film. XPS results indicate that this thin film consists of only Bi, O and La elements (see Fig. S2). After etching in HCl solution for 10 s, some ginkgo-leaf-like BiOCl nanoplates had appeared, but there were still numerous BiOCl nanoparticles (see Fig. 4b). With the etching time increased from 10 to 120 s, the nanoparticles gradually disappeared; and 105
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Fig. 2. Microstructure of typical La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl solution for 120 s: (a) low magnification SEM image; (b) high magnification SEM image, where the inset shows the image of a real ginkgo-leaf; (c, d) TEM images of a nanopetal, revealing that it consists of several ultra-thin nanosheets; (e) HRTEM image of a nanosheet; and (f) HRTEM image on the rectangle area as shown in (e).
that, when pure Bi2O3 films were used as the source for the synthesis of BiOCl films by direct etching in HCl solution, the morphology of the obtained BiOCl product had no difference compared with that of the above-mentioned La-doped BiOCl films (see Fig. S1), indicating that the doping of La did not affect the formation mechanism of BiOCl films.
more and more BiOCl nanoflakes appeared (Fig. 4b–e). When the time extended to 180 s, these nanoflakes were changed into long-arc and short-radius ones with irregular edges (see Fig. 4f and the arrows of the inset in it), which might be owing to the etching reaction between BiOCl and excessive HCl in solution. Based on these results, it can be confirmed that the formation of the present flower-like La-doped BiOCl hierarchical structures is an anisotropic growth following the Ostwald ripening mechanism. Specifically, the La-doped Bi2O3 films obtained by hot-dipping Bi2O3 films in La2O3 powder were used as the precursors to prepare the NMFs in this study. After the nucleus of La-doped BiOCl was formed, the reaction between the La-doped Bi2O3 and HCl (Bi2O3 + 2HCl → 2BiOCl + H2O) from external to internal mode would grow into La-doped BiOCl nanoparticles. These newly formed nanoparticles would further serve as seeds for the anisotropic growth of the NMFs in subsequent process via Ostwald ripening mechanism. Because BiOCl has a high intrinsically anisotropic nature [32], these nanoparticles prefer to grow into two-dimensionally plate-like structure of single-crystal nature with the etching time prolonging. Finally, flowerlike La-doped superstructures were obtained. Meanwhile, at the end of the transformation from La-doped Bi2O3 film to flower-like La-doped BiOCl hierarchical structures, the HCl in the solution would etch the edge of the La-doped BiOCl nanopetals, resulting in the relatively thin edge of the final products as shown in Fig. 2c, and even rougher edge of the products as shown in Fig. 4f and the inset in it. It should be noted
3.3. Optical properties The recorded UV–vis diffuse reflectance spectra of the La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s are shown in Fig. 5, and their corresponding plots of (αhν)1/2 versus hν are illustrated in the inset of this figure, where the band gap of the obtained La-doped BiOCl hierarchical NMFs was calculated according to the Tauc’s approach [33]. It was observed that the absorption edge of the pure BiOCl films was located at about 375 nm in the near-UV region, and the estimated band gap is about 3.312 eV. This is the intrinsic band gap absorption of BiOCl. Interestingly, there is an obvious shift of the absorption peak toward the longer wavelength up to 400 nm for the Ladoped BiOCl films prepared by La2O3 hot-dipping for 5, 10, 20, 30 and 40 min, respectively. Their corresponding band-gap values could be estimated from the intercept of the tangent to the plot as shown in the inset of Fig. 5, which are 3.275, 3.260, 3.252, 3.245 and 3.237 eV, respectively. This red-shift indicates that La doping introduces an 106
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Fig. 3. XPS data on the La-doped BiOCl hierarchical NMFs prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s: full spectrum (a), and narrow spectra of (b) Bi4f, (c) O1s, (d) Cl 2p and (e) La 3d. For comparison, the XPS spectra of Bi 4f, O 1 s and Cl 2p of the non-doped BiOCl are also presented.
ECB = EVB−Eg
impurity energy level into the band gap of BiOCl, which will be beneficial for the improvement of photocatalytic activity of BiOCl under SSI, because metal ion doping could form discrete empty energy levels below the conduction band of photocatalysts, decreasing the excitation energy for the transition of photo-generated carriers [34]. The effective band structure of the BiOCl and La-doped BiOCl samples could be deduced semi-empirically. And the band edges of BiOCl and La-doped BiOCl can be determined by Mulliken electronegativity theory [35]: EVB = χ − Ee + 0.5 × Eg
(2)
where EVB is the potential of valence band (VB) and ECB is the potential of conduction band (CB); χ is the electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale, which is about 4.5 eV [36–38]; and Eg is the band gap potentials. In this study, the band gap energy of the obtained BiOCl is 3.312 eV and the band gap energy of the La-doped BiOCl prepared by La2O3 hot-dipping for 5, 10, 20, 30, 40 min are 3.275, 3.260, 3.252, 3.245 and 3.237 eV, respectively. Hence, the calculated EVB and ECB of BiOCl are 3.516 and 0.204 eV. And the corresponding EVB and ECB for
(1)
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Fig. 4. SEM images of La-doped BiOCl nanoplates films prepared by heating Bi2O3 powder at 750 °C, hot-dipping the Bi2O3 film in La2O3 powder at 650 °C for 20 min, and then etching the La-doped Bi2O3 film in HCl solution for different times: (a) 0; (b) 10; (c) 30; (d) 60; (e) 120; and (f) 180 s. In (f), the inset shows an enlarged image of the rougher edge of the obtained La-doped BiOCl nanoplates.
the La-doped BiOCl prepared by La2O3 hot-dipping for 5, 10, 20, 30, 40 min are 3.498 and 0.223, 3.490 and 0.230, 3.486 and 0.234, 3.483 and 0.238, 3.479 and 0.242 eV, respectively. Obviously, after La doping, due to the discrete empty energy, the band gap of the La-doped BiOCl was decreased. Resultantly, the photoresponse would shift to longer length of light, thus enhancing the photo-electrical response, which finally increase the number of photogenreated carriers. 3.4. Photocatalytic activity The decolorization of RhB solution over the obtained La-doped BiOCl films in the dark and under SSI is presented in Fig. 6a. It is seen that, compared with the adsorption and photodegradation on RhB over the present catalysts, the self-photodegradation of RhB was almost negligible. Moreover, the difference in the adsorption on RhB over all the La-doped BiOCl NMFs catalysts could be also negligible. However, they would present significantly different photodegradation effects on RhB under SSI, when the catalysts were prepared under different conditions (see Fig. 6a). The photodegraded RhB could be significantly enhanced over the catalysts prepared with appropriate time for the La hot-dipping. After SSI for 240 min, the degradation of RhB over the Ladoped BiOCl catalyst prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min exhibited the highest photocatalytic activity, resulting in 70% of RhB decolorized, while only about 48% of
Fig. 5. UV–vis absorption spectra of the La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s; and the inset is their corresponding plots of (αhν)1/2 versus hν.
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Fig. 6. (a) Decolorization of RhB solution in the dark and under SSI over the obtained La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s. And (b) their corresponding plots of ln(C/C0) versus irradiation time.
catalysts was finally enhanced. In addition, the discrete empty energy levels could also serve as the trapping center of the photogenerated electrons, prolonging the lifetime of the photogenerated holes, finally improving the photocatalytic performance of BiOCl [36–38]. To further investigate the photochemical stability of the prepared La-doped BiOCl photocatalysts, repeating photocatalytic experiments were also performed. After each cycle of reaction, the catalyst was cleaned by distilled water and absolute alcohol, then dried in an oven, and reused. Five cycles of tests were carried out under the same conditions, and the results are displayed in Fig. 7b. As is seen from this figure, the degradation efficiency of RhB over the proposed optimum La-doped BiOCl catalysts only presented a slight decrease after each cycle of reaction. The reason for the slight decrease of photodegradation efficiency of RhB after each cycle of reaction may be attributed to the mass loss of catalysts due to the repeating washing. This result reveals that the optimum La-doped BiOCl catalysts have excellent photochemical stability and recyclability for the degradation of RhB during repeating use, which will be promising photocatalysts for the long-time photodegradation of organic dyes in water.
RhB was degraded over the pure BiOCl films. According to the Langmuir–Hinshelwood model, the linear relationship between ln(C/C0) versus irradiation time can be fitted, and the result is illustrated in Fig. 6b, which indicates that the degradation of RhB over the present BiOCl catalysts followed the pseudo-first kinetics. The calculated reaction rate constants were 0.00229, 0.00266, 0.00342, 0.00458, 0.00309 and 0.00198 min−1 for the photodegradation reactions of RhB over the sample without La doping (the pure BiOCl film) and the ones prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 5, 10, 20, 30 and 40 min, respectively. This result also confirms that the La-doped BiOCl sample prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min exhibited the highest photocatalytic activity. And all these data reveal that an appropriate amount of La doping is helpful to improve the photocatalytic activity of BiOCl. To investigate the main active species in the photodegradation reaction, IPA, TEOA and BQ, respectively, were also dissolved into the reaction system together with the obtained BiOCl catalyst (prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s) before light irradiation. And the corresponding experimental results are shown in Fig. 7a. As is seen from this figure, the photodegradation of RhB was significantly inhibited after the addition of TEOA, while the degradation efficiency of RhB was reduced slightly in the presence of IPA and BQ. This result reveals that the photocatalytic degradation of RhB over the present La-doped BiOCl catalysts can be mainly attributed to the photogenerated h+. Considering the results of UV–vis diffuse reflectance spectra, it can be concluded that La doping led to an increased number of photogenreated h+. Resultantly, the photocatalytic activity of the La-doped BiOCl
4. Conclusions La-doped BiOCl films of flower-like hierarchical nano-/microstructures with ginkgo-leaf-like “petals” were prepared by hot-dipping Bi2O3 films in La2O3 powder followed by etching in HCl solution at room temperature. The obtained La-doped BiOCl nanopetals consisted of numerous ultrathin nanosheets. Furthermore, the introduction of an appropriate amount of La into the BiOCl nanosheets could significantly
Fig. 7. (a) Controlled tests of the photocatalytic degradation on RhB with the addition of different radical scavengers; and (b) recycling tests on the decolorization of RhB. Both experiments are carried out by using the optimum La-doped BiOCl films as the catalysts, which were prepared through hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s.
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improve their visible-light-driven photocatalytic performance on RhB degradation. And the optimum La-doped BiOCl hierarchical NMFs prepared with La2O3 hot-dipping for 20 min exhibited the highest photocatalytic activity. The active species trapping experiments revealed that the photocatalytic degradation of RhB over the present Ladoped BiOCl catalysts can be mainly attributed to the photogenerated holes. The excellent photocatalytic activity of the La-doped BiOCl film could be attributed to the narrower band gap energy. Moreover, the Ladoped BiOCl catalysts have excellent photochemical stability and recyclability for the RhB degradation during repeating use.
[16]
[17]
[18]
[19]
[20]
Acknowledgments
[21]
This work was supported by the National Natural Science Foundation of China (grant nos. 11674035, and 11274052) and Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications).
[22]
[23]
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.materresbull.2017.10.013.
[24]
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Facile synthesis and photocatalytic activity of La-doped BiOCl hierarchical, flower-like nano-/micro-structures ⁎
Keke Xua,b, Xiuli Fua, , Zhijian Pengb,
MARK
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a State Key Laboratory of Information Photonics and Optical Communications, and School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China b School of Science, China University of Geosciences, Beijing 100083, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: BiOCl La doping Hierarchical structure Ginkgo-leaf-like structure Photocatalysis
A series of La-doped BiOCl hierarchical, flower-like nano-/micro-structures (NMFs) with ginkgo-leaf-like “petals” were prepared by etching La-doped Bi2O3 films in HCl solution at room temperature. For the La-doped Bi2O3 films, they were fabricated through hot-dipping Bi2O3 thin films in La2O3 powder at 650 °C. Microstructure examination revealed that the ginkgo-leaf-like La-doped BiOCl petals have a multilayered structure consisting of several ultra-thin nanosheets with different radii due to the external-to-internal etching process. The investigation on their formation mechanism revealed that the La-doped BiOCl hierarchical NMFs grew anisotropically through Ostwald ripening mechanism. Besides, the photodegradation of rhodamine B under simulated sunlight irradiation revealed that the La-doped BiOCl hierarchical NMFs possessed higher performance than pure BiOCl sample. And the active species trapping tests indicated that the photogenerated holes were the main active radicals for the degradation of rhodamine B.
1. Introduction As a member of V-VI-VIIA groups compound semiconductors, bismuth oxychloride (BiOCl) protrudes outstandingly among its congeners in optical, catalytic, luminescent, electrical and gas sensitive properties. Consequently, it has various promising applications in industries, like paints in cosmetic industry [1], photocatalysts for degrading many organic dyes such as rhodamine B (RhB), ciprofloxacin and eosin Y under visible-light irradiation [2,3], high-performance electrocatalysts for air electrode of Al-air batteries [4], and gas sensors to CO, CO2 and O2 [5,6]. To date, many approaches have been proposed to synthesize various forms of BiOCl materials, including electrospinning, solvothermal synthesis, hydrolysis, hydrothermal synthesis, sonochemical route, refluxing method, solution oxidation process, and electrochemical route [7–14]. And a variety of BiOCl materials with different nano-/microstructures, like one dimensional nano-rods/wires, two dimensional nano-plates/sheets and three dimensional hierarchical architectures, have been fabricated. However, most of their aggregates are powders or particles, which have some inevitable problems [15,16]. (i) They are not easily applied to continuous flow system. (ii) The suspended catalyst powders or particles tend to aggregate, especially when they are of high concentration, weakening their photocatalytic activity. And (iii)
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Corresponding authors. E-mail addresses: [email protected] (X. Fu), [email protected] (Z. Peng).
http://dx.doi.org/10.1016/j.materresbull.2017.10.013 Received 4 August 2017; Received in revised form 7 October 2017; Accepted 9 October 2017 Available online 10 October 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.
the separation of the photocatalysts with organic pollutants is very difficult and expensive; so they cannot be recycled and reused, preventing some special applications such as battery electrode or photocatalysis reaction. Therefore, many scientists have still striven to synthesize various BiOCl materials with specific nano-/micro-structures. On the other hand, although BiOCl has attracted considerable attention for potential photocatalytic application, its large band-gap (about 3.46 eV [17]) makes it useful only under ultraviolet irradiation, hindering its applications under visible light, the main part (about 96%) of the natural solar light. In order to improve the absorption and utilization efficiency of BiOCl for visible light, the modification of BiOCl has been a hot topic in this field. And it has been reported that the visible light driven photocatalytic activity of BiOCl can be improved to some extent by changing the crystal structure, band gap or surface microstructure. For example, Xie et al. [18] synthesized Sn-doped BiOCl photocatalysts at room temperature via an oxidation-reduction method, indicating that Sn(10%)-doped BiOCl sample exhibited higher photocatalytic degradation on benzoic acid and RhB than the other Sn-doped BiOCl and pure BiOCl ones due to its narrowed band gap (2.91 eV). Ding et al. [19] synthesized Er3+ doped BiOCl hierarchical microspheres by a solvothermal method. The presence of Er3+ did not affect the microstructure and morphology of BiOCl, but the Er3+ doped BiOCl microspheres exhibited an enhanced photocatalytic activity for the
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λ=1.5418 Å) in a 2θ range of 10° to 90° under a continuous scanning mode with a scanning rate of 6°/min at an incidence angle of 1° of the X-ray. The morphology of the samples was observed by field emission scanning electron microscope (SEM, S4800). The samples were also examined by transmission electron microscopy (TEM, Tecnai G2 F20 UTWIN, America) and high-resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectroscopy (XPS, non-monochromated Mg Kα radiation, photon energy 1253.6 eV) was performed to characterize the chemical state and composition of the samples, during which the spectrometer was calibrated by the binding energy of C1s line (285 eV). The UV–vis absorption spectra were recorded on a Cary 5000 UV–vis spectrometer (Varian) equipped with a DRA-CA-30I integrating sphere for solid-phase characterization.
degradation of RhB under visible light (λ > 420 nm) irradiation. The enhanced activity was attributed to the up-conversion resulted in by the dopant that could transform visible into ultraviolet light. Besides, La doping is also an effective way to improve the catalytic activity of some oxide semiconductors photocatalysts. Not only can La present as stable La3+ ion because there existed no variable valence state for it, but also improve the quantum yield of the doped oxides by prolonging the life of the photo-generated carries because of the shallow potential trap formed by its full electronic configuration [20]. For instance, Yang et al. [21] prepared La doped Bi2O3 by an impregnation method, indicating that an appropriate amount of La doping can effectively prevent the transformation of Bi2O3 from tetragonal to monoclinic phase. Their UV–vis absorption spectra revealed an extension of light absorption into the visible region (higher than 550 nm). The doped La would partially substitute bismuth in the lattice of Bi2O3, existing in a form of Bi-O-La chemical bond, and a new complex metal oxide compound (La0.176Bi0.824O1.5) appeared in the catalysts, which could inhibit the recombination between photoelectrons and holes, leading to enhanced photocatalytic quantum efficiency. Sun et al. [20] synthesized La-doped TiO2 film by a sol-gel method. La3+ dispersed into TiO2 in the form of mainly La2O3 and partly Ti-O-La bond. And La doping can inhibit the phase transformation of TiO2 by enhancing its phase transformation temperature, and is also beneficial for the formation of TiO2 in smaller particle size and thus with large surface area. However, the application of La doping into BiOCl to improve its photocatalytic properties has not been reported. Therefore, in this work, we present a simple approach to prepare Ladoped BiOCl films, which are composed of hierarchical, flower-like nano-/micro-structures (NMFs) with ginkgo-leaf-like “petals”, by etching La-doped Bi2O3 films in HCl acid at room temperature. For the applied La-doped Bi2O3 films, they were fabricated through hot-dipping Bi2O3 thin films in La2O3 powder at 650 °C. Special emphasis was paid on the effects of the hot-dipping time in La2O3 and etching time in HCl acid on the composition, structure and photocatalytic properties of the as-prepared La-doped BiOCl films. The formation mechanism was proposed. And the mechanism for their improved photocatalytic performances was also suggested.
2.3. Photocatalytic activity The photodegradation of model dye RhB under simulated sunlight irradiation (SSI) was carried out to evaluate the photocatalytic activity of the obtained La-doped BiOCl hierarchical NMFs. The samples were set on the bottom of a glass tube with the substrate surface covered with the La-doped BiOCl hierarchical NMFs toward the light (provide by an 8 W halogen lamp, which emits light with a wavelength approximately in the range of 400–790 nm). Then the glass tube was filled with 5 mmol·L−1 RhB solution, keeping the ratio of the mass of the La-doped BiOCl catalyst (mg) to the volume of the RhB solution (mL) equal to 1:1. After the light irradiation for each 10 min, 3 mL of the reactive solution was withdrawn from the reaction system for the measurement of the remnant RhB content in it by an UV–vis spectroscope (SP-752). After the measurement, the reactive solution was immediately poured back into the glass tube to keep the reaction in almost the same state. For comparison, the photocatalytic activity of the pure BiOCl sample was also examined under the same conditions. 2.4. Active species trapping experiments In order to examine the active species generated in the photocatalytic process, 1 mM benzoquinone (BQ, a quencher of superoxide radicals %O2− [22]), 1 mM triethanolamine (TEOA, a scavenger of holes h+ [22]), and 1 mM isopropanol (IPA, a capturer of hydroxyl radicals % OH [23]) were added into the reaction system, respectively, together with the prepared catalyst before light irradiation. And all the other experimental parameters during the tests were kept as those of the above RhB photodegradation experiments.
2. Experimental section 2.1. Samples preparation P-type conducting silicon wafers purchased from GRINM Advanced Materials Co. Ltd (China) were used as the substrates. Before use, the substrates were ultrasonically cleaned in absolute alcohol and de-ionized water for 20 min, respectively, and then completely dried in an open oven. All the applied chemicals were of analytical grade and used as received without further purification. Among them, HCl acid was bought from Beijing Chemical Works (China), and the others were from Xilong Chemical Co. Ltd (China). In a typical process, Bi2O3 films were first prepared onto silicon substrates by covering the substrates with Bi2O3 powder and then heating at 800 °C for 1 h in a muffle furnace (KSL-1100, Hefei Kejing Materials Technology Co. Ltd, China). After heating, extra Bi2O3 powder became a chunk, which could be peeled off from the substrate, and thus thin Bi2O3 film was left on it. Then, the Bi2O3 film was covered with La2O3 powder and heated at 650 °C for 5, 10, 20, 30 and 40 min, respectively. After that, the samples were further etched in HCl aqueous solution (HCl:H2O = 1:100) for 120 s. Finally, the etched samples were rinsed by de-ionized water and dried by N2 flow. For comparison, a non-doped, pure BiOCl film was also prepared by the same process.
3. Results and discussion 3.1. Composition and microstructure Typical XRD patterns of the La-doped BiOCl films produced from Bi2O3 films via hot-dipping in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s are shown in Fig. 1a. For comparison, that of the sample without La2O3 hot-dipping is also presented. As is seen from this figure, all the diffraction peaks of the samples, whether they were hot-dipped in La2O3 powder or not, can be assigned to those of the tetragonal phase of BiOCl with a space group of P4/nmm (JCPDS card no. 85-0861). Because there are no other diffraction peaks detected, it can be concluded that the applied Bi2O3 films were converted into BiOCl during the present etching processes. But due to the detection limit of XRD, no new phase in correlation with the doped La3+ ions could be identified. And the strong and sharp diffraction peaks indicated that the samples prepared by this approach are well crystallized. However, when Bi2O3 was hot-dipped in La2O3 powder for increasing time up to 20 min, a peak shift to large angle of 2θ could be easily observed. From the XRD patterns as shown in Fig. 1a, the lattice constants of a and c of the BiOCl phase were calculated, and the results are displayed in Fig. 1b as a function of the La2O3 hot-
2.2. Materials characterization The phase composition of the samples were identified by grazing incidence X-ray diffraction (GI-XRD, D/max-RB, Cu Kα radiation, 104
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Fig. 1. Typical results of XRD analyses: (a) XRD patterns of the La-doped BiOCl films prepared by hot-dipping Bi2O3 films in La2O3 powder at 650 °C for different times followed with etching in HCl acid for 120 s; and (b) evolution of the lattice parameters a and c of the samples. For comparison, the results of the samples without La2O3 hot-dipping are also presented.
peak at 530.42 eV should be attributed to the lattice oxygen, and the one at 532.35 eV can be ascribed to the absorbed oxygen species, such as surface physically adsorbed oxygen species (CO2, etc.), surface bridging oxygen, hydroxyl and so on [21]. The recorded Cl 2p spectrum presents two peaks at 198.38 eV (Cl 2p3/2) and 199.83 eV (Cl 2p1/2) as shown in Fig. 3d. The high-resolution XPS spectrum of La 3d shown in Fig. 3e could be fitted into a peak corresponding to the binding energy of 834.06 eV, which is close to the reported value for La 3d5/2 [24,25]. It should be noted that, the binding energies of Bi 4f, lattice oxygen, and Cl 2p3/2 in the La-doped BiOCl films have a slight increase than the ones of pure BiOCl (see its microstructure in Fig. S1 in Supplementary materials). The binding energy increase could be ascribed to the decrease of the electron cloud density around the Bi, O and Cl atoms after La doping [18,21]. Meanwhile, it could be also found that the peak intensity of the adsorbed oxygen species decreased obviously after La-doping. The above results indicate that the Bi-O bonds and the surface state of BiOCl have been influenced by La doping.
dipping time. It is seen from Fig. 1b that the lattice constants of a and c increased with the prolonged La2O3 hot-dipping time, because the position of Bi3+ ions (ionic radius: 0.103 nm) in BiOCl lattice were partially occupied by larger La3+ ions (ionic radius: 0.106 nm), which is correspondent with the Vegard's law. The microstructure of the obtained La-doped BiOCl films was examined by SEM and TEM. Typical results on the samples prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl solution for 120 s are shown in Fig. 2. The SEM images as shown in Fig. 2a and 2b revealed that the films consisted of a large quantity of hierarchical, flower-like structures with average diameter of 4 ± 0.5 μm. Such structures were composed of densely packed ginkgo-leaf-like plates as the “petals” of a flower, with a thickness of 30–50 nm, which were assembled together. All the plates in a single flowery structure seem to grow from a centre. TEM examination was further carried out to investigate the detailed crystal structure of the La-doped BiOCl film, and typical images are presented in Fig. 2c–f. As is also seen from Fig. 2c, the individual La-doped BiOCl nanopetal looks like a ginkgo-leaf as shown in the inset of Fig. 2b, which is a novel morphology never reported in literature. Interestingly, the high magnification image as shown in Fig. 2d reveals that each petal has a multilayered structure consisting of several ultra-thin nanosheets with different radii. The contrast variation along the radius direction as shown in Fig. 2c indicates that the ginkgo-leaf-like nanopetal is not homogeneous in thickness, due to the different radius of each ultra-thin nanosheet. Generally, the root of the ginkgo-leaf-like nanopetal is thicker than the brim. Moreover, the HRTEM images of a single nanosheet as shown in Fig. 2e–f present clearly lattice fringes with a spacing of 0.275 nm, which is completely corresponding with the dspacing of the [110] reflection of BiOCl phase. XPS measurements were employed to confirm the chemical composition and surface chemical state of the La-doped BiOCl films, as depicted in Fig. 3. Typical XPS survey spectrum is displayed in Fig. 3a, which undoubtedly indicates the coexistence of Bi, Cl, O and La in the present sample. But, the peak of La is much weaker than that of the others, which might be assigned to the small content of La in the sample. The as-observed carbon may come from the carbon contamination in the equipment for the measurement. Moreover, the corresponding high resolution XPS spectra of Bi 4f, O 1s, Cl 2p and La 3d of the La-doped BiOCl sample are shown in Fig. 3b-e, respectively. For comparison, the XPS spectra of Bi 4f, O 1 s and Cl 2p of the non-doped, pure BiOCl are also presented in Fig. 3b–d. From Fig. 3b, two strong peaks centered at about 159.69 and 165.00 eV corresponding to Bi 4f7/ 2 and Bi 4f5/2 could be observed, indicating that Bi element in the Ladoped BiOCl film mainly existed as Bi3+ [18]. The recorded asymmetric O 1 s XPS spectrum (Fig. 3c) of the La-doped BiOCl films could be fitted into two peaks centered at 530.42 and 532.35 eV, respectively. The
3.2. Formation mechanism In general, flower-like hierarchical structures could be usually obtained via a hierarchical assembly process or the localized Ostwald ripening process. In the assembly process, tiny nanoplates should be formed at the early stage, followed by the oriented attachment of these building blocks into 2D nanosheets. Then, the 2D nanosheets were arranged into 3D hierarchical structures [26–28]. But in the Ostwald ripening process, tiny crystalline nuclei were generated first in a supersaturated solution and grown into nanoparticles, and these newly formed nanoparticles were spontaneously aggregated to minimize their surface energy. Then these crystallized nanoparticles grew anisotropically along the 2D direction, resulting in the formation of the flakes. Finally, flower-like superstructures were formed [29–31]. In present work, all plates in a single flowery structure were obviously grown from a centre as shown in Fig. 2b and 2c. Thus, the formation of the flower-like La-doped BiOCl hierarchical structures was proposed to follow the Ostwald ripening mechanism. The time-dependent evolution experiments were further conducted to reveal the formation process of the present La-doped BiOCl hierarchical NMFs. The products collected from the reactions with different etching times in HCl solution were observed by SEM (see Fig. 4). Fig. 4a is a typical SEM image of Bi2O3 after hot-dipping in La2O3 powder at 650 °C for 20 min, showing the morphology feature of the resultant thin film. XPS results indicate that this thin film consists of only Bi, O and La elements (see Fig. S2). After etching in HCl solution for 10 s, some ginkgo-leaf-like BiOCl nanoplates had appeared, but there were still numerous BiOCl nanoparticles (see Fig. 4b). With the etching time increased from 10 to 120 s, the nanoparticles gradually disappeared; and 105
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Fig. 2. Microstructure of typical La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl solution for 120 s: (a) low magnification SEM image; (b) high magnification SEM image, where the inset shows the image of a real ginkgo-leaf; (c, d) TEM images of a nanopetal, revealing that it consists of several ultra-thin nanosheets; (e) HRTEM image of a nanosheet; and (f) HRTEM image on the rectangle area as shown in (e).
that, when pure Bi2O3 films were used as the source for the synthesis of BiOCl films by direct etching in HCl solution, the morphology of the obtained BiOCl product had no difference compared with that of the above-mentioned La-doped BiOCl films (see Fig. S1), indicating that the doping of La did not affect the formation mechanism of BiOCl films.
more and more BiOCl nanoflakes appeared (Fig. 4b–e). When the time extended to 180 s, these nanoflakes were changed into long-arc and short-radius ones with irregular edges (see Fig. 4f and the arrows of the inset in it), which might be owing to the etching reaction between BiOCl and excessive HCl in solution. Based on these results, it can be confirmed that the formation of the present flower-like La-doped BiOCl hierarchical structures is an anisotropic growth following the Ostwald ripening mechanism. Specifically, the La-doped Bi2O3 films obtained by hot-dipping Bi2O3 films in La2O3 powder were used as the precursors to prepare the NMFs in this study. After the nucleus of La-doped BiOCl was formed, the reaction between the La-doped Bi2O3 and HCl (Bi2O3 + 2HCl → 2BiOCl + H2O) from external to internal mode would grow into La-doped BiOCl nanoparticles. These newly formed nanoparticles would further serve as seeds for the anisotropic growth of the NMFs in subsequent process via Ostwald ripening mechanism. Because BiOCl has a high intrinsically anisotropic nature [32], these nanoparticles prefer to grow into two-dimensionally plate-like structure of single-crystal nature with the etching time prolonging. Finally, flowerlike La-doped superstructures were obtained. Meanwhile, at the end of the transformation from La-doped Bi2O3 film to flower-like La-doped BiOCl hierarchical structures, the HCl in the solution would etch the edge of the La-doped BiOCl nanopetals, resulting in the relatively thin edge of the final products as shown in Fig. 2c, and even rougher edge of the products as shown in Fig. 4f and the inset in it. It should be noted
3.3. Optical properties The recorded UV–vis diffuse reflectance spectra of the La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s are shown in Fig. 5, and their corresponding plots of (αhν)1/2 versus hν are illustrated in the inset of this figure, where the band gap of the obtained La-doped BiOCl hierarchical NMFs was calculated according to the Tauc’s approach [33]. It was observed that the absorption edge of the pure BiOCl films was located at about 375 nm in the near-UV region, and the estimated band gap is about 3.312 eV. This is the intrinsic band gap absorption of BiOCl. Interestingly, there is an obvious shift of the absorption peak toward the longer wavelength up to 400 nm for the Ladoped BiOCl films prepared by La2O3 hot-dipping for 5, 10, 20, 30 and 40 min, respectively. Their corresponding band-gap values could be estimated from the intercept of the tangent to the plot as shown in the inset of Fig. 5, which are 3.275, 3.260, 3.252, 3.245 and 3.237 eV, respectively. This red-shift indicates that La doping introduces an 106
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Fig. 3. XPS data on the La-doped BiOCl hierarchical NMFs prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s: full spectrum (a), and narrow spectra of (b) Bi4f, (c) O1s, (d) Cl 2p and (e) La 3d. For comparison, the XPS spectra of Bi 4f, O 1 s and Cl 2p of the non-doped BiOCl are also presented.
ECB = EVB−Eg
impurity energy level into the band gap of BiOCl, which will be beneficial for the improvement of photocatalytic activity of BiOCl under SSI, because metal ion doping could form discrete empty energy levels below the conduction band of photocatalysts, decreasing the excitation energy for the transition of photo-generated carriers [34]. The effective band structure of the BiOCl and La-doped BiOCl samples could be deduced semi-empirically. And the band edges of BiOCl and La-doped BiOCl can be determined by Mulliken electronegativity theory [35]: EVB = χ − Ee + 0.5 × Eg
(2)
where EVB is the potential of valence band (VB) and ECB is the potential of conduction band (CB); χ is the electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale, which is about 4.5 eV [36–38]; and Eg is the band gap potentials. In this study, the band gap energy of the obtained BiOCl is 3.312 eV and the band gap energy of the La-doped BiOCl prepared by La2O3 hot-dipping for 5, 10, 20, 30, 40 min are 3.275, 3.260, 3.252, 3.245 and 3.237 eV, respectively. Hence, the calculated EVB and ECB of BiOCl are 3.516 and 0.204 eV. And the corresponding EVB and ECB for
(1)
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Fig. 4. SEM images of La-doped BiOCl nanoplates films prepared by heating Bi2O3 powder at 750 °C, hot-dipping the Bi2O3 film in La2O3 powder at 650 °C for 20 min, and then etching the La-doped Bi2O3 film in HCl solution for different times: (a) 0; (b) 10; (c) 30; (d) 60; (e) 120; and (f) 180 s. In (f), the inset shows an enlarged image of the rougher edge of the obtained La-doped BiOCl nanoplates.
the La-doped BiOCl prepared by La2O3 hot-dipping for 5, 10, 20, 30, 40 min are 3.498 and 0.223, 3.490 and 0.230, 3.486 and 0.234, 3.483 and 0.238, 3.479 and 0.242 eV, respectively. Obviously, after La doping, due to the discrete empty energy, the band gap of the La-doped BiOCl was decreased. Resultantly, the photoresponse would shift to longer length of light, thus enhancing the photo-electrical response, which finally increase the number of photogenreated carriers. 3.4. Photocatalytic activity The decolorization of RhB solution over the obtained La-doped BiOCl films in the dark and under SSI is presented in Fig. 6a. It is seen that, compared with the adsorption and photodegradation on RhB over the present catalysts, the self-photodegradation of RhB was almost negligible. Moreover, the difference in the adsorption on RhB over all the La-doped BiOCl NMFs catalysts could be also negligible. However, they would present significantly different photodegradation effects on RhB under SSI, when the catalysts were prepared under different conditions (see Fig. 6a). The photodegraded RhB could be significantly enhanced over the catalysts prepared with appropriate time for the La hot-dipping. After SSI for 240 min, the degradation of RhB over the Ladoped BiOCl catalyst prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min exhibited the highest photocatalytic activity, resulting in 70% of RhB decolorized, while only about 48% of
Fig. 5. UV–vis absorption spectra of the La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s; and the inset is their corresponding plots of (αhν)1/2 versus hν.
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Fig. 6. (a) Decolorization of RhB solution in the dark and under SSI over the obtained La-doped BiOCl films prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for different times followed by etching in HCl acid for 120 s. And (b) their corresponding plots of ln(C/C0) versus irradiation time.
catalysts was finally enhanced. In addition, the discrete empty energy levels could also serve as the trapping center of the photogenerated electrons, prolonging the lifetime of the photogenerated holes, finally improving the photocatalytic performance of BiOCl [36–38]. To further investigate the photochemical stability of the prepared La-doped BiOCl photocatalysts, repeating photocatalytic experiments were also performed. After each cycle of reaction, the catalyst was cleaned by distilled water and absolute alcohol, then dried in an oven, and reused. Five cycles of tests were carried out under the same conditions, and the results are displayed in Fig. 7b. As is seen from this figure, the degradation efficiency of RhB over the proposed optimum La-doped BiOCl catalysts only presented a slight decrease after each cycle of reaction. The reason for the slight decrease of photodegradation efficiency of RhB after each cycle of reaction may be attributed to the mass loss of catalysts due to the repeating washing. This result reveals that the optimum La-doped BiOCl catalysts have excellent photochemical stability and recyclability for the degradation of RhB during repeating use, which will be promising photocatalysts for the long-time photodegradation of organic dyes in water.
RhB was degraded over the pure BiOCl films. According to the Langmuir–Hinshelwood model, the linear relationship between ln(C/C0) versus irradiation time can be fitted, and the result is illustrated in Fig. 6b, which indicates that the degradation of RhB over the present BiOCl catalysts followed the pseudo-first kinetics. The calculated reaction rate constants were 0.00229, 0.00266, 0.00342, 0.00458, 0.00309 and 0.00198 min−1 for the photodegradation reactions of RhB over the sample without La doping (the pure BiOCl film) and the ones prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 5, 10, 20, 30 and 40 min, respectively. This result also confirms that the La-doped BiOCl sample prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min exhibited the highest photocatalytic activity. And all these data reveal that an appropriate amount of La doping is helpful to improve the photocatalytic activity of BiOCl. To investigate the main active species in the photodegradation reaction, IPA, TEOA and BQ, respectively, were also dissolved into the reaction system together with the obtained BiOCl catalyst (prepared by hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s) before light irradiation. And the corresponding experimental results are shown in Fig. 7a. As is seen from this figure, the photodegradation of RhB was significantly inhibited after the addition of TEOA, while the degradation efficiency of RhB was reduced slightly in the presence of IPA and BQ. This result reveals that the photocatalytic degradation of RhB over the present La-doped BiOCl catalysts can be mainly attributed to the photogenerated h+. Considering the results of UV–vis diffuse reflectance spectra, it can be concluded that La doping led to an increased number of photogenreated h+. Resultantly, the photocatalytic activity of the La-doped BiOCl
4. Conclusions La-doped BiOCl films of flower-like hierarchical nano-/microstructures with ginkgo-leaf-like “petals” were prepared by hot-dipping Bi2O3 films in La2O3 powder followed by etching in HCl solution at room temperature. The obtained La-doped BiOCl nanopetals consisted of numerous ultrathin nanosheets. Furthermore, the introduction of an appropriate amount of La into the BiOCl nanosheets could significantly
Fig. 7. (a) Controlled tests of the photocatalytic degradation on RhB with the addition of different radical scavengers; and (b) recycling tests on the decolorization of RhB. Both experiments are carried out by using the optimum La-doped BiOCl films as the catalysts, which were prepared through hot-dipping Bi2O3 film in La2O3 powder at 650 °C for 20 min followed by etching in HCl acid for 120 s.
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improve their visible-light-driven photocatalytic performance on RhB degradation. And the optimum La-doped BiOCl hierarchical NMFs prepared with La2O3 hot-dipping for 20 min exhibited the highest photocatalytic activity. The active species trapping experiments revealed that the photocatalytic degradation of RhB over the present Ladoped BiOCl catalysts can be mainly attributed to the photogenerated holes. The excellent photocatalytic activity of the La-doped BiOCl film could be attributed to the narrower band gap energy. Moreover, the Ladoped BiOCl catalysts have excellent photochemical stability and recyclability for the RhB degradation during repeating use.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (grant nos. 11674035, and 11274052) and Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications).
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.materresbull.2017.10.013.
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