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bismuth oxide formate p-n heterojunctions with significantly enhanced photocatalytic performance under visible light
Journal of Colloid and Interface Science 548 (2019) 12–19
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Facile construction of flower-like bismuth oxybromide/bismuth oxide formate p-n heterojunctions with significantly enhanced photocatalytic performance under visible light Shijie Li a, Jialin Chen a, Wei Jiang a,⇑, Yanping Liu b, Yaming Ge a,⇑, Jianshe Liu c a Key Laboratory of Key Technical Factors in Zhejiang Seafood Health Hazards, Institute of Innovation & Application, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China b College of Marine Science and Technology, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China c State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 January 2019 Revised 3 April 2019 Accepted 8 April 2019 Available online 8 April 2019 Keywords: BiOBr/BiOCOOH p-n Heterojunction Anion exchange Photocatalysis Visible light
a b s t r a c t Visible-light harvesting ability and charge separation efficiency are two pivotal factors for the design and construction of photocatalysts with an efficient ability for degrading toxic pollutants. Herein, visiblelight-driven (VLD) BiOBr/BiOCOOH p-n heterojunction photocatalysts were prepared via an in-situ anion-exchange route. Through controlling the addition of KBr, we synthesized a series of BiOBr/ BiOCOOH p-n heterojunctions with a different BiOBr loading. During the process, BiOBr production and homogeneous deposition on BiOCOOH with close interfacial interactions were realized by employing BiOCOOH microspheres as the self-sacrificing template. Compared to bare BiOBr and BiOCOOH, such p-n heterojunctions displayed dramatically strengthened performance in decomposing the industrial dye (rhodamine B, RhB) and antibiotic (tetracycline chloride, TC) under the irradiation of visible light. Among them, BiOBr/BiOCOOH p-n heterojunction with a BiOBr/BiOCOOH theoretical molar ratio of 0.6/0.4 (0.6Br-Bi) achieved the highest performance. Moreover, 0.6Br-Bi showed a good durability, indicating BiOBr/BiOCOOH p-n heterojunction possessed an excellently stable photocatalytic activity. Such an efficient and stable photocatalytic performance was mainly due to the formation of p-n heterojunctions which can profoundly improve the visible-light absorption and significantly depress the recombination of charge carriers. Trapping experiments and ESR tests verified that superoxide free radicals
⇑ Corresponding authors. E-mail addresses: [email protected] (W. Jiang), [email protected] (Y. Ge). https://doi.org/10.1016/j.jcis.2019.04.024 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
S. Li et al. / Journal of Colloid and Interface Science 548 (2019) 12–19
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+ (O 2 ) and photogenerated hole (h ) played a significant role in RhB degradation. This research affords a promising p-n heterojunction catalyst for wastewater treatment. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction The formidable environmental pollution across the world has sparked the scholars’ appetite to explore new technologies for environmental remediation. Photocatalysis technology that can make use of clean and inexhaustible solar energy to decompose toxic pollutants has been regarded as an efficacious technology to remedy environment [1–5]. To take full advantage of sunlight, the photocatalysts are supposed to possess a wide photoresponsive range and high charge separation efficiency [6]. However, many single-phase photocatalysts still suffer from the insufficient optical absorption and/or high rapid recombination rate of charge carriers [7–9]. Encouragingly, constructing heterojunction photocatalysts by hybridizing different semiconductors can surmount their shortcomings for obtaining improved photocatalytic performance compared to the single-phase catalysts [7,8,10–30]. BiOCOOH, as an n-type semiconductor, is reported as a good photocatalyst in the decomposition of toxic pollutants [31–35]. Benefiting from the special crystalline structure built from [Bi2O2]2+ sheets and formic acid, BiOCOOH displays a high separation rate of photo-induced electron-hole pairs. Despite this, suffer from its relatively large band gap (3.4 eV), BiOCOOH can hardly absorb visible-light [31,36]. Constructing heterojunction photocatalysts by coupling BiOCOOH and narrow-band-gap semiconductors is a feasible and effective strategy to solve this dilemma. In addition to widening the range of light absorption, this approach could promote the efficient separation of charge carriers, thus endowing photocatalysts with strengthened photocatalytic ability [37,38]. As a consequence, some BiOCOOH containing n-n heterojunctions (e.g. C3N4/BiOCOOH [37], Ag2CO3/BiOCOOH [38]) have been developed. Indeed, in contrast to n-n heterojunctions, p-n heterojunctions can promote the separation of electrons and holes more efficiently due to the establishment of internal electric field, hence greatly strengthening the photocatalytic property [39]. However, current research on BiOCOOH-based p-n heterojunctions is very limited [40–42]. Therefore, further exploration of novel BiOCOOH-based p-n heterojunctions with distinct photocatalytic property is of vital importance for both scientific research and practical applications. p-type bismuth oxybromide (BiOBr), a kind of V-VI-VII layered ternary oxide, has been regarded as an outstanding VLD photocatalyst by virtue of its distinctive features, such as narrow band gap of 2.7 eV, superior electron-hole separation efficiency, and good chemical stability [31,43–51]. More importantly, BiOBr has the open layered structure composed of the [Bi2O2] slabs intercalated by Br atoms, and the similar layered architectures are conducive to establishing the heterostructure with BiOCOOH. Considering the proper energy band potential, strong visible light absorption, and layered architecture, BiOBr is expected to be a good candidate for fabricating a novel p-n heterostructure to ameliorate the visible-light harvesting capability and photocatalytic ability of BiOCOOH. Additionally, no research concerns with the construction and the photocatalytic performance of BiOBr/BiOCOOH p-n heterojunctions. Motived by the above facts, flower-like BiOBr/BiOCOOH p-n heterojunctions were fabricated via a facile procedure, in which BiOBr was in-situ grown on the surface of BiOCOOH microspheres. And the photocatalytic performance of BiOBr/BiOCOOH was testified by degrading RhB dye and TC antibiotic under the irradiation
of visible light. It was revealed that the establishment of p-n heterojunction between BiOCOOH and BiOBr greatly impeded the recombination of charge carriers, thus dramatically upgrading the photocatalytic performance of BiOCOOH. The exceptional photocatalytic activity and reusability of BiOBr/BiOCOOH p-n heterojunctions implicates their potentials for practical wastewater treatment.
2. Experiment 2.1. Reagents N,N-dimethyformamide (DMF), Bismuth nitrate (Bi(NO3)35H2O), KBr, p-benzoquinone (BQ), glycerol, ammonium oxalate (AO), RhB, isopropanol (IPA), and TC were obtained from Chinese Sinopharm. All reagents were used with no further disposal. 2.2. Synthesis of catalysts 2.2.1. Fabrication of BiOCOOH Typically, 1.45 g Bi(NO3)35H2O was firstly dissolved in a solution of 20 mL DMF and 50 mL glycerol, followed by adding 10 mL deionized water, and the resultant solution was stirred for 25 min. Subsequently, the obtained solution was sealed in a 100 mL autoclave, and then reacted at 160 °C for 24 h in an electric oven. After that, the collected solids were washed by distilled water and ethanol for five times, and dried at 70 °C overnight in a vacuum oven. 2.2.2. Fabrication of BiOBr/BiOCOOH BiOBr/BiOCOOH heterojunctions were fabricated via an in-situ anion-exchange route. Firstly, 0.27 g BiOCOOH was added to 50 mL deionized water and ultrasonicated for 2 min to make it evenly dispersed. Secondly, a certain amount of KBr was introduced into the above suspension under magnetic stirring for 1 h, and then the obtained mixture was sealed in a 100 mL autoclave reacted at 140 °C for 12 h. Finally, the obtained solids were washed, and dried in a vacuum oven. The BiOBr/BiOCOOH heterojunctions with the adding amount of 0.1 mmol, 0.3 mmol, 0.6 mmol or 0.9 mmol of KBr were separately recorded as 0.1BrBi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi. Pure BiOBr was synthesized by introducing 1 mmol KBr into 1 mmol Bi(NO3)35H2O solution. 2.3. Characterization The crystalline properties of as-prepared catalysts were tested by X-ray diffraction (XRD, Bruker D8 Advance) in the reflection mode (Cu Ka = 1.5406 Å). The microstructure was identified by scanning electron microscopy (SEM, Hitachi S–4800) and transmission electron microscopy (TEM, JEM–2010F), and the element constitution and distribution were characterized by energy-disperse X-ray spectroscopy (EDS) and elemental mappings. Photoluminescence (PL) spectra of all catalysts were identified by a fluorescence spectrophotometer (Hitachi F-7000). The optical properties of all catalysts were detected on a Shimadzu UV-2600 spectrophotome ter. The signals for DMPO-O 2 and DMPO- OH was detected on a Bruker ESR 300E spectrometer.
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2.4. Photocatalytic tests Photocatalytic decomposition of industrial dye RhB or antibiotic TC was conducted in a photocatalytic reactor under the illumination of visible light from a 300 W Xe lamp (Beijing NBET Technology, HSX-UV300) with a 400 nm cutoff filter [52–54]. Briefly, the catalyst (50 mg) was introduced into a RhB (10 mg/L, 100 mL), or TC (15 mg/L, 50 mL) solution. Before light irradiation, the mixed solution was continuously stirred for half an hour in darkness to reach the adsorption-desorption equilibrium. During the illumination, the reactor was controlled at room temperature with the assistance of flowing water. Finally, the samples withdrawn from the reaction solution at certain intervals were examined by detecting the absorbance at the characteristic band (554 nm for RhB, and 357 nm for TC) using an UV-2600 spectrophotometer. Total organic carbon (TOC) values during RhB (30 mg/L, 250 mL) or TC (30 mg/L, 250 mL) degradation over 150 mg of 0.6Br-Bi, BiOCOOH, or BiOBr were recorded and analyzed to evaluate the mineralization capability of the samples. 3. Results and discussion 3.1. Characterization The XRD patterns of all obtained samples are displayed in Fig. 1. The diffraction peaks of pure BiOCOOH, and BiOBr are in accordance with the BiOCOOH standard data (JCPDS 35-0939) [42], and BiOBr standard data (JCPDS 09-0393) [47]. For BiOBr/BiOCOOH heterojunctions, except for the peaks belonging to BiOCOOH, the diffraction peaks assigned to the (1 0 1), (1 1 0), (2 1 2) planes of BiOBr (JCPDS card No. 09-0393) can be found, indicating the existence of BiOBr phase. Additionally, the intensity of characteristic peaks for BiOBr gradually becomes stronger with increasing the molar ratio of KBr to BiOCOOH. No signals of other crystals in the XRD pattern were detected, implying the high purity of the products. Thus, the as-prepared BiOBr/BiOCOOH heterojunctions consist of the BiOBr and BiOCOOH two phases. The morphology information on BiOCOOH, and BiOBr/BiOCOOH heterojunctions were obtained from SEM and TEM images (Fig. 2). Pristine BiOCOOH is composed of micro-spherical assembly (diameter: 1.6–3.5 lm) (Fig. 2a). The in-situ generation of BiOBr depositing on the surface of BiOCOOH has only marginal effect on the
Fig. 1. XRD patterns of BiOCOOH, BiOBr, and BiOBr/BiOCOOH heterojunctions (0.1Br-Bi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi).
morphology of the heterojunctions, which all display the microspherical morphology with diameters in the range of 1.5–3.5 lm (Figs. S1 and 2). Specifically, 0.6Br-Bi shows a similar hierarchical structure to the BiOCOOH (Fig. 2b). The nanoscale morphology of 0.6Br-Bi was further visualized by using TEM, which demonstrates the flower-like morphology that is built from numerous nanoplates (Fig. 2c). Fig. S2 shows the high-resolution TEM image of 0.6Br-Bi, where two adjacent lattice fringes with distances of 0.272 nm and 0.283 nm corresponds to the (1 1 0) plane of BiOCOOH and the (1 0 2) plane of BiOBr, respectively. Additionally, the presence of BiOBr was further verified with EDS spectra (Fig. 3), EDX elemental mapping (Fig. 2d–h), as well as the XRD pattern (Fig. 1). It is evident that there is a homogeneous distribution of Bi, Br, O, and C elements in 0.6Br-Bi, implying the successful fabrication of BiOBr/BiOCOOH. The light absorption of BiOBr, BiOCOOH, and BiOBr/BiOCOOH heterojunctions were examined by UV–Vis DRS and the result is displayed in Fig. 4a. BiOCOOH displays intense photo-absorption in the UV region with a distinct absorption edge about 370 nm (band gap: 3.40 eV), reflecting that BiOCOOH can hardly absorb visible light, which accords well with the reported results [40,42,55]. In contrast, BiOBr exhibits effective photo-absorption in the visible light region, and its absorption edge is approximately 462 nm (band gap: 2.68 eV), in line with the previous results [47]. Encouragingly, the as-obtained BiOBr/BiOCOOH heterojunctions exhibit intense visible-light absorption. On the basis of these results, the band positions of BiOCOOH and BiOBr were estimated by the empirical formula.
EVB ¼ X E0 þ 0:5Eg
ð1Þ
ECB ¼ EVB Eg
ð2Þ
Here Eg, EVB, E0, ECB and X separately represent the band gap, valence band potential, the energy of free electrons (4.5 eV), conduction band potential, and the electronegativity of the semiconductor. Accordingly, the ECB and EVB of BiOCOOH are estimated as –0.67 and 2.73 eV [40,42], while those of BiOBr are 0.34 and 3.02 eV [49]. 3.2. Photocatalytic property Toxic organic contaminants, including industrial dye RhB and antibiotic TC, were selected as the targets to determine the photocatalytic properties of BiOBr/BiOCOOH heterojunctions under the irradiation of visible light. The degradation curves of RhB are presented in Fig. 5a. Blank test revealed that no RhB broke down without the catalyst after 1 h of visible-light illumination. Pristine BiOCOOH displayed a very low photodegradation efficiency of RhB (12.7%) because of its wide band gap. Under the same condition, approximately 51.6% of RhB was degraded by using pristine BiOBr as the catalyst in 60 min of reaction. Comparatively, the as-prepared BiOBr/BiOCOOH heterojunctions exhibited markedly enhanced photocatalytic performance, which is mainly attributed to the unique hierarchical hetero-structure that realizes the rapid charge transfer and thus significantly impedes the recombination of electrons and holes. Specifically, 0.6Br-Bi displayed the highest photocatalytic property, in which 100% of RhB was degraded in 50 min. Moreover, the BET surface areas of the samples were measured and the results were shown in Table S1. It is noted that the BET surface areas of BiOCOOH, 0.1Br-Bi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi are 26.13, 27.58, 28.36, 27.74, and 25.62 m2g1, respectively. Since the BET surface areas of 0.6Br-Bi is not the largest among these samples, the surface area does not dominantly account for the photocatalytic performance of BiOBr/BiOCOOH. Besides, the RhB photodegradation efficiency achieved by using the physically mixed sample (63 wt% BiOBr + 37 wt% BiOCOOH) was much lower
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Fig. 2. SEM images of BiOCOOH (a) and 0.6Br-Bi (b); TEM image of 0.6Br-Bi (c); EDX elemental mapping images of 0.6Br-Bi (d–h).
Fig. 3. EDS spectrum of 0.6Br-Bi.
Fig. 4. UV–Vis DRS of all obtained samples.
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Fig. 6. Photodegradation of TC by various samples under visible light.
photocatalytic activity of 0.6Br-Bi in decomposing toxic pollutants, suggesting its huge potential for practical application in wastewater treatment. To evaluate the mineralization capability of 0.6Br-Bi, the TOC values during the degradation of RhB (30 mg/L, 250 mL) and TC (30 mg/L, 250 mL) were monitored and analyzed. Fig. 7a presented
Fig. 5. (a) Photodegradation of RhB by BiOBr, BiOCOOH, BiOBr/BiOCOOH heterojunctions and the physical mixture of BiOBr and BiOCOOH under visible light; (b) Photodegradation kinetics of RhB.
than that of 0.6Br-Bi, reflecting that the formation of tightly contacted interface is favorable for enhancing the activity. Additionally, the reaction kinetics of the RhB degradation process was studied by employing the Langmuir-Hinshelwood (L-H) model. It was observed that the reaction process conformed to the apparent first-order model (Fig. 5b). The k values for various catalysts were presented in the following order: 0.6Br-Bi > 0.3Br-Bi > 0.9BrBi > 0.1Br-Bi > BiOBr > Mixture > BiOCOOH. Remarkably, the k value of 0.6Br-Bi (0.0846 min1) is the largest, about 37.2, 6.3, and 7.1 folds higher than that of pristine BiOCOOH (0.0022 min1), BiOBr (0.0116 min1), or the mixed sample (0.0105 min1). The photocatalytic performance of 0.6Br-Bi was further compared to other catalysts such as commercial TiO2 (P25), Ag2O/ TaON [56], CNT/BiOI [57] and Ag/BiOCOOH [58] (Fig. S3). Obviously, 0.6Br-Bi had a much higher RhB degradation efficiency than other photocatalysts, demonstrating the powerful photocatalytic capability. To further examine the photooxidation ability of 0.6Br-Bi, TC antibiotic, which can cause reproductive abnormalities to humans, was chosen as the photodegradation model. As depicted in Fig. 6, approximately 83.7% of TC was degraded in 120 min of reaction for 0.6Br-Bi, while only 4.4%, 31.6%, and 25.8% of TC were degraded for BiOCOOH, BiOBr, and the mixture. The results validate the high
Fig. 7. (a) The TOC removal rate of RhB over the as-prepared samples after 4 h irradiation, (b) recycling tests for photodegradation of RhB over 0.6Br-Bi.
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the TOC removal rate of RhB over the as-prepared samples after 4 h of irradiation. As revealed, the mineralization ratio of RhB for 0.6Br-Bi (87.3%) is 14.3 times of that for BiOCOOH (6.1%) and 3.1 times of that for BiOBr (27.9%). Similarly, 0.6Br-Bi also achieved a high mineralization ratio of TC (72.4%) after 4 h of reaction (Fig. S4). These results manifest the strong mineralization capability of 0.6Br-Bi. Photostability of the photocatalyst is a pivotal factor for the practical application, thus the recycling experiments were implemented to assess the stability of 0.6Br-Bi. As shown in Fig. 7b, the photocatalytic activity of 0.6Br-Bi displayed no significant deactivation (only approximately 17.2% loss) after ten consecutive runs for photodegrading RhB. Furthermore, the XRD pattern of the recycled 0.6Br-Bi was almost unchanged, compared to that of the fresh one (Fig. S5). The results demonstrate the excellent stability of 0.6Br-Bi, bodying for its huge potential value in environmental remediation. 3.3. Photocatalytic mechanism In order to determine the exact roles of possibly produced active species and further clarify the photocatalytic mechanism of BiOBr/BiOCOOH, the trapping experiments were conducted. As shown in Fig. 8a, the RhB degradation efficiency by using 0.6BrBi in 60 min of reaction was 100% without any quenchers. For comparison, the introduction of IPA brought about the partial inhibition of the degradation efficiency, indicating that OH exerts a minor influence on RhB removal. Instead, the involvement of BQ or AO pronouncedly suppressed RhB decomposition, with RhB degradation efficiency of only 70.7% or 87.5%. The results assure + the critical roles of O 2 and h species in the photodegradation process over BiOBr/BiOCOOH. The electron spin resonance (ESR) spin-trap with DMPO technique was further employed to identify the active species formed by 0.6Br-Bi under visible light illumination (Figs. 8b and S2) [59]. Apparently, signals of DMPO-O 2 were detected under the irradiation of visible light, reflecting that O 2 radicals were produced during the photocatalytic process (Fig. 8b). However, no signals of DMPO-OH appeared, indicating that no OH was generated (Fig. S6). Therefore, the trapping experiments and ESR tests + demonstrate that O 2 and h species dominantly account for the pollutant degradation over BiOBr/BiOCOOH. The photoluminescence (PL) spectra were employed to further inspect the separation and transfer rate of photo-generated charge carriers, which was closely related to the photocatalytic property
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of the photocatalyst [11,60,61]. Generally, the weak PL intensity represents the boosted separation process and thus the improved photocatalytic activity [62–64]. Fig. 9 shows the PL spectra of pure BiOCOOH and BiOBr/BiOCOOH heterojunctions under the excitation wavelength of 300 nm. Pure BiOCOOH reflects a strong emission peak centered at ca. 370 nm [42,45]. For BiOBr/BiOCOOH heterojunctions, their PL spectra are similar to that of BiOCOOH, but the PL peak intensity is pronouncedly decreased, revealing the efficient separation and transfer of charge carriers through the phase interface. Of note, 0.6Br-Bi possesses the lowest PL intensity, which reflects the possibly lowest charge recombination rate and thereby signifies the highest photocatalytic activity. On the basis of the above-mentioned analyses, two major factors are supposed to account for the superior photocatalytic performance of BiOBr/BiOCOOH. On the one hand, the hierarchical architectures of BiOBr/BiOCOOH (Figs. 2 and S1) with numerous pores can facilitate the absorption of light through multiple reflection and offer plenty of transport paths and reactive sites, boosting the degradation of contaminants [9,11]. On the other hand, the BiOBr/BiOCOOH p-n heterojunction established is a fascinating architecture for charge transfer and separation. A schematic diagram of the photocatalytic mechanism of BiOBr/BiOCOOH under
Fig. 9. PL spectra of BiOCOOH and BiOBr/BiOCOOH heterojunctions.
Fig. 8. (a) Influences of various scavengers on RhB degradation performance by using 0.6Br-Bi, (b) DMPO spin-trapping ESR spectra in methanol dispersion of 0.6Br-Bi for DMPO-O 2.
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Fig. 10. Proposed photocatalysis mechanism of BiOBr/BiOCOOH p-n heterojunction photocatalyst toward toxic pollutant under visible light.
the illumination of visible light is brought up and depicted in Fig. 10. The Fermi level (EF) and CB of n-type BiOCOOH are more negative than those of p-type BiOBr prior to the establishment of the BiOBr/BiOCOOH p-n heterostructure. As the p-n heterostructure is created between BiOCOOH and BiOBr, the EF of BiOBr drifts upward and that of BiOCOOH moves downward until the EF level of two constituents reaches an equilibrium. Correspondingly, the band positions of BiOBr and BiOCOOH move along with the shift of EF. Consequently, the CB and VB potentials of BiOCOOH are more positive than those of BiOBr, leading to the formation of the internal electric field. Such type-II band alignment created in BiOBr/ BiOCOOH with internal electric field is favorable to the separation of electron-hole pairs [10,39,65,66]. BiOCOOH cannot respond to visible light, and thus no electron-hole pairs can be produced in BiOCOOH under the irradiation of visible light, whereas BiOBr can be driven by visible light to produce electrons and holes. The photo-induced electrons in the CB of BiOBr could be readily injected into that of BiOCOOH, leaving the photo-induced holes store in the VB of BiOBr. This process profoundly promotes the separation of electrons and holes, further alleviating the photodegradation ability [7,67,68]. Meanwhile, the assembling electrons on CB of BiOCOOH participate in the reduction of O2 for the formation of O 2 radicals. As a result, the active oxidizing species of O2 , and + h mainly participate in the decomposition of toxic contaminants (Fig. 8). Benefiting from the above merits, BiOBr/BiOCOOH p-n heterojunctions furnishes powerful photocatalytic capability in removal of refractory pollutants. 4. Conclusions In summary, flower-like BiOBr/BiOCOOH p-n heterojunctions with tightly interfacial contact were constructed via a readily achievable in-situ anion-exchange approach. Compared to pristine BiOCOOH and BiOBr, the VLD photocatalytic activities of BiOBr/ BiOCOOH heterojunctions are consumedly strengthened. The 0.6Br-Bi heterojunction has the optimal photocatalytic ability in degrading the RhB dye and TC antibiotic. The superior photocatalytic activity of BiOBr/BiOCOOH could be ascribed to the optimized light absorption and the suppressed recombination of photo-generated electrons and holes, resulting from the establishment of p-n heterojunction between BiOCOOH and BiOBr. Moreover, 0.6Br-Bi owns high stability and strong mineralization capability, and thus can be expected to serve as a good candidate
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Regular Article
Facile construction of flower-like bismuth oxybromide/bismuth oxide formate p-n heterojunctions with significantly enhanced photocatalytic performance under visible light Shijie Li a, Jialin Chen a, Wei Jiang a,⇑, Yanping Liu b, Yaming Ge a,⇑, Jianshe Liu c a Key Laboratory of Key Technical Factors in Zhejiang Seafood Health Hazards, Institute of Innovation & Application, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China b College of Marine Science and Technology, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China c State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 January 2019 Revised 3 April 2019 Accepted 8 April 2019 Available online 8 April 2019 Keywords: BiOBr/BiOCOOH p-n Heterojunction Anion exchange Photocatalysis Visible light
a b s t r a c t Visible-light harvesting ability and charge separation efficiency are two pivotal factors for the design and construction of photocatalysts with an efficient ability for degrading toxic pollutants. Herein, visiblelight-driven (VLD) BiOBr/BiOCOOH p-n heterojunction photocatalysts were prepared via an in-situ anion-exchange route. Through controlling the addition of KBr, we synthesized a series of BiOBr/ BiOCOOH p-n heterojunctions with a different BiOBr loading. During the process, BiOBr production and homogeneous deposition on BiOCOOH with close interfacial interactions were realized by employing BiOCOOH microspheres as the self-sacrificing template. Compared to bare BiOBr and BiOCOOH, such p-n heterojunctions displayed dramatically strengthened performance in decomposing the industrial dye (rhodamine B, RhB) and antibiotic (tetracycline chloride, TC) under the irradiation of visible light. Among them, BiOBr/BiOCOOH p-n heterojunction with a BiOBr/BiOCOOH theoretical molar ratio of 0.6/0.4 (0.6Br-Bi) achieved the highest performance. Moreover, 0.6Br-Bi showed a good durability, indicating BiOBr/BiOCOOH p-n heterojunction possessed an excellently stable photocatalytic activity. Such an efficient and stable photocatalytic performance was mainly due to the formation of p-n heterojunctions which can profoundly improve the visible-light absorption and significantly depress the recombination of charge carriers. Trapping experiments and ESR tests verified that superoxide free radicals
⇑ Corresponding authors. E-mail addresses: [email protected] (W. Jiang), [email protected] (Y. Ge). https://doi.org/10.1016/j.jcis.2019.04.024 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
S. Li et al. / Journal of Colloid and Interface Science 548 (2019) 12–19
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+ (O 2 ) and photogenerated hole (h ) played a significant role in RhB degradation. This research affords a promising p-n heterojunction catalyst for wastewater treatment. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction The formidable environmental pollution across the world has sparked the scholars’ appetite to explore new technologies for environmental remediation. Photocatalysis technology that can make use of clean and inexhaustible solar energy to decompose toxic pollutants has been regarded as an efficacious technology to remedy environment [1–5]. To take full advantage of sunlight, the photocatalysts are supposed to possess a wide photoresponsive range and high charge separation efficiency [6]. However, many single-phase photocatalysts still suffer from the insufficient optical absorption and/or high rapid recombination rate of charge carriers [7–9]. Encouragingly, constructing heterojunction photocatalysts by hybridizing different semiconductors can surmount their shortcomings for obtaining improved photocatalytic performance compared to the single-phase catalysts [7,8,10–30]. BiOCOOH, as an n-type semiconductor, is reported as a good photocatalyst in the decomposition of toxic pollutants [31–35]. Benefiting from the special crystalline structure built from [Bi2O2]2+ sheets and formic acid, BiOCOOH displays a high separation rate of photo-induced electron-hole pairs. Despite this, suffer from its relatively large band gap (3.4 eV), BiOCOOH can hardly absorb visible-light [31,36]. Constructing heterojunction photocatalysts by coupling BiOCOOH and narrow-band-gap semiconductors is a feasible and effective strategy to solve this dilemma. In addition to widening the range of light absorption, this approach could promote the efficient separation of charge carriers, thus endowing photocatalysts with strengthened photocatalytic ability [37,38]. As a consequence, some BiOCOOH containing n-n heterojunctions (e.g. C3N4/BiOCOOH [37], Ag2CO3/BiOCOOH [38]) have been developed. Indeed, in contrast to n-n heterojunctions, p-n heterojunctions can promote the separation of electrons and holes more efficiently due to the establishment of internal electric field, hence greatly strengthening the photocatalytic property [39]. However, current research on BiOCOOH-based p-n heterojunctions is very limited [40–42]. Therefore, further exploration of novel BiOCOOH-based p-n heterojunctions with distinct photocatalytic property is of vital importance for both scientific research and practical applications. p-type bismuth oxybromide (BiOBr), a kind of V-VI-VII layered ternary oxide, has been regarded as an outstanding VLD photocatalyst by virtue of its distinctive features, such as narrow band gap of 2.7 eV, superior electron-hole separation efficiency, and good chemical stability [31,43–51]. More importantly, BiOBr has the open layered structure composed of the [Bi2O2] slabs intercalated by Br atoms, and the similar layered architectures are conducive to establishing the heterostructure with BiOCOOH. Considering the proper energy band potential, strong visible light absorption, and layered architecture, BiOBr is expected to be a good candidate for fabricating a novel p-n heterostructure to ameliorate the visible-light harvesting capability and photocatalytic ability of BiOCOOH. Additionally, no research concerns with the construction and the photocatalytic performance of BiOBr/BiOCOOH p-n heterojunctions. Motived by the above facts, flower-like BiOBr/BiOCOOH p-n heterojunctions were fabricated via a facile procedure, in which BiOBr was in-situ grown on the surface of BiOCOOH microspheres. And the photocatalytic performance of BiOBr/BiOCOOH was testified by degrading RhB dye and TC antibiotic under the irradiation
of visible light. It was revealed that the establishment of p-n heterojunction between BiOCOOH and BiOBr greatly impeded the recombination of charge carriers, thus dramatically upgrading the photocatalytic performance of BiOCOOH. The exceptional photocatalytic activity and reusability of BiOBr/BiOCOOH p-n heterojunctions implicates their potentials for practical wastewater treatment.
2. Experiment 2.1. Reagents N,N-dimethyformamide (DMF), Bismuth nitrate (Bi(NO3)35H2O), KBr, p-benzoquinone (BQ), glycerol, ammonium oxalate (AO), RhB, isopropanol (IPA), and TC were obtained from Chinese Sinopharm. All reagents were used with no further disposal. 2.2. Synthesis of catalysts 2.2.1. Fabrication of BiOCOOH Typically, 1.45 g Bi(NO3)35H2O was firstly dissolved in a solution of 20 mL DMF and 50 mL glycerol, followed by adding 10 mL deionized water, and the resultant solution was stirred for 25 min. Subsequently, the obtained solution was sealed in a 100 mL autoclave, and then reacted at 160 °C for 24 h in an electric oven. After that, the collected solids were washed by distilled water and ethanol for five times, and dried at 70 °C overnight in a vacuum oven. 2.2.2. Fabrication of BiOBr/BiOCOOH BiOBr/BiOCOOH heterojunctions were fabricated via an in-situ anion-exchange route. Firstly, 0.27 g BiOCOOH was added to 50 mL deionized water and ultrasonicated for 2 min to make it evenly dispersed. Secondly, a certain amount of KBr was introduced into the above suspension under magnetic stirring for 1 h, and then the obtained mixture was sealed in a 100 mL autoclave reacted at 140 °C for 12 h. Finally, the obtained solids were washed, and dried in a vacuum oven. The BiOBr/BiOCOOH heterojunctions with the adding amount of 0.1 mmol, 0.3 mmol, 0.6 mmol or 0.9 mmol of KBr were separately recorded as 0.1BrBi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi. Pure BiOBr was synthesized by introducing 1 mmol KBr into 1 mmol Bi(NO3)35H2O solution. 2.3. Characterization The crystalline properties of as-prepared catalysts were tested by X-ray diffraction (XRD, Bruker D8 Advance) in the reflection mode (Cu Ka = 1.5406 Å). The microstructure was identified by scanning electron microscopy (SEM, Hitachi S–4800) and transmission electron microscopy (TEM, JEM–2010F), and the element constitution and distribution were characterized by energy-disperse X-ray spectroscopy (EDS) and elemental mappings. Photoluminescence (PL) spectra of all catalysts were identified by a fluorescence spectrophotometer (Hitachi F-7000). The optical properties of all catalysts were detected on a Shimadzu UV-2600 spectrophotome ter. The signals for DMPO-O 2 and DMPO- OH was detected on a Bruker ESR 300E spectrometer.
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2.4. Photocatalytic tests Photocatalytic decomposition of industrial dye RhB or antibiotic TC was conducted in a photocatalytic reactor under the illumination of visible light from a 300 W Xe lamp (Beijing NBET Technology, HSX-UV300) with a 400 nm cutoff filter [52–54]. Briefly, the catalyst (50 mg) was introduced into a RhB (10 mg/L, 100 mL), or TC (15 mg/L, 50 mL) solution. Before light irradiation, the mixed solution was continuously stirred for half an hour in darkness to reach the adsorption-desorption equilibrium. During the illumination, the reactor was controlled at room temperature with the assistance of flowing water. Finally, the samples withdrawn from the reaction solution at certain intervals were examined by detecting the absorbance at the characteristic band (554 nm for RhB, and 357 nm for TC) using an UV-2600 spectrophotometer. Total organic carbon (TOC) values during RhB (30 mg/L, 250 mL) or TC (30 mg/L, 250 mL) degradation over 150 mg of 0.6Br-Bi, BiOCOOH, or BiOBr were recorded and analyzed to evaluate the mineralization capability of the samples. 3. Results and discussion 3.1. Characterization The XRD patterns of all obtained samples are displayed in Fig. 1. The diffraction peaks of pure BiOCOOH, and BiOBr are in accordance with the BiOCOOH standard data (JCPDS 35-0939) [42], and BiOBr standard data (JCPDS 09-0393) [47]. For BiOBr/BiOCOOH heterojunctions, except for the peaks belonging to BiOCOOH, the diffraction peaks assigned to the (1 0 1), (1 1 0), (2 1 2) planes of BiOBr (JCPDS card No. 09-0393) can be found, indicating the existence of BiOBr phase. Additionally, the intensity of characteristic peaks for BiOBr gradually becomes stronger with increasing the molar ratio of KBr to BiOCOOH. No signals of other crystals in the XRD pattern were detected, implying the high purity of the products. Thus, the as-prepared BiOBr/BiOCOOH heterojunctions consist of the BiOBr and BiOCOOH two phases. The morphology information on BiOCOOH, and BiOBr/BiOCOOH heterojunctions were obtained from SEM and TEM images (Fig. 2). Pristine BiOCOOH is composed of micro-spherical assembly (diameter: 1.6–3.5 lm) (Fig. 2a). The in-situ generation of BiOBr depositing on the surface of BiOCOOH has only marginal effect on the
Fig. 1. XRD patterns of BiOCOOH, BiOBr, and BiOBr/BiOCOOH heterojunctions (0.1Br-Bi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi).
morphology of the heterojunctions, which all display the microspherical morphology with diameters in the range of 1.5–3.5 lm (Figs. S1 and 2). Specifically, 0.6Br-Bi shows a similar hierarchical structure to the BiOCOOH (Fig. 2b). The nanoscale morphology of 0.6Br-Bi was further visualized by using TEM, which demonstrates the flower-like morphology that is built from numerous nanoplates (Fig. 2c). Fig. S2 shows the high-resolution TEM image of 0.6Br-Bi, where two adjacent lattice fringes with distances of 0.272 nm and 0.283 nm corresponds to the (1 1 0) plane of BiOCOOH and the (1 0 2) plane of BiOBr, respectively. Additionally, the presence of BiOBr was further verified with EDS spectra (Fig. 3), EDX elemental mapping (Fig. 2d–h), as well as the XRD pattern (Fig. 1). It is evident that there is a homogeneous distribution of Bi, Br, O, and C elements in 0.6Br-Bi, implying the successful fabrication of BiOBr/BiOCOOH. The light absorption of BiOBr, BiOCOOH, and BiOBr/BiOCOOH heterojunctions were examined by UV–Vis DRS and the result is displayed in Fig. 4a. BiOCOOH displays intense photo-absorption in the UV region with a distinct absorption edge about 370 nm (band gap: 3.40 eV), reflecting that BiOCOOH can hardly absorb visible light, which accords well with the reported results [40,42,55]. In contrast, BiOBr exhibits effective photo-absorption in the visible light region, and its absorption edge is approximately 462 nm (band gap: 2.68 eV), in line with the previous results [47]. Encouragingly, the as-obtained BiOBr/BiOCOOH heterojunctions exhibit intense visible-light absorption. On the basis of these results, the band positions of BiOCOOH and BiOBr were estimated by the empirical formula.
EVB ¼ X E0 þ 0:5Eg
ð1Þ
ECB ¼ EVB Eg
ð2Þ
Here Eg, EVB, E0, ECB and X separately represent the band gap, valence band potential, the energy of free electrons (4.5 eV), conduction band potential, and the electronegativity of the semiconductor. Accordingly, the ECB and EVB of BiOCOOH are estimated as –0.67 and 2.73 eV [40,42], while those of BiOBr are 0.34 and 3.02 eV [49]. 3.2. Photocatalytic property Toxic organic contaminants, including industrial dye RhB and antibiotic TC, were selected as the targets to determine the photocatalytic properties of BiOBr/BiOCOOH heterojunctions under the irradiation of visible light. The degradation curves of RhB are presented in Fig. 5a. Blank test revealed that no RhB broke down without the catalyst after 1 h of visible-light illumination. Pristine BiOCOOH displayed a very low photodegradation efficiency of RhB (12.7%) because of its wide band gap. Under the same condition, approximately 51.6% of RhB was degraded by using pristine BiOBr as the catalyst in 60 min of reaction. Comparatively, the as-prepared BiOBr/BiOCOOH heterojunctions exhibited markedly enhanced photocatalytic performance, which is mainly attributed to the unique hierarchical hetero-structure that realizes the rapid charge transfer and thus significantly impedes the recombination of electrons and holes. Specifically, 0.6Br-Bi displayed the highest photocatalytic property, in which 100% of RhB was degraded in 50 min. Moreover, the BET surface areas of the samples were measured and the results were shown in Table S1. It is noted that the BET surface areas of BiOCOOH, 0.1Br-Bi, 0.3Br-Bi, 0.6Br-Bi, and 0.9Br-Bi are 26.13, 27.58, 28.36, 27.74, and 25.62 m2g1, respectively. Since the BET surface areas of 0.6Br-Bi is not the largest among these samples, the surface area does not dominantly account for the photocatalytic performance of BiOBr/BiOCOOH. Besides, the RhB photodegradation efficiency achieved by using the physically mixed sample (63 wt% BiOBr + 37 wt% BiOCOOH) was much lower
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Fig. 2. SEM images of BiOCOOH (a) and 0.6Br-Bi (b); TEM image of 0.6Br-Bi (c); EDX elemental mapping images of 0.6Br-Bi (d–h).
Fig. 3. EDS spectrum of 0.6Br-Bi.
Fig. 4. UV–Vis DRS of all obtained samples.
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Fig. 6. Photodegradation of TC by various samples under visible light.
photocatalytic activity of 0.6Br-Bi in decomposing toxic pollutants, suggesting its huge potential for practical application in wastewater treatment. To evaluate the mineralization capability of 0.6Br-Bi, the TOC values during the degradation of RhB (30 mg/L, 250 mL) and TC (30 mg/L, 250 mL) were monitored and analyzed. Fig. 7a presented
Fig. 5. (a) Photodegradation of RhB by BiOBr, BiOCOOH, BiOBr/BiOCOOH heterojunctions and the physical mixture of BiOBr and BiOCOOH under visible light; (b) Photodegradation kinetics of RhB.
than that of 0.6Br-Bi, reflecting that the formation of tightly contacted interface is favorable for enhancing the activity. Additionally, the reaction kinetics of the RhB degradation process was studied by employing the Langmuir-Hinshelwood (L-H) model. It was observed that the reaction process conformed to the apparent first-order model (Fig. 5b). The k values for various catalysts were presented in the following order: 0.6Br-Bi > 0.3Br-Bi > 0.9BrBi > 0.1Br-Bi > BiOBr > Mixture > BiOCOOH. Remarkably, the k value of 0.6Br-Bi (0.0846 min1) is the largest, about 37.2, 6.3, and 7.1 folds higher than that of pristine BiOCOOH (0.0022 min1), BiOBr (0.0116 min1), or the mixed sample (0.0105 min1). The photocatalytic performance of 0.6Br-Bi was further compared to other catalysts such as commercial TiO2 (P25), Ag2O/ TaON [56], CNT/BiOI [57] and Ag/BiOCOOH [58] (Fig. S3). Obviously, 0.6Br-Bi had a much higher RhB degradation efficiency than other photocatalysts, demonstrating the powerful photocatalytic capability. To further examine the photooxidation ability of 0.6Br-Bi, TC antibiotic, which can cause reproductive abnormalities to humans, was chosen as the photodegradation model. As depicted in Fig. 6, approximately 83.7% of TC was degraded in 120 min of reaction for 0.6Br-Bi, while only 4.4%, 31.6%, and 25.8% of TC were degraded for BiOCOOH, BiOBr, and the mixture. The results validate the high
Fig. 7. (a) The TOC removal rate of RhB over the as-prepared samples after 4 h irradiation, (b) recycling tests for photodegradation of RhB over 0.6Br-Bi.
S. Li et al. / Journal of Colloid and Interface Science 548 (2019) 12–19
the TOC removal rate of RhB over the as-prepared samples after 4 h of irradiation. As revealed, the mineralization ratio of RhB for 0.6Br-Bi (87.3%) is 14.3 times of that for BiOCOOH (6.1%) and 3.1 times of that for BiOBr (27.9%). Similarly, 0.6Br-Bi also achieved a high mineralization ratio of TC (72.4%) after 4 h of reaction (Fig. S4). These results manifest the strong mineralization capability of 0.6Br-Bi. Photostability of the photocatalyst is a pivotal factor for the practical application, thus the recycling experiments were implemented to assess the stability of 0.6Br-Bi. As shown in Fig. 7b, the photocatalytic activity of 0.6Br-Bi displayed no significant deactivation (only approximately 17.2% loss) after ten consecutive runs for photodegrading RhB. Furthermore, the XRD pattern of the recycled 0.6Br-Bi was almost unchanged, compared to that of the fresh one (Fig. S5). The results demonstrate the excellent stability of 0.6Br-Bi, bodying for its huge potential value in environmental remediation. 3.3. Photocatalytic mechanism In order to determine the exact roles of possibly produced active species and further clarify the photocatalytic mechanism of BiOBr/BiOCOOH, the trapping experiments were conducted. As shown in Fig. 8a, the RhB degradation efficiency by using 0.6BrBi in 60 min of reaction was 100% without any quenchers. For comparison, the introduction of IPA brought about the partial inhibition of the degradation efficiency, indicating that OH exerts a minor influence on RhB removal. Instead, the involvement of BQ or AO pronouncedly suppressed RhB decomposition, with RhB degradation efficiency of only 70.7% or 87.5%. The results assure + the critical roles of O 2 and h species in the photodegradation process over BiOBr/BiOCOOH. The electron spin resonance (ESR) spin-trap with DMPO technique was further employed to identify the active species formed by 0.6Br-Bi under visible light illumination (Figs. 8b and S2) [59]. Apparently, signals of DMPO-O 2 were detected under the irradiation of visible light, reflecting that O 2 radicals were produced during the photocatalytic process (Fig. 8b). However, no signals of DMPO-OH appeared, indicating that no OH was generated (Fig. S6). Therefore, the trapping experiments and ESR tests + demonstrate that O 2 and h species dominantly account for the pollutant degradation over BiOBr/BiOCOOH. The photoluminescence (PL) spectra were employed to further inspect the separation and transfer rate of photo-generated charge carriers, which was closely related to the photocatalytic property
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of the photocatalyst [11,60,61]. Generally, the weak PL intensity represents the boosted separation process and thus the improved photocatalytic activity [62–64]. Fig. 9 shows the PL spectra of pure BiOCOOH and BiOBr/BiOCOOH heterojunctions under the excitation wavelength of 300 nm. Pure BiOCOOH reflects a strong emission peak centered at ca. 370 nm [42,45]. For BiOBr/BiOCOOH heterojunctions, their PL spectra are similar to that of BiOCOOH, but the PL peak intensity is pronouncedly decreased, revealing the efficient separation and transfer of charge carriers through the phase interface. Of note, 0.6Br-Bi possesses the lowest PL intensity, which reflects the possibly lowest charge recombination rate and thereby signifies the highest photocatalytic activity. On the basis of the above-mentioned analyses, two major factors are supposed to account for the superior photocatalytic performance of BiOBr/BiOCOOH. On the one hand, the hierarchical architectures of BiOBr/BiOCOOH (Figs. 2 and S1) with numerous pores can facilitate the absorption of light through multiple reflection and offer plenty of transport paths and reactive sites, boosting the degradation of contaminants [9,11]. On the other hand, the BiOBr/BiOCOOH p-n heterojunction established is a fascinating architecture for charge transfer and separation. A schematic diagram of the photocatalytic mechanism of BiOBr/BiOCOOH under
Fig. 9. PL spectra of BiOCOOH and BiOBr/BiOCOOH heterojunctions.
Fig. 8. (a) Influences of various scavengers on RhB degradation performance by using 0.6Br-Bi, (b) DMPO spin-trapping ESR spectra in methanol dispersion of 0.6Br-Bi for DMPO-O 2.
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Fig. 10. Proposed photocatalysis mechanism of BiOBr/BiOCOOH p-n heterojunction photocatalyst toward toxic pollutant under visible light.
the illumination of visible light is brought up and depicted in Fig. 10. The Fermi level (EF) and CB of n-type BiOCOOH are more negative than those of p-type BiOBr prior to the establishment of the BiOBr/BiOCOOH p-n heterostructure. As the p-n heterostructure is created between BiOCOOH and BiOBr, the EF of BiOBr drifts upward and that of BiOCOOH moves downward until the EF level of two constituents reaches an equilibrium. Correspondingly, the band positions of BiOBr and BiOCOOH move along with the shift of EF. Consequently, the CB and VB potentials of BiOCOOH are more positive than those of BiOBr, leading to the formation of the internal electric field. Such type-II band alignment created in BiOBr/ BiOCOOH with internal electric field is favorable to the separation of electron-hole pairs [10,39,65,66]. BiOCOOH cannot respond to visible light, and thus no electron-hole pairs can be produced in BiOCOOH under the irradiation of visible light, whereas BiOBr can be driven by visible light to produce electrons and holes. The photo-induced electrons in the CB of BiOBr could be readily injected into that of BiOCOOH, leaving the photo-induced holes store in the VB of BiOBr. This process profoundly promotes the separation of electrons and holes, further alleviating the photodegradation ability [7,67,68]. Meanwhile, the assembling electrons on CB of BiOCOOH participate in the reduction of O2 for the formation of O 2 radicals. As a result, the active oxidizing species of O2 , and + h mainly participate in the decomposition of toxic contaminants (Fig. 8). Benefiting from the above merits, BiOBr/BiOCOOH p-n heterojunctions furnishes powerful photocatalytic capability in removal of refractory pollutants. 4. Conclusions In summary, flower-like BiOBr/BiOCOOH p-n heterojunctions with tightly interfacial contact were constructed via a readily achievable in-situ anion-exchange approach. Compared to pristine BiOCOOH and BiOBr, the VLD photocatalytic activities of BiOBr/ BiOCOOH heterojunctions are consumedly strengthened. The 0.6Br-Bi heterojunction has the optimal photocatalytic ability in degrading the RhB dye and TC antibiotic. The superior photocatalytic activity of BiOBr/BiOCOOH could be ascribed to the optimized light absorption and the suppressed recombination of photo-generated electrons and holes, resulting from the establishment of p-n heterojunction between BiOCOOH and BiOBr. Moreover, 0.6Br-Bi owns high stability and strong mineralization capability, and thus can be expected to serve as a good candidate
for wastewater treatment. This work affords a facile strategy to achieve hierarchical p-n heterojunction photocatalyst with powerful capability for environmental remediation. Acknowledgments This work has been financially supported by the National Natural Science Foundation of China (51708504), the Public Projects of Zhejiang Province (LGN18E080003), Zhejiang Provincial Natural Science Foundation of China (LQ17C030001), the Science and Technology Project of Zhoushan (2017C41006), and the Key Subject Open Fund of Zhejiang Province (20140205). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.04.024. References [1] L. Zhang, C. Lin, D. Zhang, L. Gong, Y. Zhu, Q. Xu, H. Li, Z. Xia, Adv. Mater. (2018), https://doi.org/10.1002/adma.201805252. [2] P. Singh, A. Borthakur, J. Clean. Prod. 196 (2018) 1669–1680. [3] D. Jassby, T.Y. Cath, H. Buisson, Nat. Nanotechnol. 13 (2018) 670–672. [4] Y. Jia, S. Li, J. Gao, G. Zhu, F. Zhang, X. Shi, Y. Huang, C. Liu, Appl. Catal., B 240 (2019) 241–252. [5] Y. Liu, G.Q. Zhu, J.Z. Gao, R.L. Zhu, M. Hojamberdiev, C.H. Wang, X.M. Wei, P. Liu, Appl. Catal., B 205 (2017) 421–432. [6] G. Zhang, L. Lin, G. Li, Y. Zhang, A. Savateev, S. Zafeiratos, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 57 (2018) 9372–9376. [7] S. Chen, Y. Qi, C. Li, F. Zhang, Joule 2 (2018) 2260–2288. [8] S. Li, S. Hu, W. Jiang, Y. Zhou, J. Liu, Z. Wang, J. Colloid Interface Sci. 530 (2018) 171–178. [9] Y. Ye, Z. Zang, T. Zhou, F. Dong, S. Lu, X. Tang, W. Wei, Y. Zhang, J. Catal. 357 (2018) 100–107. [10] R. Ye, J. Zhao, B.B. Wickemeyer, F.D. Toste, G.A. Somorjai, Nature Catal. 1 (2018) 318–325. [11] S. Li, X. Shen, J. Liu, L. Zhang, Environ. Sci. Nano 4 (2017) 1155–1167. [12] S.A. Khaneghah, A. Habibi-Yangjeh, D. Seifzadeh, J. Taiwan Inst. Chem. Eng. 87 (2018) 98–111. [13] J. Zheng, F. Chang, M. Jiao, Q. Xu, B. Deng, X. Hu, J. Colloid Interface Sci. 510 (2018) 20–31. [14] Y. Wang, H. Wang, A. Xu, Z. Song, J. Mater. Sci.: Mater. Electron. 29 (2018) 16691–16701. [15] N.-C. Zheng, T. Ouyang, Y. Chen, Z. Wang, D.-Y. Chen, Z.-Q. Liu, Catal. Sci. Technol. 9 (2019) 1357–1364. [16] X. Li, W. Zhang, J. Li, G. Jiang, Y. Zhou, S. Lee, F. Dong, Appl. Catal., B 241 (2019) 187–195. [17] J. Li, W. Zhang, M. Ran, Y. Sun, H. Huang, F. Dong, Appl. Catal., B 243 (2019) 313–321.
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