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Facet engineered TiO2 hollow sphere for the visible-light-mediated degradation of antibiotics via ligand-to-metal charge transfer
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Facet engineered TiO2 hollow sphere for the visible-light-mediated degradation of antibiotics via ligand-to-metal charge transfer Sai Zhanga, Zhengliang Yina, Liangxu Xiec, Jianjian Yib, Wenjie Tanga, Tao Tanga, Jinyu Chena, Shunsheng Caoa,∗ a
Research School of Polymer Materials, School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, China c Institute of Bioinformatics and Medical Engineering, School of Electrical and Information Engineering, Jiangsu University of Technology, Changzhou, 213001, China b
ARTICLE INFO
ABSTRACT
Keywords: 101}/{001} facet heterojunction Ligand-to-metal charge transfer Hollow TiO2 Photocatalysis
Efficient removal of tetracycline (TC) under visible-light irradiation over TiO2-based photocatalysts remains a challenge based on the fact that the reported photocatalytic systems still suffer from weak visible-light absorption and/or inefficient charge separation. Herein, we constructed {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT) via a gentle NaF treatment, in which the hollow mesoporous feature can trap incident light for a long time to improve photons efficiently. Meanwhile, the as-formed facet heterojunction significantly facilitates the charge separation. As a result, the 001-HT exhibits a high removal rate (~90.1%) of TC under visible-light irradiation, beyond the values of many reported TiO2-based photocatalysts. Most importantly, we further expound the ligand-to-metal charge transfer mechanism towards TiO2-assisted degradation of TC under visible-light irradiation, which effectively clarifies the confusion about the origin of pure TiO2 visible-light activity towards TC degradation because both TiO2 and TC do not exhibit any visible-light catalytic activity. Therefore, this work provides a new insight in revealing the mechanism of visible-light-mediated TC degradation over pure TiO2 photocatalyst.
1. Introduction Residual tetracycline (TC) in nature seriously threatens the ecological environment due to its high toxicity and persistence [1,2]. Although TiO2-assisted photocatalysis is a promising tool to remove refractory TC [3–7], the efficient degradation of TC only can be achieved under strong light irradiation (e.g., ultraviolet light, simulated solar light, etc.) because the robust macromolecular ring structure of TC is hard to be broken [8–10]. Therefore, how to efficiently degrade TC under visible-light irradiation over TiO2-based photocatalysts remains a great challenge. To construct a visible-light TiO2 photocatalyst with high-performance, both strong visible-light absorption and efficient charge separation are required [11–13]. However, previous investigations have proved that the existed doping/modifying strategies are too difficult to realize such requirements because excess dopants or additives act as electron-hole recombination centers, decreasing the visible-light activity of TiO2 [14,15]. Building a heterojunction with another semiconductor seems to be an effective method to improve the charge separation [16,17], however, the high quality interface is required to ∗
achieve efficient charge transfer since the interfacial defects result in the formation of new electron/hole recombination centers, evidently increasing the difficulty of material preparation [18]. By contrast, in situ formation of phase or facet heterojunction in single semiconductor avoids such a drawback (e.g., P25 and {101}/{001} facets co-exposed TiO2) [19,20]. Compared with phase heterojunction, facet heterojunction shows a higher potential in improving photocatalytic efficiency because facets can significantly affect the adsorption and activation of reactants based on the different surface atomic configuration and coordination [21]. Especially, the high-energy {001} facet of TiO2 exhibits higher photocatalytic performance than low-energy {101} facet [22]. For instance, Zhao et al. proved that dioxygen was more strongly adsorbed on {001} facet rather than {101} facet of TiO2, and thus promoted the reduction of dioxygen [23]. However, the previously reported facet engineered TiO2 were bulk nanoparticles with extremely low surface area (~2.8 m2 g−1) [22–24], leading to a few active sites. In addition, harsh synthesis conditions (e.g., high temperature, high pressure and HF) were required to prepare the exposed {001} facet [25,26]. Therefore, exploring a facile method to construct a facet heterojunction of TiO2 with high surface area is promising for the efficient
Corresponding author. E-mail address: [email protected] (S. Cao).
https://doi.org/10.1016/j.ceramint.2019.12.142 Received 14 November 2019; Received in revised form 6 December 2019; Accepted 14 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sai Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.142
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removal of TC under visible-light irradiation. Here, we synthesize {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT) via a gentle NaF treatment, effectively overcoming the requirement of harsh experimental conditions. The hollow/mesoporous structure not only can boost the accessibility of reactants because of its high surface area, but also trap incident light for a long time to efficiently realize the use of photons. Besides, the {101}/{001} facets heterojunction significantly facilitates the charge separation, resulting in an efficient removal (90.1%) of TC under visible-light irradiation, beyond the values of the most TiO2-based photocatalysts (Table S1). Especially, we detail the process of the visible-light-mediated degradation of TC because both TiO2 and TC do not exhibit any visiblelight catalytic activity. Accordingly, we further ascertain the charge transfer mechanism between TC and TiO2. Therefore, this work provides a new insight in revealing the visible-light-mediated mechanism of TC over pure TiO2 photocatalyst.
filtered through 1.22 μm Millipore filter. The filtrates were analyzed by UV–vis spectra using Shimadzu UV-2600 spectrophotometer (λ = 357 nm [3]). After each photodegradation experiment, the sample was separated by centrifugation, washed with distilled water, and ultimately dried at 50 °C for the next test. 3. Results and discussion 3.1. Material synthesis and characterization The fabrication process of 001-HT is illustrated in Fig. 1a. A gentle NaF treatment was selected for the formation of {001} facet on TiO2 hollow sphere at room temperature, avoiding the broken of TiO2 hollow structure. Briefly, the CPS@TiO2 core-shell composites were firstly dispersed into NaF aqueous solutions (10 mM) at pH 3.5 adjusted by HCl. Such acid environment promotes the ligand exchange (≡Ti–OH + F−→≡Ti–F + OH−) on the surface of CPS@TiO2 [29], which is confirmed by the FT-IR spectra of 001-HT exhibiting a weaker peak of O–H stretching vibrations at 3364 cm−1 after NaF treatment (Fig. S1). As a result, {001} facet is easy to be exposed on the surface of TiO2 hollow sphere because of the decreased surface energy after the fluorination [30]. In comparison with hollow TiO2 without NaF treatment (inset of Fig. 1b), the 001-HT holds a rougher surface (inset of Fig. 1e), whilst 001-HT is observed to be composed of relatively larger crystalline grains (Fig. 1c and f) because the surface fluorination promotes the growth and crystallization of TiO2 [31], which is further ascertained by BET measurement (e.g., higher surface area and bigger pore diameter (Fig. S2)). High-resolution TEM (HRTEM) image of HT (Fig. 1d) shows that the clear lattice fringes with spacing of 0.35 nm correspond to the (101) planes, indicating that {101} facet is exposed on the surface of HT [28]. The 001-HT (Fig. 1g) with lattice spacing of 0.235 nm is indexed to the (004) planes, demonstrating the exposure of {001} facet on the surface of 001-HT [28]. Moreover, the lattice fringes of 0.35 and 0.47 nm are ascribed to the (101) and (002) planes, respectively, suggesting that the bottom surface exposed by truncation is bound by a {001} facet [32]. In addition, their angle value is 68.3°, which is consistent with the theoretical value of the angle between {101} and {001} facets [32]. All results ascertain that {001} and {101} facets have been formed in 001-HT sample. The crystallographic structures of HT and 001-HT were further studied by XRD. Fig. 2a shows that both HT and 001-HT are anatase that agrees with standard crystal of JCPDS (No. 21-1272) [33]. Additionally, the 001-HT presents the stronger peaks (004) than HT, further revealing that 001-HT is preferentially oriented along the direction due to surface alignment with {001} facet [28,34]. X-ray photoelectron spectroscopy (XPS) was used to obtain the binding energies of HT and 001-HT. From XPS peak of F 1s (Fig. 2b), the disappeared signal of F 1s confirms that the surface fluoride specie of 001-HT has been removed completely after calcination (450 °C). For 001-HT, the XPS peaks of Ti 2p and O 1s both manifest a positive shift (Fig. 2c and d for Ti 2p and O 1s, respectively) than corresponding peaks of HT, which are attributed to the increased surface oxygen vacancies on {001} facet owing to the lower coordination number of Ti with oxygen [32,35].
2. Experimental 2.1. Reagents and chemicals Tetrabutyl titanate (TBT) and hexadecyl trimethyl ammonium bromide (CTAB) were obtained from J&K Chemical Company Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co, Ltd. (China). 2.2. Synthesis of catalysts The CPS@TiO2 core-shell composites prepared according our previous works [15,27] were re-dispersed into NaF aqueous solution (10 mM) at pH 3.5 adjusted by HCl solution to exchange ligands (≡Ti–OH + F−→≡Ti–F + OH−) for 30 min [28]. The sample was centrifuged, washed with ethanol, dried at 60 °C and finally calcinated at 450 °C for 2 h under air atmosphere, preparing the {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT). For comparison, the {101} facet-exposed TiO2 hollow sphere were obtained without NaF treatment (HT). 2.3. Characterizations Power X-ray diffraction (XRD) patterns of the samples were collected on a Rigaku Ultima IV diffractometer. The morphology of the samples was investigated using JSM-7800F Schottky Field Emission Scanning Electron Microscope (SEM) with an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were achieved with JEOL JEM-2100 high-resolution transmission electron microscopes. The specific surface area and pore size analysis were performed by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods through a micromeritics ASAP 2460. The solid diffuse reflection spectrum was collected via a Shimadzu UV-2600 spectrophotometer. Photocurrent was conducted in 0.5 M Na2SO4 aqueous solution (purified with nitrogen gas) using an electrochemical workstation (CHI-660E electrochemical analyzer, China) at 25 °C. 2.4. Photocatalytic degradation experiments
3.2. Photocatalytic performances
10 mg sample was dispersed into a quartz photoreactor containing 50 mL of TC solution (10 mg L−1). A 300 W Xenon lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd.) equipped with a 420 nm cutoff filter was employed as visible light source. The solution was stirred magnetically in dark for 30 min to achieve adsorption-desorption equilibrium before visible-light irradiation. At the given time intervals, 3 mL of the suspension was withdrawn, centrifuged and then
In order to demonstrate the positive role of {001} facet, the photocatalytic degradation of TC over HT and 001-HT was carried out under visible-light irradiation. Fig. 3a shows that TC itself is extremely hard to be degraded under visible-light irradiation. After 20 min of visible-light irradiation, HT degrades ~35.5% TC, while 001-HT exhibits ~74.4% degradation rate of TC, suggesting twice higher than
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Fig. 1. The fabrication process of HT and 001-HT (a), SEM, TEM and HRTEM images of HT (b–d) and 001-HT (e–g).
that of HT. When the irradiation time is prolonged to 120 min, the degradation of TC over 001-HT is as high as ~90.1%, beyond most TiO2-based photocatalysts including HT without NaF treatment (Table S1). The excellent activity will become clear when the higher concentration of TC is used. The results show that 001-HT still maintains a high degradation (~81%) of TC even at high concentration of TC (40 mg L−1) after 120 min under visible-light irradiation (Fig. 3b), while only ~52% TC is degraded over HT. The enhanced photocatalytic activity of 001-HT is further investigated by evaluating the degradation of tetracycline hydrochloride (TCH) and oxytetracycline (OTC) under visible-light irradiation (Fig. 3c and d). the results show that the higher removal rates of TCH and OTC over 001-HT are achieved than HT, strongly confirming the positive effect of the {001} facet. The stability of photocatalyst determines its potential application [36–38]. Previous studies have proved that doping elements or decorating additional components are unstable because they are often reduced, oxidized or falling off during photocatalysis [14], resulting in the decreased photocatalytic performance under long-time irradiation. By contrast, the degradation of TC keeps almost changeless even after five cycles (Fig. 3e) and the hollow structure remains very well (Fig. S3), indicating a remarkable recyclability.
3.3. Visible-light-mediated TC degradation mechanism The active species generated in photocatalytic process are of significance to understand the mechanism of visible-light-mediated degradation of TC [3]. Thus, various scavenger agents including N2, triethanolamine (TEOA) and t-butanol (t-BuOH) were used to capture superoxide radicals (·O2−), holes (h+) and hydroxyl radicals (·OH), respectively (Fig. 4a and b) [4,21]. The addition of t-BuOH shows negligible influence on TC degradation, suggesting that ·OH does not play a main role in degrading TC. Interestingly, the removal rate of TC is increased slightly (from 80.5% to 82.7%) after adding TEOA, which is probably responsible for the fact that TEOA increases alkalinity (pH value changed from 5.4 to 9) of the photoreaction system, promoting the self-degradation of TC [39]. By contrast, the degradation rate of TC is significantly inhibited when N2 is bubbled into the photocatalytic system, indicating the decisive role of ·O2− in removing TC. However, considering that TiO2 itself cannot absorb visible light due to the large bandgap (~3.2 eV), neither HT nor 001-HT can produce ·O2− under visible-light irradiation, which is further confirmed by electron spin resonance (ESR) measurement using spin-trap agents (5,5-diemthyl-1pyrroline N-oxide (DMPO)) in HT and 001-HT aqueous solution
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Fig. 2. XRD patterns (a), XPS spectra of F 1s (b), Ti 2p (c) and O 1s (d).
Fig. 3. Photocatalytic performances of HT and 001-HT for degrading TC (a, b), TCH (c) and OTC (d); Cycle test of degrading TC over 001-HT (e) (λ > 420 nm; Cat.: 0.2 mg mL−1; TC: 10 and 40 mg L−1, TCH and OTC: 10 mg L−1).
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Fig. 4. Degradation rate of TC over 001-HT with different scavengers (λ > 420 nm, Cat.: 0.2 mg mL−1, TC: 40 mg L−1, scavengers: 1 mmol) (a, b); ESR spectra of DMPO-·O2− (c) and DMPO-·OH (d) adducts in various samples aqueous dispersion.
(Fig. 4c). Accordingly, TC may be involved in the production of ·O2− during photoreaction process because TC has a small energy gap (~2.33 eV) between HOMO and LUMO according to density functional theory (DFT) calculation [40]. Briefly, TC molecules adsorbed on the surface of TiO2 will sensitize TiO2 and shift the ultra-violet of TiO2 to visible-light-driven photocatalysis, which is convincedly confirmed by ESR test (the strong signal of ·O2− in Fig. 4c) [41] for the collected samples (denoted as HT/TC and 001-HT/TC) after the adsorption-desorption equilibrium. Moreover, the peak intensity of ·O2− in 001-HT/ TC system is stronger than that of HT/TC, suggesting the more ·O2− generated in 001-HT/TC system [42]. Incidentally, no ·OH is detected in either HT/TC or 001-HT/TC systems (Fig. 4d). These results ascertain that the formation of TiO2 and TC complexes leads to visible-lightmediated degradation of TC through O2− pathway. To deepen the understanding of visible-light-mediated degradation of TC, the sensitizing mechanism was further investigated in this work, as illustrated in Fig. 5a and b. Generally, organic species can sensitize TiO2 via two pathways including dye sensitization and ligand-to-metal charge transfer (LMCT) [43]. Almost all workers including S. Leong [8] and S. Wu [44] insisted that the dye sensitization, in which electron was photoexcited from HUMO to LUMO of TC and further transferred to the conduction band (CB) of TiO2 [45], was responsible for the TC degradation under visible-light irradiation. Unfortunately, these previous works did not further expound the role of LMCT in degrading TC, therefore, the further investigation is necessary to clarify the LMCT mechanism because the TiO2/TC composites also may take part in TC degradation. Based on LMCT sensitizing mechanism, the photoexcited
electron in HUMO of TC is transferred directly to the CB of TiO2 due to the formation of TiO2/TC composite [43], which is strongly confirmed by UV–vis absorption spectra of HT and 001-HT after adsorbing TC, producing new absorption band and red shift (Fig. 5c) due to the LMCT process [41] between TC and surface-bond Ti (IV) ion (Fig. 6). The degradation performance of TC was further detailed by investigating the function of the incident light wavelength (λ0) to deeply understand the role of LMCT. Fig. 5d shows that the irradiation wavelength-dependent degradation rate of TC agrees well with the photo-absorption edge of LMCT rather than TC itself, strongly convincing that LMCT plays a decisive role in degrading TC via the visible-light-mediated mechanism, as shown in Fig. 6. In brief, the coordination composite of TC and TiO2 firstly is formed on surface of TiO2 via hydroxyl groups, and then photogenerated electrons from the ground state of TC are directly transferred to the CB of TiO2, leading to the production of ·O2− that drives the degradation of TC. The enhanced visible-light activity of 001-HT was further detailed as follow: firstly, the stronger intensity (Fig. 5c) of 001-HT/TC in the visible light range promotes the production of photogenerated electrons from TC because the number of unsaturated 5c-Ti atoms on the surface of {001} facet holds twice higher than {101} facet [28], generating more OH groups ascertained by Zeta potential (-38.47 and -21.9 mV for 001-HT and HT, respectively) and FT-IR (Fig. S4) [46,47] and thus forming more TiO2/TC composites. Secondly, Fig. 7a shows that the 001-HT/TC exhibits stronger photocurrent response and smaller arc radius than HT/TC under visible-light irradiation (λ > 420 nm), indicating the higher transfer efficiency of photogenerated electrons and
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Fig. 5. Illustration of dye sensitization (a) and LMCT sensitization (b); UV–vis absorption spectra of various samples (c); UV–vis absorption spectra of TC (40 mg L−1) and 001-HT/TC, and degradation rate of TC plotted as a function of wavelength of the incident light (a bandpass filter of λ ± 10 nm) (d).
3.4. Transformation intermediates and possible pathway The intermediates of TC were detected by mass spectrometry during the degradation process and a possible degradation pathway was proposed, as shown in Fig. 8. The TC is firstly transferred to TC1 through the demethylation of the dimethylamino group and replacement of the acylamino by the carbinol group [52]. TC1 is further transferred to TC2 by dehydration and the loss of carbinol and acylamino group, respectively [52]. Then, TC2 undergoes cleavage of the carboatomic ring, generating TC3, which is subjected to decarboxylation and transferred to TC4. Ultimately, TC5 is generated by another cleavage of the carboatomic ring in TC4 and the above ring-opening products are decomposed into CO2 and H2O. Most importantly, the polyhydric intermediates can also form complexes with TiO2 via their hydroxy group to induce LMCT. Therefore, the degradation can be maintained until the intermediates cannot form stable complex with TiO2. As a result, after 120 min under visible-light irradiation, the solution holds a high concentration of TC5 because LMCT can not be initiated.
Fig. 6. The mechanism of visible-light-mediated TiO2 photocatalyzed TC degradation.
4. Conclusion
stronger LMCT efficiency [48], which are further ascertained by photoluminescence emission (prolonged lifetime). The enhanced charge separation and prolonged lifetime are mainly ascribed to the formation of {101}/{001} facet heterojunction (Fig. 7d) that have been demonstrated by previous works and computational results [49–51]. It is worth mentioning that only 001-HT can degrade TC completely when the TiO2 itself is excited by simulated solar-light irradiation (Fig. S5), proving the positive effect of facet heterojunction.
In summary, a {101} and {001} facets co-exposed TiO2 hollow photocatalyst has been successfully synthesized via a gentle NaF treatment. The formation of coordination complexes between TC molecule and surface-bound Ti (IV) ion induces strong visible-light absorption via LMCT. The {001}/{101} facet heterojunction also facilitates the charge migration, extending the lifetime of photogenerated
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Fig. 7. Transient photocurrent performance of various samples under visible light (a); EIS plots of HT and 001-HT (b); PL emission spectra (c); Illustration of {101}/ {001} facet heterojunction (d).
Fig. 8. Schematic diagram illustration of the proposed degradation pathway of TC.
electrons. As a result, 001-HT maintains a high charge separation efficiency and strong visible-light absorption, achieving 90.1% removal rate of TC under visible-light irradiation. This work opens a new sight for visible-light-mediated TiO2 photocatalysis.
Acknowledgements We appreciate the financial support provided for this research by the National Natural Science Foundation of China (21876069) and the Six Talent Peaks Project in Jiangsu (XCL-018).
Declaration of competing interest
Appendix A. Supplementary data
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.142. 7
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Facet engineered TiO2 hollow sphere for the visible-light-mediated degradation of antibiotics via ligand-to-metal charge transfer Sai Zhanga, Zhengliang Yina, Liangxu Xiec, Jianjian Yib, Wenjie Tanga, Tao Tanga, Jinyu Chena, Shunsheng Caoa,∗ a
Research School of Polymer Materials, School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, China c Institute of Bioinformatics and Medical Engineering, School of Electrical and Information Engineering, Jiangsu University of Technology, Changzhou, 213001, China b
ARTICLE INFO
ABSTRACT
Keywords: 101}/{001} facet heterojunction Ligand-to-metal charge transfer Hollow TiO2 Photocatalysis
Efficient removal of tetracycline (TC) under visible-light irradiation over TiO2-based photocatalysts remains a challenge based on the fact that the reported photocatalytic systems still suffer from weak visible-light absorption and/or inefficient charge separation. Herein, we constructed {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT) via a gentle NaF treatment, in which the hollow mesoporous feature can trap incident light for a long time to improve photons efficiently. Meanwhile, the as-formed facet heterojunction significantly facilitates the charge separation. As a result, the 001-HT exhibits a high removal rate (~90.1%) of TC under visible-light irradiation, beyond the values of many reported TiO2-based photocatalysts. Most importantly, we further expound the ligand-to-metal charge transfer mechanism towards TiO2-assisted degradation of TC under visible-light irradiation, which effectively clarifies the confusion about the origin of pure TiO2 visible-light activity towards TC degradation because both TiO2 and TC do not exhibit any visible-light catalytic activity. Therefore, this work provides a new insight in revealing the mechanism of visible-light-mediated TC degradation over pure TiO2 photocatalyst.
1. Introduction Residual tetracycline (TC) in nature seriously threatens the ecological environment due to its high toxicity and persistence [1,2]. Although TiO2-assisted photocatalysis is a promising tool to remove refractory TC [3–7], the efficient degradation of TC only can be achieved under strong light irradiation (e.g., ultraviolet light, simulated solar light, etc.) because the robust macromolecular ring structure of TC is hard to be broken [8–10]. Therefore, how to efficiently degrade TC under visible-light irradiation over TiO2-based photocatalysts remains a great challenge. To construct a visible-light TiO2 photocatalyst with high-performance, both strong visible-light absorption and efficient charge separation are required [11–13]. However, previous investigations have proved that the existed doping/modifying strategies are too difficult to realize such requirements because excess dopants or additives act as electron-hole recombination centers, decreasing the visible-light activity of TiO2 [14,15]. Building a heterojunction with another semiconductor seems to be an effective method to improve the charge separation [16,17], however, the high quality interface is required to ∗
achieve efficient charge transfer since the interfacial defects result in the formation of new electron/hole recombination centers, evidently increasing the difficulty of material preparation [18]. By contrast, in situ formation of phase or facet heterojunction in single semiconductor avoids such a drawback (e.g., P25 and {101}/{001} facets co-exposed TiO2) [19,20]. Compared with phase heterojunction, facet heterojunction shows a higher potential in improving photocatalytic efficiency because facets can significantly affect the adsorption and activation of reactants based on the different surface atomic configuration and coordination [21]. Especially, the high-energy {001} facet of TiO2 exhibits higher photocatalytic performance than low-energy {101} facet [22]. For instance, Zhao et al. proved that dioxygen was more strongly adsorbed on {001} facet rather than {101} facet of TiO2, and thus promoted the reduction of dioxygen [23]. However, the previously reported facet engineered TiO2 were bulk nanoparticles with extremely low surface area (~2.8 m2 g−1) [22–24], leading to a few active sites. In addition, harsh synthesis conditions (e.g., high temperature, high pressure and HF) were required to prepare the exposed {001} facet [25,26]. Therefore, exploring a facile method to construct a facet heterojunction of TiO2 with high surface area is promising for the efficient
Corresponding author. E-mail address: [email protected] (S. Cao).
https://doi.org/10.1016/j.ceramint.2019.12.142 Received 14 November 2019; Received in revised form 6 December 2019; Accepted 14 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sai Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.142
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removal of TC under visible-light irradiation. Here, we synthesize {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT) via a gentle NaF treatment, effectively overcoming the requirement of harsh experimental conditions. The hollow/mesoporous structure not only can boost the accessibility of reactants because of its high surface area, but also trap incident light for a long time to efficiently realize the use of photons. Besides, the {101}/{001} facets heterojunction significantly facilitates the charge separation, resulting in an efficient removal (90.1%) of TC under visible-light irradiation, beyond the values of the most TiO2-based photocatalysts (Table S1). Especially, we detail the process of the visible-light-mediated degradation of TC because both TiO2 and TC do not exhibit any visiblelight catalytic activity. Accordingly, we further ascertain the charge transfer mechanism between TC and TiO2. Therefore, this work provides a new insight in revealing the visible-light-mediated mechanism of TC over pure TiO2 photocatalyst.
filtered through 1.22 μm Millipore filter. The filtrates were analyzed by UV–vis spectra using Shimadzu UV-2600 spectrophotometer (λ = 357 nm [3]). After each photodegradation experiment, the sample was separated by centrifugation, washed with distilled water, and ultimately dried at 50 °C for the next test. 3. Results and discussion 3.1. Material synthesis and characterization The fabrication process of 001-HT is illustrated in Fig. 1a. A gentle NaF treatment was selected for the formation of {001} facet on TiO2 hollow sphere at room temperature, avoiding the broken of TiO2 hollow structure. Briefly, the CPS@TiO2 core-shell composites were firstly dispersed into NaF aqueous solutions (10 mM) at pH 3.5 adjusted by HCl. Such acid environment promotes the ligand exchange (≡Ti–OH + F−→≡Ti–F + OH−) on the surface of CPS@TiO2 [29], which is confirmed by the FT-IR spectra of 001-HT exhibiting a weaker peak of O–H stretching vibrations at 3364 cm−1 after NaF treatment (Fig. S1). As a result, {001} facet is easy to be exposed on the surface of TiO2 hollow sphere because of the decreased surface energy after the fluorination [30]. In comparison with hollow TiO2 without NaF treatment (inset of Fig. 1b), the 001-HT holds a rougher surface (inset of Fig. 1e), whilst 001-HT is observed to be composed of relatively larger crystalline grains (Fig. 1c and f) because the surface fluorination promotes the growth and crystallization of TiO2 [31], which is further ascertained by BET measurement (e.g., higher surface area and bigger pore diameter (Fig. S2)). High-resolution TEM (HRTEM) image of HT (Fig. 1d) shows that the clear lattice fringes with spacing of 0.35 nm correspond to the (101) planes, indicating that {101} facet is exposed on the surface of HT [28]. The 001-HT (Fig. 1g) with lattice spacing of 0.235 nm is indexed to the (004) planes, demonstrating the exposure of {001} facet on the surface of 001-HT [28]. Moreover, the lattice fringes of 0.35 and 0.47 nm are ascribed to the (101) and (002) planes, respectively, suggesting that the bottom surface exposed by truncation is bound by a {001} facet [32]. In addition, their angle value is 68.3°, which is consistent with the theoretical value of the angle between {101} and {001} facets [32]. All results ascertain that {001} and {101} facets have been formed in 001-HT sample. The crystallographic structures of HT and 001-HT were further studied by XRD. Fig. 2a shows that both HT and 001-HT are anatase that agrees with standard crystal of JCPDS (No. 21-1272) [33]. Additionally, the 001-HT presents the stronger peaks (004) than HT, further revealing that 001-HT is preferentially oriented along the direction due to surface alignment with {001} facet [28,34]. X-ray photoelectron spectroscopy (XPS) was used to obtain the binding energies of HT and 001-HT. From XPS peak of F 1s (Fig. 2b), the disappeared signal of F 1s confirms that the surface fluoride specie of 001-HT has been removed completely after calcination (450 °C). For 001-HT, the XPS peaks of Ti 2p and O 1s both manifest a positive shift (Fig. 2c and d for Ti 2p and O 1s, respectively) than corresponding peaks of HT, which are attributed to the increased surface oxygen vacancies on {001} facet owing to the lower coordination number of Ti with oxygen [32,35].
2. Experimental 2.1. Reagents and chemicals Tetrabutyl titanate (TBT) and hexadecyl trimethyl ammonium bromide (CTAB) were obtained from J&K Chemical Company Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co, Ltd. (China). 2.2. Synthesis of catalysts The CPS@TiO2 core-shell composites prepared according our previous works [15,27] were re-dispersed into NaF aqueous solution (10 mM) at pH 3.5 adjusted by HCl solution to exchange ligands (≡Ti–OH + F−→≡Ti–F + OH−) for 30 min [28]. The sample was centrifuged, washed with ethanol, dried at 60 °C and finally calcinated at 450 °C for 2 h under air atmosphere, preparing the {101} and {001} facets co-exposed TiO2 hollow sphere (001-HT). For comparison, the {101} facet-exposed TiO2 hollow sphere were obtained without NaF treatment (HT). 2.3. Characterizations Power X-ray diffraction (XRD) patterns of the samples were collected on a Rigaku Ultima IV diffractometer. The morphology of the samples was investigated using JSM-7800F Schottky Field Emission Scanning Electron Microscope (SEM) with an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were achieved with JEOL JEM-2100 high-resolution transmission electron microscopes. The specific surface area and pore size analysis were performed by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods through a micromeritics ASAP 2460. The solid diffuse reflection spectrum was collected via a Shimadzu UV-2600 spectrophotometer. Photocurrent was conducted in 0.5 M Na2SO4 aqueous solution (purified with nitrogen gas) using an electrochemical workstation (CHI-660E electrochemical analyzer, China) at 25 °C. 2.4. Photocatalytic degradation experiments
3.2. Photocatalytic performances
10 mg sample was dispersed into a quartz photoreactor containing 50 mL of TC solution (10 mg L−1). A 300 W Xenon lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd.) equipped with a 420 nm cutoff filter was employed as visible light source. The solution was stirred magnetically in dark for 30 min to achieve adsorption-desorption equilibrium before visible-light irradiation. At the given time intervals, 3 mL of the suspension was withdrawn, centrifuged and then
In order to demonstrate the positive role of {001} facet, the photocatalytic degradation of TC over HT and 001-HT was carried out under visible-light irradiation. Fig. 3a shows that TC itself is extremely hard to be degraded under visible-light irradiation. After 20 min of visible-light irradiation, HT degrades ~35.5% TC, while 001-HT exhibits ~74.4% degradation rate of TC, suggesting twice higher than
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Fig. 1. The fabrication process of HT and 001-HT (a), SEM, TEM and HRTEM images of HT (b–d) and 001-HT (e–g).
that of HT. When the irradiation time is prolonged to 120 min, the degradation of TC over 001-HT is as high as ~90.1%, beyond most TiO2-based photocatalysts including HT without NaF treatment (Table S1). The excellent activity will become clear when the higher concentration of TC is used. The results show that 001-HT still maintains a high degradation (~81%) of TC even at high concentration of TC (40 mg L−1) after 120 min under visible-light irradiation (Fig. 3b), while only ~52% TC is degraded over HT. The enhanced photocatalytic activity of 001-HT is further investigated by evaluating the degradation of tetracycline hydrochloride (TCH) and oxytetracycline (OTC) under visible-light irradiation (Fig. 3c and d). the results show that the higher removal rates of TCH and OTC over 001-HT are achieved than HT, strongly confirming the positive effect of the {001} facet. The stability of photocatalyst determines its potential application [36–38]. Previous studies have proved that doping elements or decorating additional components are unstable because they are often reduced, oxidized or falling off during photocatalysis [14], resulting in the decreased photocatalytic performance under long-time irradiation. By contrast, the degradation of TC keeps almost changeless even after five cycles (Fig. 3e) and the hollow structure remains very well (Fig. S3), indicating a remarkable recyclability.
3.3. Visible-light-mediated TC degradation mechanism The active species generated in photocatalytic process are of significance to understand the mechanism of visible-light-mediated degradation of TC [3]. Thus, various scavenger agents including N2, triethanolamine (TEOA) and t-butanol (t-BuOH) were used to capture superoxide radicals (·O2−), holes (h+) and hydroxyl radicals (·OH), respectively (Fig. 4a and b) [4,21]. The addition of t-BuOH shows negligible influence on TC degradation, suggesting that ·OH does not play a main role in degrading TC. Interestingly, the removal rate of TC is increased slightly (from 80.5% to 82.7%) after adding TEOA, which is probably responsible for the fact that TEOA increases alkalinity (pH value changed from 5.4 to 9) of the photoreaction system, promoting the self-degradation of TC [39]. By contrast, the degradation rate of TC is significantly inhibited when N2 is bubbled into the photocatalytic system, indicating the decisive role of ·O2− in removing TC. However, considering that TiO2 itself cannot absorb visible light due to the large bandgap (~3.2 eV), neither HT nor 001-HT can produce ·O2− under visible-light irradiation, which is further confirmed by electron spin resonance (ESR) measurement using spin-trap agents (5,5-diemthyl-1pyrroline N-oxide (DMPO)) in HT and 001-HT aqueous solution
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Fig. 2. XRD patterns (a), XPS spectra of F 1s (b), Ti 2p (c) and O 1s (d).
Fig. 3. Photocatalytic performances of HT and 001-HT for degrading TC (a, b), TCH (c) and OTC (d); Cycle test of degrading TC over 001-HT (e) (λ > 420 nm; Cat.: 0.2 mg mL−1; TC: 10 and 40 mg L−1, TCH and OTC: 10 mg L−1).
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Fig. 4. Degradation rate of TC over 001-HT with different scavengers (λ > 420 nm, Cat.: 0.2 mg mL−1, TC: 40 mg L−1, scavengers: 1 mmol) (a, b); ESR spectra of DMPO-·O2− (c) and DMPO-·OH (d) adducts in various samples aqueous dispersion.
(Fig. 4c). Accordingly, TC may be involved in the production of ·O2− during photoreaction process because TC has a small energy gap (~2.33 eV) between HOMO and LUMO according to density functional theory (DFT) calculation [40]. Briefly, TC molecules adsorbed on the surface of TiO2 will sensitize TiO2 and shift the ultra-violet of TiO2 to visible-light-driven photocatalysis, which is convincedly confirmed by ESR test (the strong signal of ·O2− in Fig. 4c) [41] for the collected samples (denoted as HT/TC and 001-HT/TC) after the adsorption-desorption equilibrium. Moreover, the peak intensity of ·O2− in 001-HT/ TC system is stronger than that of HT/TC, suggesting the more ·O2− generated in 001-HT/TC system [42]. Incidentally, no ·OH is detected in either HT/TC or 001-HT/TC systems (Fig. 4d). These results ascertain that the formation of TiO2 and TC complexes leads to visible-lightmediated degradation of TC through O2− pathway. To deepen the understanding of visible-light-mediated degradation of TC, the sensitizing mechanism was further investigated in this work, as illustrated in Fig. 5a and b. Generally, organic species can sensitize TiO2 via two pathways including dye sensitization and ligand-to-metal charge transfer (LMCT) [43]. Almost all workers including S. Leong [8] and S. Wu [44] insisted that the dye sensitization, in which electron was photoexcited from HUMO to LUMO of TC and further transferred to the conduction band (CB) of TiO2 [45], was responsible for the TC degradation under visible-light irradiation. Unfortunately, these previous works did not further expound the role of LMCT in degrading TC, therefore, the further investigation is necessary to clarify the LMCT mechanism because the TiO2/TC composites also may take part in TC degradation. Based on LMCT sensitizing mechanism, the photoexcited
electron in HUMO of TC is transferred directly to the CB of TiO2 due to the formation of TiO2/TC composite [43], which is strongly confirmed by UV–vis absorption spectra of HT and 001-HT after adsorbing TC, producing new absorption band and red shift (Fig. 5c) due to the LMCT process [41] between TC and surface-bond Ti (IV) ion (Fig. 6). The degradation performance of TC was further detailed by investigating the function of the incident light wavelength (λ0) to deeply understand the role of LMCT. Fig. 5d shows that the irradiation wavelength-dependent degradation rate of TC agrees well with the photo-absorption edge of LMCT rather than TC itself, strongly convincing that LMCT plays a decisive role in degrading TC via the visible-light-mediated mechanism, as shown in Fig. 6. In brief, the coordination composite of TC and TiO2 firstly is formed on surface of TiO2 via hydroxyl groups, and then photogenerated electrons from the ground state of TC are directly transferred to the CB of TiO2, leading to the production of ·O2− that drives the degradation of TC. The enhanced visible-light activity of 001-HT was further detailed as follow: firstly, the stronger intensity (Fig. 5c) of 001-HT/TC in the visible light range promotes the production of photogenerated electrons from TC because the number of unsaturated 5c-Ti atoms on the surface of {001} facet holds twice higher than {101} facet [28], generating more OH groups ascertained by Zeta potential (-38.47 and -21.9 mV for 001-HT and HT, respectively) and FT-IR (Fig. S4) [46,47] and thus forming more TiO2/TC composites. Secondly, Fig. 7a shows that the 001-HT/TC exhibits stronger photocurrent response and smaller arc radius than HT/TC under visible-light irradiation (λ > 420 nm), indicating the higher transfer efficiency of photogenerated electrons and
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Fig. 5. Illustration of dye sensitization (a) and LMCT sensitization (b); UV–vis absorption spectra of various samples (c); UV–vis absorption spectra of TC (40 mg L−1) and 001-HT/TC, and degradation rate of TC plotted as a function of wavelength of the incident light (a bandpass filter of λ ± 10 nm) (d).
3.4. Transformation intermediates and possible pathway The intermediates of TC were detected by mass spectrometry during the degradation process and a possible degradation pathway was proposed, as shown in Fig. 8. The TC is firstly transferred to TC1 through the demethylation of the dimethylamino group and replacement of the acylamino by the carbinol group [52]. TC1 is further transferred to TC2 by dehydration and the loss of carbinol and acylamino group, respectively [52]. Then, TC2 undergoes cleavage of the carboatomic ring, generating TC3, which is subjected to decarboxylation and transferred to TC4. Ultimately, TC5 is generated by another cleavage of the carboatomic ring in TC4 and the above ring-opening products are decomposed into CO2 and H2O. Most importantly, the polyhydric intermediates can also form complexes with TiO2 via their hydroxy group to induce LMCT. Therefore, the degradation can be maintained until the intermediates cannot form stable complex with TiO2. As a result, after 120 min under visible-light irradiation, the solution holds a high concentration of TC5 because LMCT can not be initiated.
Fig. 6. The mechanism of visible-light-mediated TiO2 photocatalyzed TC degradation.
4. Conclusion
stronger LMCT efficiency [48], which are further ascertained by photoluminescence emission (prolonged lifetime). The enhanced charge separation and prolonged lifetime are mainly ascribed to the formation of {101}/{001} facet heterojunction (Fig. 7d) that have been demonstrated by previous works and computational results [49–51]. It is worth mentioning that only 001-HT can degrade TC completely when the TiO2 itself is excited by simulated solar-light irradiation (Fig. S5), proving the positive effect of facet heterojunction.
In summary, a {101} and {001} facets co-exposed TiO2 hollow photocatalyst has been successfully synthesized via a gentle NaF treatment. The formation of coordination complexes between TC molecule and surface-bound Ti (IV) ion induces strong visible-light absorption via LMCT. The {001}/{101} facet heterojunction also facilitates the charge migration, extending the lifetime of photogenerated
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Fig. 7. Transient photocurrent performance of various samples under visible light (a); EIS plots of HT and 001-HT (b); PL emission spectra (c); Illustration of {101}/ {001} facet heterojunction (d).
Fig. 8. Schematic diagram illustration of the proposed degradation pathway of TC.
electrons. As a result, 001-HT maintains a high charge separation efficiency and strong visible-light absorption, achieving 90.1% removal rate of TC under visible-light irradiation. This work opens a new sight for visible-light-mediated TiO2 photocatalysis.
Acknowledgements We appreciate the financial support provided for this research by the National Natural Science Foundation of China (21876069) and the Six Talent Peaks Project in Jiangsu (XCL-018).
Declaration of competing interest
Appendix A. Supplementary data
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.142. 7
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