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Green synthesis of 3D tripyramid TiO2 architectures with assistance of aloe extracts for highly efficient photocatalytic degradation of antibiotic ciprofloxacin
Applied Catalysis B: Environmental 260 (2020) 118149
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Green synthesis of 3D tripyramid TiO2 architectures with assistance of aloe extracts for highly efficient photocatalytic degradation of antibiotic ciprofloxacin ⁎⁎
Yanjing Lia,b, Yunzhi Fua,c, , Mingshan Zhub,
T
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a
College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, PR China School of Environment, Jinan University, Guangzhou, 510632, PR China c Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, Hainan University, Haikou 570228, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Antibiotic ciprofloxacin TiO2 Aloe Tripyramid nanostructure Photocatalysis
The large consumption and discharge of antibiotic ciprofloxacin (CIP) force us to find an effective technology to removal of CIP. Herein, three dimensional (3D) tripyramid TiO2 architectures were eco-friendliness and facile synthesized with the assistance of aloe extracts and hydrothermal method and then were used in the photocatalytic removal of CIP pollutants. It’s found that residual biologically active components on the surface of TiO2 induced to the formation of 3D tripyramid architectures. The optimized composite showed a removal efficiency of 90% within 60 min. Compared to rod-like TiO2, 3D tripyramid TiO2 architectures displayed 3.4 times for photocatalytic degradation of CIP under UV–vis light irradiation. The present investigation initiates a green and eco-friendliness path for the construction of high-performance hierarchical catalysts in the application of wastewater purification.
1. Introduction Because of the continuous input and persistence into the aquatic and soil environments, antibiotics have been considered as the emerging pharmaceutical pollutants over the past few years [1–4]. Among various antibiotics, ciprofloxacin (CIP) is a famous fluoroquinolone antibiotic, which has been used for treating bacterial infections. It is widely found in aquatic environments, causing the enormous attention of the research to focus on the remove of CIP from aquatic environments [5,6]. Advanced oxidation process (AOP) is one of the most effective technologies for the removal of toxic and persistent contaminants and photocatalysis is an efficient yet green technology among AOPs due to its effective utilization of solar energy [7–10]. Up to present, semiconductor-based photocatalysts, traditionally represented by TiO2, have been investigated to meet the requirements of antibiotic CIP elimination [11–15]. On the other hand, it's well known that the catalytic reactions occur on the surface of nanostructured photocatalysts and their catalytic activities are strong dependent on their morphologies [16–19]. This is because the distinct surface-to-volume ratio of nanostructures and the amount of catalytically active sites, such as corners, edges, steps, etc.
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highly dominate the activation and adsorption/desorption of the involved catalytic reaction [17–21]. Therefore, it is an issue of considerable concern to synthesize photocatalysts with fascinating hierarchical properties [22]. Traditionally, these multi-dimensional architectures are achieved by multi-step assembly, template-sacrificial dissolution, chemically induced self-transformation and post-synthesis treatment, etc. [22]. In these cases, high-cost, abundant surfactants, toxic organic solvents and long-time consume procedures are needed, which limit the broad application of these advanced hierarchical architectures. In this regard, a facile and eco-friendliness route for preparation of hierarchical photocatalysts for the efficient removal of CIP is very imperative. Recently, a green technology for the synthesis of metal and metal oxide materials was raising more and more interest with the aid of plant extracts [23–26]. We all know that plant extracts are composed of many biologically active components, including polysaccharides vitamins, polyphenols, flavonoids, proteins, heterocyclic and carbonyl compounds and so on. These biologically active components can act as capping agent and template, which plan an important role in the synthesis of designed nanoarchitectures [24,24,25,26]. Inspired by the above elaboration, we herein reported a green synthesis of 3D
Corresponding author. Corresponding author at: College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, PR China. E-mail addresses: [email protected] (Y. Fu), [email protected] (M. Zhu).
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https://doi.org/10.1016/j.apcatb.2019.118149 Received 30 June 2019; Received in revised form 20 August 2019; Accepted 31 August 2019 Available online 03 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Scheme of the fabrication process of TP-TiO2 and RL-TiO2.
amorphous TiO2 (RLA-TiO2). For the synthesis of TP-TiO2, the above products were dispersed again into 70 mL of deionized water and then transferred to a 100 mL Teflon lining. Keep the reactor in a 180 °C air drying furnace for 3 h. After that, leave it to cool to room temperature and separate the product with ethanol for three times, followed by drying in a 60 °C vacuum drying chamber for 12 h. For the synthesis of RL-TiO2, the RLA-TiO2 products were directly calcined at 500 °C for 3 h in air, resulting RLTiO2 nanostructures.
tripyramid TiO2 (TP-TiO2) architectures with the assistance of aloe extracts and hydrothermal method. As shown in Scheme 1, we firstly obtained aloe extracts from the fresh aloe leaf, and then used them to be the solvent for the synthesis of amorphous TiO2. Rod-like morphology of TiO2 (RL-TiO2) nanostructure was formed in this amorphous TiO2. To get an anatase TiO2, a hydrothermal process was further carried out and then an interesting architecture with 3D TP-TiO2 nanostructure was obtained. Finally, such as-prepared 3D TP-TiO2 architectures showed effective degradation of antibiotic CIP pollutants under simulation solar light irradiation. The present results offer an eco-friendliness and lowcost protocol for the synthesis of advanced architectures in the application of antibiotics pollutants elimination.
2.4. Photocatalytic performance The samples of TP-TiO2 and RL-TiO2 were used as catalysts to catalyze the degradation of ciprofloxacin under UV–vis light condition. 50 mL CIP (32.6 μM) and 5 mg photocatalysts were magnetically stirred under darkness for at least 30 min to achieve adsorption equilibrium before photocatalytic reaction. During photocatalytic reaction, 2 mL above dispersions were taken every 5 min or 10 min for 10 consecutive times. The sample was centrifuged to remove catalyst, and then the absorbance at 275 nm was detected by UV–vis spectrophotometer. According to Lambert-Beer Law, the concentration change of CIP (Ct/ C0) can be represented by the corresponding absorbance. In the cycle experiments, after the completion of the first catalytic experiment, all catalysts were collected and put into the next catalytic experiment, and the experiments were repeated five times. For active species trap experiments, the concentration of CIP was 29 μM, superoxide dismutase (SOD, 0.16 g L−1) and potassium iodide (KI, 6.67 mM) were used as effective scavengers of superoxide radical (•O2-) and hole, respectively. Isopropyl alcohol (0.87 mM) was utilized as scavenger of hydroxyl radical (•OH). Other conditions were consistent with the above photocatalytic experiments.
2. Experimental section 2.1. Materials Fresh aloe was picked from PuTian, Fujian, China. TiCl3 (15.0%∼20.0% with 30% excess HCl), charcoal activated powder, absolute ethyl alcohol (C2H5OH), potassium iodide (99%, AR), isopropyl alcohol (99.5%, AR) were purchased from Sinopharm group chemical reagent Co., Ltd. Ciprofloxacin hydrochloride monohydrate (98%) was purchased from Tokyo Chemical Industry Co., Ltd. Superoxide dismutase (BR, 2500～7000 u mg−1, from pig blood protein) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. All chemicals were used without purification. 2.2. Preparation of aloe extracts (ALE) Typically, 100 g of peeled aloe were mixed with 200 mL of water, and then the mixture was completely milked using soybean milk machine. After that, the mixture was put under ultrasonic treatment for 20 min. After ultrasonic treatment, the mixture was filtered to remove residues and 4.0 g active carbon powder was added into as-prepared initial extract under stirring for 8 h in order to decolorize. Finally, the extract was obtained by filtration treatment again and stored at 4 °C environment for later use.
2.5. Characterization The surface morphologies and sizes of photocatalysts were observed and analyzed using a FESEM (Hitachi SU8220 Hitachi, Tokyo, Japan). The samples were prepared by diluted with ultrapure water and then dropped on the silicon sheet and dried by the infrared lamp. TEM measurements and HAADF-STEM-EDX analyses were performed on a FETEM (FEI Tecnai G2 F20) instrument with an acceleration voltage of 200kv. The crystal phase analysis of TiO2 powder is obtained by an Xray diffractometer (BRUKER D8 Advance), use a wide angle (2θ = 1080°) detection mode. UV − vis diffuse reflectance spectra were obtained on UV-3600 UV-VIS-NIR spectrophotometer (Shimadzu). X-ray photo-electron spectroscopy (XPS) was measured on a ESCALAB 250Xi XPS System (Thermo Fisher Scientific). The specific surface area and pore diameter were determined by the TriStar II 3020 specific surface and porosity analyzer. Photocatalytic reaction of the light source from 300 W Xe lamp (Perfectlight PLS-SXE300) with a filter, a light source
2.3. Preparation of TiO2 photocatalyst First, 30 mL of ALE was centrifuged at 20,000 rpm for 10 min to remove sediment, and then 60 μL TiCl3 was injected into 20 mL ALE in the 50 mL single-port bottle under magnetic stirring for 60 min. After that, milky white liquid was produced which showing that the rod-like precursor (RLP) colloid was synthesized in ALE. At last, the milky white RLP colloid was centrifuged at 800 rpm for 10 min. And the supernatant was carefully discarded and the RLP was dispersed again by adding ultrapure water and further purification more than two times with ultrapure water and ethanol, respectively, resulting in rod-like 2
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titanium source (TiCl3) into ultra-pure water. As shown in Fig. S2, only small size of nanoparticles of amorphous TiO2 was obtained. This result confirms the aloe extract plays an important role in the synthesis of TiO2 with specific morphology. Fig. 1C shows the HRTEM image of as-prepared RLA-TiO2, confirming that an amorphous structure was formed in the as-prepared samples. Further analysis of RLA-TiO2 was investigated by HAADFSTEM-EDX (Fig. 1D–H). It can be seen that beside Ti and O elements were observed in the as-synthesized RLA-TiO2, the elements of C and N were also detected. These elements might be derived from amino acids and polysaccharides in aloe meat, similar to the other reported result [28]. The concentrations of titanium source and aloe extracts were also carried out to investigate the optimum condition RLA-TiO2. Firstly, with the increase of titanium source concentration, the yield of rod particles first increased and then decreased (Fig. S3). If the amount of titanium source is overload or insufficient, the protective effect of aloe extracts or the yield of rod-like micelle will be reduced, respectively. In addition, different concentrations of aloe extracts were used as the reaction environment to prepare TiO2 particles. When the ratio of aloe leaf flesh to water mass was 1:2, the yield of rod-shaped particles was the highest, while the concentration of aloe leaf flesh extract was too high or too low, the yields of rod-shaped particles were greatly reduced and many flower-shaped particles were generated (Fig. S4). This indicates that the concentration of active components in plant extracts is the key to the formation of rod-like precursors. Furthermore, reasonable reaction time is also an important factor to obtain high yield rodshaped granules (Fig. S5). Insufficient reflection in a relatively short time, the particle shape is slender and uneven. When the reaction time was 1 h, the morphology of the rod particles was uniform, but as the reaction continued, obvious cracks appeared on the rod particles. The active components in aloe extract on the formation of rod particles were further studied by FT-IR spectroscopy. As shown in Fig. S6a, for aloe extract, the strong peaks appear in 2927 and 1710 cm−1 were assigned to −OH stretching vibration and C]O stretching vibration, respectively. The strong broad peaks between 3733 to 2522 cm−1 were assigned to νNH or νOH [29,30]. In addition, the bending vibration of amidogen group can be seen at 1612 cm−1, which giving evidence of the amidogen group existence [31]. Signals observed in the spectra of hydroxy and ether groups belonging to glycosyl residue can be seen in 1032 cm−1 (νOH) and 1080 cm−1 (νC-O-C). These results
with the wavelength range of 320–780 nm was provided for the reaction. Electron paramagnetic resonance (EPR) experiments were performed using the Bruker E500 instrument to detect the formation of free radicals in the reaction [27]. DMPO (5, 5-dimethyl-l-pyrrolidine noxide) was used as the spin trap, and the light source was ultraviolet light. 2.6. LC–MS analysis The transformation products formed during CIP degradation were studied by LC–MS. Ciprofloxacin solution of 12 mg L−1 was used for sampling at different catalytic times. The samples were tested using Thermos TSQ Quantum Ultra. Chromatographic separation was performed on an ultimate XB C18 2.1 × 100 mm, 3 μm HPLC column. The mobile phase is composed of 0.1% formic acid in water and acetonitrile. During operation, acetonitrile always accounts for 22%. The flow velocity was 0.3 mL min−1. The separated samples were imported into the mass spectrometry system and ionized using ESI as an ion source in a positive mode. The scan range was set from m/z 50 to m/z1000. The capillary and vaporizer temperatures were set at 370 °C and 240 °C, respectively. The CIP transformation was recorded with Shimadzu Essentia Prep LC-16 P HPLC instrument. SinoPak C18 4.6 × 250 mm 5 μm HPLC column was used for separation. The mobile phase is consisted of acetonitrile: aqueous phase = 2:8 (the aqueous phase is the mixture of 20 mmol L−1 NaH2PO4:20 mmol L−1 Na2HPO4 = 1.5:1). The flow rate was 0.5 mL min−1 and all compounds were monitored at the wavelength of 275 nm. 3. Results and discussion 3.1. Morphology and structure of the samples The RLA-TiO2 was synthesized directly from the mixture of aloe extract and water at room temperature. The morphology of prepared RLA-TiO2 was studied by SEM and TEM Fig. 1A and B. With assistance of aloe extract, rod-like shape was formed in the as-synthesized samples. The enlarge SEM image of RLA-TiO2 was shown in Fig. S1 and the analysis shows that the length of particles is about 2.5 μm. We also investigate the as-synthesized amorphous TiO2 by using traditional hydrolytic deposition method, viz. by adding sodium bicarbonate and
Fig. 1. SEM (A), TEM (B), HRTEM (C), and STEM (D) images of RLA-TiO2. (E–H) EDS elemental mapping images showing the Ti, O, C, and N atoms of the area of (D), respectively. 3
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XPS spectra of RL-TiO2 and TP-TiO2 were further recorded, which are shown in Fig. 2C–D. In the sample of TP-TiO2, the peaks attributed to the binding energy of Ti 2p3/2 and Ti 2p1/2 were easily observed at 458.5 and 464.3 eV, respectively [37]. The binding energy between two peaks was 5.8 eV, which was consistent with the binding energy corresponding of Ti4+ in TiO2 [37,44]. The O 1s spectrum of TP-TiO2 can be divided into two peaks at 529.8 and 531.8 eV, which corresponds to Ti–O and −OH on the surface. It’s found that both the peaks of Ti 2p and O 1s in TP-TiO2 were shifted to lower binding energy compared to these of in RL-TiO2. Theoretically, the decrease of binding energy is due to the enhancement of electron shielding effect caused by the increase of electron concentration around Ti and O atoms [45,46]. Beside the main contents of Ti and O, there is a peak at 399.8 eV in the TP-TiO2, which is assigned to N 1s signal, while no peak has been found in the sample of RL-TiO2 (Fig. S8). This result represents the presence of amino group on the surface of TP-TiO2 after hydrothermal treatment. The morphologies and crystallization of as-prepared samples after calcination and hydrothermal process were investigated by SEM, TEM and HRTEM images. As shown in Fig. 3A and B, the rod-like shape of TiO2 was kept after calcinations process. Furthermore, HRTEM image shows clear lattice fringes with d-spacing of 0.35 nm in the selected area of sample (Fig. 3C and D), which was attributed to (101) planes of anatase TiO2. This result further confirms the well crystallization of TiO2 after calcinations process. Interesting, after RLA-TiO2 were treated by hydrothermal process, the morphology of rod was changed to 3D tripyramid structure. As shown in Fig. 3E and F, homogenous 3D tripyramid structures of TiO2 were obtained after hydrothermal treatment at 180 °C with 3 h. The enlarge image of TEM shows that the 3D tripyramid structure was constructed by small size TiO2 particles (ca. 10 nm, Fig. 3G). HRTEM image further shows the well crystallization of TiO2 and the lattice fringes were mainly 0.35 nm, corresponding to (101) crystal planes of anatase TiO2 (Fig. 3H). Further analysis of TP-TiO2 by HAADF-STEMEDX (Fig. 3I–K) showed that obvious Ti and O signals were obtained, indicating that Ti and O elements were the main components of 3D tripyramid structure. The different synthetic conditions including hydrothermal reaction time and temperature were investigated Figs. S9 and S10. As shown in Fig. S9, the reaction precursor in this system was rod-like particles with regular morphology before the reaction started. With the increase of reaction time, the morphology of the rod-like titanium dioxide particles gradually changes. When the reaction is carried out for 0.5 h, the middle part of the rod-like titanium dioxide particles is dented, and a large number of small particles can be observed. With the reaction time is 3 h, TP-TiO2 is successful synthesized. On the other hand, the hydrothermal reaction temperature also plays an important role in the synthesis of nanostructures. At 180 °C, homogeneous 3D tripyramid structures were easily obtained. When the temperature was decrease, the tendency of rod-like precursor to 3D tripyramid particles gradually decreased (Fig. S10). For a closed hydrothermal reactor, the external high temperature environment will generate great heat and pressure in the reactor, forming an extreme reaction atmosphere. It is speculated that the rod-like precursor will lose water and gradually crystallize under high temperature and high pressure in the closed hydrothermal reactor. In beginning, the solubility of the rod-like precursor is similar to that of the crystal. During the reaction process, the high temperature and pressure conditions in the hydrothermal reactor upset the equilibrium and caused the TiO2 atoms to rearrange to form the crystal phase. The process includes a dissolution and crystallization equilibrium, which conforms to the following thermodynamic equation [47]:
provide a visual evidence of amino acid and glycosyl occurrence in the aloe extracts. For the samples of RLA-TiO2 (Fig. S6b), the disappeared νOH peak at 2927 and 1710 cm−1 in carboxyl group, together with the appearance of asymmetrical stretching vibration and the symmetrical stretching vibration (assigned to carboxyl group) at 1611 cm−1 and 1402 cm−1 give the evidence of the absence of free carboxyl group around the Ti4+, which may owing to the Ti4+ replacing the H+ in carboxyl group [32–34]. It may be pointed out that the νasym (COO) (1611 cm−1) is lower than the parent acid (1710 cm−1) is an indication of carboxyl group involvement in chelation and form coordination compound. The amidogen νN-H expansion vibration peak appears in the form of a wide and strong peak at 3600 to 2700 cm−1 and has a peak at 3300 cm−1, belonging to RLA-TiO2, which is shifted to a lower frequency. The reduction of the stretching frequency of amino group may be related to the intermolecular hydrogen bond [35,36]. Therefore, we speculate that the active ingredients in aloe extract play a decisive role in the synthesis of RLA-TiO2. First, amino acids and Ti4+ form coordination compounds, which then aggregate through hydrogen bond interactions. In the process of coordination compound aggregation, amino acids and sugars are used as the capping ligands for the encapsulation. Finally, rod-shaped particles with regular morphology are formed. The component and surface information on RLA-TiO2 were further analyzed by XPS spectra, which are shown in Fig. S7. Firstly, in the survey spectra of XPS, Ti, O, C, and N signals were detected (Fig. S7A). Secondly, for Ti 2p, two peaks were observed at 458.6 and 464.4 eV, which can be assigned to Ti 2p3/2 and Ti 2p1/2, respectively [37]. In the spectrum of O 1s, three peaks at 529.8, 531.5, and 532.9 eV were detected, which corresponds to O2−, −OH, and chemically adsorbed water as well as the ether group (COe), respectively [37]. For C 1s, three peaks can be fitted at 284.8, 286.3, 288.5 eV, corresponding to CeC and CH bonds, CO, and COee], respectively [37–39]. Note that, N signal was also detected, and the peak at 399.6 eV was represented the NHe bond. We speculate that it comes from an amino acid in aloe extract [40]. The other two lower peaks are corresponded to chemically modified –N2 (400–402 eV) [41]. By analyzing the XPS results, we speculated that amino acids and sugars existed on the surface of RLATiO2, which once again demonstrated that amino acids and sugars could play an important role in encapsulating the synthesis of rodshaped titanium dioxide. These results combined with above investigations of STEM-EDS and FT-IR solidly proved that amino, carboxyl and hydroxyl groups were detected on the surface of RLA-TiO2, which play the key factors for the formation of rod-like morphology. Generally, the amorphous TiO2 has negligible photocatalytic activity owing to its poor crystallization. To obtain the well-crystallization of TiO2, calcination and hydrothermal methods were used, respectively. Firstly, FT-IR spectra of samples were studied after calcination and hydrothermal methods. Fig. 2A shows a broad band around 3400 cm−1, which is caused by the stretching vibration of the hydroxyl group on the surface of TiO2 [42]. Besides, peaks at 471 and 464 cm−1 are corresponded to the stretching vibration of TiO2. Compare to RLA-TiO2 (Fig. S6b), organic groups of amino, carboxyl and hydroxyl groups were removed from the surface of samples after calcination and hydrothermal process. XRD pattern is an important characterization to evaluate the well crystallization of samples. As shown in Fig. 2B, very poor crystal phase was observed in the RLA-TiO2 precursor. After calcination and hydrothermal process, well crystallized peaks were observed in both samples of RL-TiO2 and TP-TiO2. The distinct peaks at 2θ = 25.10°, 37.84°, 47.70°, 53.91°, 54.58° and 62.55°, were correspond well to the anatase phase of TiO2 of (101), (004), (200), (105), (211), and (204) planes, respectively [43]. There are no other impurity peaks were detected, suggesting that the purity of the anatase TiO2 were obtained after calcination and hydrothermal process. Note that, compared with the TPTiO2, the half width of peak (101) in RL-TiO2 is narrower, indicating that its grain size is larger.
ΔG = ΔGS − C
(1)
Herein, ΔGS-C refers to in situ crystallization energy of RLA-TiO2 particles, which is less than zero and means the process is crystallization direction. RLA-TiO2 contains a certain proportion of amino acids and sugars, which will bind some TiO2 units in the crystallization process, 4
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Fig. 2. (A) FT-IR spectra of TP-TiO2 (a) and RL-TiO2 (b); (B) XRD patterns of the samples of RLA-TiO2 (a), RL-TiO2 (b), and TP-TiO2 (c). (C and D) XPS spectra of TPTiO2 and RL-TiO2 of O 1s (C) and Ti 2p (D).
Fig. 3. SEM (A and E) images of RL-TiO2 (A) and TP-TiO2 (E), TEM (B, C, F, and G) and HRTEM (D and H) images of RL-TiO2 (B–D) and TP-TiO2 (E–H), STEM image of TP-TiO2 (I), EDS images of Ti (J) and O (K) elements in the area of image I.
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Fig. 4. (A) Nitrogen adsorption–desorption isotherm of TP-TiO2, (B) UV–vis DRS spectra of (a) RL-TiO2 and (b) TP-TiO2.
where C is concentration of the CIP molecules, t is reaction time, and k is the rate constant. It can be found that the rate constants of TP-TiO2 (4.03 × 10−2 min−1) is 3.4 times than that of RL-TiO2 (1.19 × 10−2 min−1), which further confirm that TP-TiO2 presents superior photocatalytic performance for the elimination of CIP pollutants. In the practical application, the stability of photocatalyst is a very important factor. To evaluate the stability of as-prepared TP-TiO2, cyclic experiments for the photodegradation of CIP were performed. As shown in Fig. 6A, TP-TiO2 showed negligibility decrease of photocatalytic activity with five cyclic experiments. Moreover, the SEM image shows that 3D tripyramid structures also same as the morphology of the original TP-TiO2, which is shown in Fig. 6B. These results indicate the as-prepared TP-TiO2 kept stable photocatalytic during long time photo-irradiation reactions. Moreover, by detecting the XRD pattern before and after the photocatalytic degradation of CIP of TP-TiO2, it was found that the position of the characteristic peak did not change, further indicating that the crystallization state of catalyst remains stable during photocatalysis (Fig. S12). Generally, in the photocatalytic system of TiO2, the oxidation ability of photogenic holes and the reduction ability of photogenic electrons are very strong. The photogenic hole can oxidize OH-/ H2O to form % OH, and the photogenic electron can reduce O2 to form %O2−, as shown in the following equation (3)-(5).
allowing them to crystallize in tiny areas. At the same time, convection occurs in the liquid center during the hydrothermal process, generating additional driving forces in the liquid reaction system inside the hydrothermal reactor. Due to the strong impact force generated by convection and the relatively large stressed area in the longitudinal position of the rod-like particles, RLA-TiO2 gradually fractured from the middle in the crystallization process and finally constructs 3D tripyramid structures. 3.2. Optical absorption properties of photocatalysts The 3D TiO2 structure constitutes the TiO2 interconnection network, and the existence of pores in the particles significantly increases the surface area of the samples. Fig. 4A shows the nitrogen adsorptiondesorption isotherm of TP-TiO2. The result shows the sample with hysteresis loop, indicating the distribution of pores in the TP-TiO2. This non-dense particle arrangement increases the number of active sites involved in the reaction and improves the catalytic efficiency. The specific surface area of TP-TiO2 is 84 m² g−1, while the specific surface area of RL-TiO2 is only 65 m² g−1. The optical properties of samples were studied by UV–Vis diffuse reflectance spectroscopy (DRS). In general, the absorption degree of photocatalyst is an important factor to reflect its photocatalytic performance. Fig. 4B shows the UV–Vis DRS spectra of TP-TiO2 and RLTiO2 with a wavelength range of 200–800 nm. Both the absorbance of two catalysts showed obvious absorption edge at 387 nm, showing the typical TiO2 absorption and with the band gap of 3.2 eV. 3.3. Photocatalytic activity and stability under ultraviolet-visible light
(3)
e− + O2 → •O2−
(4)
h+ + H2 O→ •O H+ H+
(5)
In order to explore the reaction mechanism in the photocatalytic degradation process of TP-TiO2, and to find the active species which played the decisive role in the degradation process of CIP, a series of active species trap experiments were conducted, which is shown in Fig. 7A. The effects of different kinds of trapping agents on CIP degradation were studied, so as to judge the contribution of each reactive oxygen group in the reaction process. It can be seen that when superoxide dismutase (SOD) was added, the degradation rate of CIP was significantly reduced due to the scavenging of superoxide radicals. Similarly, when potassium iodide (KI) is added, the holes are cleared and the degradation rate of CIP is greatly slowed down. These results indicate that superoxide radicals and photogenic holes are the key factors for CIP degradation. However, when isopropanol was added, the degradation rate of CIP was almost constant with without scavenger, indicating that hydroxyl radical had little contribution to the photodegradation of CIP. In order to further understand the generation of active radicals during irradiation, DOMO was used to detect %O2− radical. As shown in Fig. 7B, 11 characteristic peaks of DMPO-%O2− adducts could be observed after ultraviolet light irradiation, but the related characteristic peaks did not appear under dark conditions. This result suggests that superoxide radicals do exist in the photocatalytic system with TP-TiO2 as catalyst.
Based on the above optical properties, we evaluated the photocatalytic performance of TP-TiO2 and RL-TiO2 by photocatalytic degradation of antibiotic CIP. The photodegradation of CIP over different photocatalysts was monitored by measuring the real-time UV–vis absorption of CIP at 275 nm, as shown in Fig. S11. Firstly, pure CIP without any photocatalyst kept stable under UV–vis light irradiation. By using as-prepared TiO2 as photocatalysts, the absorptions of CIP molecules in the solution were decrease with the increase of photo-irradiation time, indicating the degradation of CIP molecules. The ratios of photocatalytic degradation of CIP were summarized in Fig. 5A. By using RL-TiO2 as photocatalyst, 52% CIP molecules were removed under 60 min UV–vis light irradiation. However, when TPTiO2 was used as the photocatalyst, 90% CIP pollutants were degraded under the same conditions. To show the detail photocatalytic degradation rate, the correlation between ln(C/C0) and the reaction time t are shown as plotted in Fig. 5B. The nice linear correlation between ln (C/C0) and t indicates that the decomposition reaction of CIP photocatalyzed by as-prepared TiO2-based catalysts follows the first-order kinetics:
− dC /dt = kC
TiO2 + hv → e− + h+
(2) 6
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Fig. 5. (A) Photocatalytic degradation CIP curves and (B) kinetic curves of CIP degradation with different photocatalysts and without catalyst.
intermediate CIP-5 (m/z 245) [49] and finally with completely degradation to CO2, H2O, and other small molecules products.
Based on the above analysis, the basic mechanism for the degradation of CIP was proposed (Fig. 8). Firstly, after excitation of TiO2 under UV–vis light irradiation, generated electrons from the valance band (VB) to conduction band (CB). Secondly, adsorbed oxygen molecules on the surface of TP-TiO2 will be reduced by these electrons to form %O2− species. On one hand, the generation of %O2− inhibits electron-hole recombination, thus improving the quantum yield of photocatalytic reactions. On the other hand, these %O2− is an important active oxidative group, CIP molecules will be degraded to smaller molecules and finally to CO2, H2O, and others. At the same time, the generated holes also as the main oxidative components, it also participates in the degradation of CIP. The transformation products formed during CIP degradation were studied by LC–MS. Fig. S13 show the representative chromatograms of the solution after reaction for 0, 30 and 60 min. At 0 min, a typical ion peak for pure CIP at retention time of 8.79 min was observed in the original solution. Different ion peaks were detected, indicating the formation of different intermediates after 30 min of light irradiation with TP-TiO2. Lastly, the intensities of these peaks decrease after 60 min of light irradiation with TP-TiO2, suggesting the mineralization of CIP molecules. The representative intermediates after 30 min light irradiation together with the corresponding mass spectra, were shown in Table S1. The possible photocatalytic degradation pathway of CIP was shown in Fig. 8, by referencing relevant reports and identifying intermediate products [48]. The oxidative degradation of the piperazine moiety is the main degradation pathway. Firstly, Piperazine was oxidized and lost a formaldehyde group, generating two isomers with m/z of 334 (CIP-1). Secondly, CIP-1 offs formaldehyde to produce CIP-2 (m/z 306). Thirdly, CIP-2 underwent secondary amine nitrogen loss, oxidation and formaldehyde loss from the amine side chain, and successively formed CIP-3 (m/z 291) and CIP-4 (m/z 263). This part of the transformation process was consistent with the results of CIP degradation in the literature [48]. Lastly, CIP-4 reduced and defluorinated to form
4. Conclusions In conclusion, a green and eco-friendliness technology for the synthesis of rod-shaped amorphous TiO2 with high yield in aloe extract environment was designed and then 3D anatase TP-TiO2 structures were obtained by further hydrothermal crystallization. The form of small particle accumulation in 3D TP-TiO2 makes each other to form an interactive network with special configurations and high specific surface area, which can provide more active sites for catalytic reactions. Under UV–vis light irradiation, the as-prepared 3D TP-TiO2 architectures for the photocatalytic degradation rate of CIP reached to 4.03 × 10−2 min−1. Compare to rod-like TiO2, the 3D tripyramid architectures greatly improved the efficiency of photocatalytic operation of TP-TiO2. Moreover, such architectures also displayed well stability for the degradation of CIP molecules. According to the analysis of photocatalytic mechanism, %O2− and holes were the main driving agent for CIP degradation under UV–vis light irradiation. The present studies showed a green and facile method to construct advanced architectures with high photocatalytic performance, which have responded to the current concept of economic and environmental protection and has great potential for further research. Declaration of Competing Interest 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. Acknowledgment This work was sponsored by the National Natural Science
Fig. 6. (A) Regeneration photodegradation of CIP with TP-TiO2 as catalyst. (B) SEM image of TP-TiO2 after photocatalytic reactions. 7
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Fig. 7. (A) Photocatalytic disinfection of CIP by TP-TiO2 respectively with different scavengers: superoxide dismutase (SOD, 0.16 g L−1), isopropyl alcohol (0.87 mM) and potassium iodide (KI, 6.67 mM). (B) The production of •O2- in TP-TiO2 system under different light and dark conditions.
Fig. 8. Schematic diagram of photocatalytic process and the degradation pathway of CIP molecules.
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Green synthesis of 3D tripyramid TiO2 architectures with assistance of aloe extracts for highly efficient photocatalytic degradation of antibiotic ciprofloxacin ⁎⁎
Yanjing Lia,b, Yunzhi Fua,c, , Mingshan Zhub,
T
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a
College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, PR China School of Environment, Jinan University, Guangzhou, 510632, PR China c Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, Hainan University, Haikou 570228, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Antibiotic ciprofloxacin TiO2 Aloe Tripyramid nanostructure Photocatalysis
The large consumption and discharge of antibiotic ciprofloxacin (CIP) force us to find an effective technology to removal of CIP. Herein, three dimensional (3D) tripyramid TiO2 architectures were eco-friendliness and facile synthesized with the assistance of aloe extracts and hydrothermal method and then were used in the photocatalytic removal of CIP pollutants. It’s found that residual biologically active components on the surface of TiO2 induced to the formation of 3D tripyramid architectures. The optimized composite showed a removal efficiency of 90% within 60 min. Compared to rod-like TiO2, 3D tripyramid TiO2 architectures displayed 3.4 times for photocatalytic degradation of CIP under UV–vis light irradiation. The present investigation initiates a green and eco-friendliness path for the construction of high-performance hierarchical catalysts in the application of wastewater purification.
1. Introduction Because of the continuous input and persistence into the aquatic and soil environments, antibiotics have been considered as the emerging pharmaceutical pollutants over the past few years [1–4]. Among various antibiotics, ciprofloxacin (CIP) is a famous fluoroquinolone antibiotic, which has been used for treating bacterial infections. It is widely found in aquatic environments, causing the enormous attention of the research to focus on the remove of CIP from aquatic environments [5,6]. Advanced oxidation process (AOP) is one of the most effective technologies for the removal of toxic and persistent contaminants and photocatalysis is an efficient yet green technology among AOPs due to its effective utilization of solar energy [7–10]. Up to present, semiconductor-based photocatalysts, traditionally represented by TiO2, have been investigated to meet the requirements of antibiotic CIP elimination [11–15]. On the other hand, it's well known that the catalytic reactions occur on the surface of nanostructured photocatalysts and their catalytic activities are strong dependent on their morphologies [16–19]. This is because the distinct surface-to-volume ratio of nanostructures and the amount of catalytically active sites, such as corners, edges, steps, etc.
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highly dominate the activation and adsorption/desorption of the involved catalytic reaction [17–21]. Therefore, it is an issue of considerable concern to synthesize photocatalysts with fascinating hierarchical properties [22]. Traditionally, these multi-dimensional architectures are achieved by multi-step assembly, template-sacrificial dissolution, chemically induced self-transformation and post-synthesis treatment, etc. [22]. In these cases, high-cost, abundant surfactants, toxic organic solvents and long-time consume procedures are needed, which limit the broad application of these advanced hierarchical architectures. In this regard, a facile and eco-friendliness route for preparation of hierarchical photocatalysts for the efficient removal of CIP is very imperative. Recently, a green technology for the synthesis of metal and metal oxide materials was raising more and more interest with the aid of plant extracts [23–26]. We all know that plant extracts are composed of many biologically active components, including polysaccharides vitamins, polyphenols, flavonoids, proteins, heterocyclic and carbonyl compounds and so on. These biologically active components can act as capping agent and template, which plan an important role in the synthesis of designed nanoarchitectures [24,24,25,26]. Inspired by the above elaboration, we herein reported a green synthesis of 3D
Corresponding author. Corresponding author at: College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, PR China. E-mail addresses: [email protected] (Y. Fu), [email protected] (M. Zhu).
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https://doi.org/10.1016/j.apcatb.2019.118149 Received 30 June 2019; Received in revised form 20 August 2019; Accepted 31 August 2019 Available online 03 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Scheme of the fabrication process of TP-TiO2 and RL-TiO2.
amorphous TiO2 (RLA-TiO2). For the synthesis of TP-TiO2, the above products were dispersed again into 70 mL of deionized water and then transferred to a 100 mL Teflon lining. Keep the reactor in a 180 °C air drying furnace for 3 h. After that, leave it to cool to room temperature and separate the product with ethanol for three times, followed by drying in a 60 °C vacuum drying chamber for 12 h. For the synthesis of RL-TiO2, the RLA-TiO2 products were directly calcined at 500 °C for 3 h in air, resulting RLTiO2 nanostructures.
tripyramid TiO2 (TP-TiO2) architectures with the assistance of aloe extracts and hydrothermal method. As shown in Scheme 1, we firstly obtained aloe extracts from the fresh aloe leaf, and then used them to be the solvent for the synthesis of amorphous TiO2. Rod-like morphology of TiO2 (RL-TiO2) nanostructure was formed in this amorphous TiO2. To get an anatase TiO2, a hydrothermal process was further carried out and then an interesting architecture with 3D TP-TiO2 nanostructure was obtained. Finally, such as-prepared 3D TP-TiO2 architectures showed effective degradation of antibiotic CIP pollutants under simulation solar light irradiation. The present results offer an eco-friendliness and lowcost protocol for the synthesis of advanced architectures in the application of antibiotics pollutants elimination.
2.4. Photocatalytic performance The samples of TP-TiO2 and RL-TiO2 were used as catalysts to catalyze the degradation of ciprofloxacin under UV–vis light condition. 50 mL CIP (32.6 μM) and 5 mg photocatalysts were magnetically stirred under darkness for at least 30 min to achieve adsorption equilibrium before photocatalytic reaction. During photocatalytic reaction, 2 mL above dispersions were taken every 5 min or 10 min for 10 consecutive times. The sample was centrifuged to remove catalyst, and then the absorbance at 275 nm was detected by UV–vis spectrophotometer. According to Lambert-Beer Law, the concentration change of CIP (Ct/ C0) can be represented by the corresponding absorbance. In the cycle experiments, after the completion of the first catalytic experiment, all catalysts were collected and put into the next catalytic experiment, and the experiments were repeated five times. For active species trap experiments, the concentration of CIP was 29 μM, superoxide dismutase (SOD, 0.16 g L−1) and potassium iodide (KI, 6.67 mM) were used as effective scavengers of superoxide radical (•O2-) and hole, respectively. Isopropyl alcohol (0.87 mM) was utilized as scavenger of hydroxyl radical (•OH). Other conditions were consistent with the above photocatalytic experiments.
2. Experimental section 2.1. Materials Fresh aloe was picked from PuTian, Fujian, China. TiCl3 (15.0%∼20.0% with 30% excess HCl), charcoal activated powder, absolute ethyl alcohol (C2H5OH), potassium iodide (99%, AR), isopropyl alcohol (99.5%, AR) were purchased from Sinopharm group chemical reagent Co., Ltd. Ciprofloxacin hydrochloride monohydrate (98%) was purchased from Tokyo Chemical Industry Co., Ltd. Superoxide dismutase (BR, 2500～7000 u mg−1, from pig blood protein) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. All chemicals were used without purification. 2.2. Preparation of aloe extracts (ALE) Typically, 100 g of peeled aloe were mixed with 200 mL of water, and then the mixture was completely milked using soybean milk machine. After that, the mixture was put under ultrasonic treatment for 20 min. After ultrasonic treatment, the mixture was filtered to remove residues and 4.0 g active carbon powder was added into as-prepared initial extract under stirring for 8 h in order to decolorize. Finally, the extract was obtained by filtration treatment again and stored at 4 °C environment for later use.
2.5. Characterization The surface morphologies and sizes of photocatalysts were observed and analyzed using a FESEM (Hitachi SU8220 Hitachi, Tokyo, Japan). The samples were prepared by diluted with ultrapure water and then dropped on the silicon sheet and dried by the infrared lamp. TEM measurements and HAADF-STEM-EDX analyses were performed on a FETEM (FEI Tecnai G2 F20) instrument with an acceleration voltage of 200kv. The crystal phase analysis of TiO2 powder is obtained by an Xray diffractometer (BRUKER D8 Advance), use a wide angle (2θ = 1080°) detection mode. UV − vis diffuse reflectance spectra were obtained on UV-3600 UV-VIS-NIR spectrophotometer (Shimadzu). X-ray photo-electron spectroscopy (XPS) was measured on a ESCALAB 250Xi XPS System (Thermo Fisher Scientific). The specific surface area and pore diameter were determined by the TriStar II 3020 specific surface and porosity analyzer. Photocatalytic reaction of the light source from 300 W Xe lamp (Perfectlight PLS-SXE300) with a filter, a light source
2.3. Preparation of TiO2 photocatalyst First, 30 mL of ALE was centrifuged at 20,000 rpm for 10 min to remove sediment, and then 60 μL TiCl3 was injected into 20 mL ALE in the 50 mL single-port bottle under magnetic stirring for 60 min. After that, milky white liquid was produced which showing that the rod-like precursor (RLP) colloid was synthesized in ALE. At last, the milky white RLP colloid was centrifuged at 800 rpm for 10 min. And the supernatant was carefully discarded and the RLP was dispersed again by adding ultrapure water and further purification more than two times with ultrapure water and ethanol, respectively, resulting in rod-like 2
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titanium source (TiCl3) into ultra-pure water. As shown in Fig. S2, only small size of nanoparticles of amorphous TiO2 was obtained. This result confirms the aloe extract plays an important role in the synthesis of TiO2 with specific morphology. Fig. 1C shows the HRTEM image of as-prepared RLA-TiO2, confirming that an amorphous structure was formed in the as-prepared samples. Further analysis of RLA-TiO2 was investigated by HAADFSTEM-EDX (Fig. 1D–H). It can be seen that beside Ti and O elements were observed in the as-synthesized RLA-TiO2, the elements of C and N were also detected. These elements might be derived from amino acids and polysaccharides in aloe meat, similar to the other reported result [28]. The concentrations of titanium source and aloe extracts were also carried out to investigate the optimum condition RLA-TiO2. Firstly, with the increase of titanium source concentration, the yield of rod particles first increased and then decreased (Fig. S3). If the amount of titanium source is overload or insufficient, the protective effect of aloe extracts or the yield of rod-like micelle will be reduced, respectively. In addition, different concentrations of aloe extracts were used as the reaction environment to prepare TiO2 particles. When the ratio of aloe leaf flesh to water mass was 1:2, the yield of rod-shaped particles was the highest, while the concentration of aloe leaf flesh extract was too high or too low, the yields of rod-shaped particles were greatly reduced and many flower-shaped particles were generated (Fig. S4). This indicates that the concentration of active components in plant extracts is the key to the formation of rod-like precursors. Furthermore, reasonable reaction time is also an important factor to obtain high yield rodshaped granules (Fig. S5). Insufficient reflection in a relatively short time, the particle shape is slender and uneven. When the reaction time was 1 h, the morphology of the rod particles was uniform, but as the reaction continued, obvious cracks appeared on the rod particles. The active components in aloe extract on the formation of rod particles were further studied by FT-IR spectroscopy. As shown in Fig. S6a, for aloe extract, the strong peaks appear in 2927 and 1710 cm−1 were assigned to −OH stretching vibration and C]O stretching vibration, respectively. The strong broad peaks between 3733 to 2522 cm−1 were assigned to νNH or νOH [29,30]. In addition, the bending vibration of amidogen group can be seen at 1612 cm−1, which giving evidence of the amidogen group existence [31]. Signals observed in the spectra of hydroxy and ether groups belonging to glycosyl residue can be seen in 1032 cm−1 (νOH) and 1080 cm−1 (νC-O-C). These results
with the wavelength range of 320–780 nm was provided for the reaction. Electron paramagnetic resonance (EPR) experiments were performed using the Bruker E500 instrument to detect the formation of free radicals in the reaction [27]. DMPO (5, 5-dimethyl-l-pyrrolidine noxide) was used as the spin trap, and the light source was ultraviolet light. 2.6. LC–MS analysis The transformation products formed during CIP degradation were studied by LC–MS. Ciprofloxacin solution of 12 mg L−1 was used for sampling at different catalytic times. The samples were tested using Thermos TSQ Quantum Ultra. Chromatographic separation was performed on an ultimate XB C18 2.1 × 100 mm, 3 μm HPLC column. The mobile phase is composed of 0.1% formic acid in water and acetonitrile. During operation, acetonitrile always accounts for 22%. The flow velocity was 0.3 mL min−1. The separated samples were imported into the mass spectrometry system and ionized using ESI as an ion source in a positive mode. The scan range was set from m/z 50 to m/z1000. The capillary and vaporizer temperatures were set at 370 °C and 240 °C, respectively. The CIP transformation was recorded with Shimadzu Essentia Prep LC-16 P HPLC instrument. SinoPak C18 4.6 × 250 mm 5 μm HPLC column was used for separation. The mobile phase is consisted of acetonitrile: aqueous phase = 2:8 (the aqueous phase is the mixture of 20 mmol L−1 NaH2PO4:20 mmol L−1 Na2HPO4 = 1.5:1). The flow rate was 0.5 mL min−1 and all compounds were monitored at the wavelength of 275 nm. 3. Results and discussion 3.1. Morphology and structure of the samples The RLA-TiO2 was synthesized directly from the mixture of aloe extract and water at room temperature. The morphology of prepared RLA-TiO2 was studied by SEM and TEM Fig. 1A and B. With assistance of aloe extract, rod-like shape was formed in the as-synthesized samples. The enlarge SEM image of RLA-TiO2 was shown in Fig. S1 and the analysis shows that the length of particles is about 2.5 μm. We also investigate the as-synthesized amorphous TiO2 by using traditional hydrolytic deposition method, viz. by adding sodium bicarbonate and
Fig. 1. SEM (A), TEM (B), HRTEM (C), and STEM (D) images of RLA-TiO2. (E–H) EDS elemental mapping images showing the Ti, O, C, and N atoms of the area of (D), respectively. 3
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XPS spectra of RL-TiO2 and TP-TiO2 were further recorded, which are shown in Fig. 2C–D. In the sample of TP-TiO2, the peaks attributed to the binding energy of Ti 2p3/2 and Ti 2p1/2 were easily observed at 458.5 and 464.3 eV, respectively [37]. The binding energy between two peaks was 5.8 eV, which was consistent with the binding energy corresponding of Ti4+ in TiO2 [37,44]. The O 1s spectrum of TP-TiO2 can be divided into two peaks at 529.8 and 531.8 eV, which corresponds to Ti–O and −OH on the surface. It’s found that both the peaks of Ti 2p and O 1s in TP-TiO2 were shifted to lower binding energy compared to these of in RL-TiO2. Theoretically, the decrease of binding energy is due to the enhancement of electron shielding effect caused by the increase of electron concentration around Ti and O atoms [45,46]. Beside the main contents of Ti and O, there is a peak at 399.8 eV in the TP-TiO2, which is assigned to N 1s signal, while no peak has been found in the sample of RL-TiO2 (Fig. S8). This result represents the presence of amino group on the surface of TP-TiO2 after hydrothermal treatment. The morphologies and crystallization of as-prepared samples after calcination and hydrothermal process were investigated by SEM, TEM and HRTEM images. As shown in Fig. 3A and B, the rod-like shape of TiO2 was kept after calcinations process. Furthermore, HRTEM image shows clear lattice fringes with d-spacing of 0.35 nm in the selected area of sample (Fig. 3C and D), which was attributed to (101) planes of anatase TiO2. This result further confirms the well crystallization of TiO2 after calcinations process. Interesting, after RLA-TiO2 were treated by hydrothermal process, the morphology of rod was changed to 3D tripyramid structure. As shown in Fig. 3E and F, homogenous 3D tripyramid structures of TiO2 were obtained after hydrothermal treatment at 180 °C with 3 h. The enlarge image of TEM shows that the 3D tripyramid structure was constructed by small size TiO2 particles (ca. 10 nm, Fig. 3G). HRTEM image further shows the well crystallization of TiO2 and the lattice fringes were mainly 0.35 nm, corresponding to (101) crystal planes of anatase TiO2 (Fig. 3H). Further analysis of TP-TiO2 by HAADF-STEMEDX (Fig. 3I–K) showed that obvious Ti and O signals were obtained, indicating that Ti and O elements were the main components of 3D tripyramid structure. The different synthetic conditions including hydrothermal reaction time and temperature were investigated Figs. S9 and S10. As shown in Fig. S9, the reaction precursor in this system was rod-like particles with regular morphology before the reaction started. With the increase of reaction time, the morphology of the rod-like titanium dioxide particles gradually changes. When the reaction is carried out for 0.5 h, the middle part of the rod-like titanium dioxide particles is dented, and a large number of small particles can be observed. With the reaction time is 3 h, TP-TiO2 is successful synthesized. On the other hand, the hydrothermal reaction temperature also plays an important role in the synthesis of nanostructures. At 180 °C, homogeneous 3D tripyramid structures were easily obtained. When the temperature was decrease, the tendency of rod-like precursor to 3D tripyramid particles gradually decreased (Fig. S10). For a closed hydrothermal reactor, the external high temperature environment will generate great heat and pressure in the reactor, forming an extreme reaction atmosphere. It is speculated that the rod-like precursor will lose water and gradually crystallize under high temperature and high pressure in the closed hydrothermal reactor. In beginning, the solubility of the rod-like precursor is similar to that of the crystal. During the reaction process, the high temperature and pressure conditions in the hydrothermal reactor upset the equilibrium and caused the TiO2 atoms to rearrange to form the crystal phase. The process includes a dissolution and crystallization equilibrium, which conforms to the following thermodynamic equation [47]:
provide a visual evidence of amino acid and glycosyl occurrence in the aloe extracts. For the samples of RLA-TiO2 (Fig. S6b), the disappeared νOH peak at 2927 and 1710 cm−1 in carboxyl group, together with the appearance of asymmetrical stretching vibration and the symmetrical stretching vibration (assigned to carboxyl group) at 1611 cm−1 and 1402 cm−1 give the evidence of the absence of free carboxyl group around the Ti4+, which may owing to the Ti4+ replacing the H+ in carboxyl group [32–34]. It may be pointed out that the νasym (COO) (1611 cm−1) is lower than the parent acid (1710 cm−1) is an indication of carboxyl group involvement in chelation and form coordination compound. The amidogen νN-H expansion vibration peak appears in the form of a wide and strong peak at 3600 to 2700 cm−1 and has a peak at 3300 cm−1, belonging to RLA-TiO2, which is shifted to a lower frequency. The reduction of the stretching frequency of amino group may be related to the intermolecular hydrogen bond [35,36]. Therefore, we speculate that the active ingredients in aloe extract play a decisive role in the synthesis of RLA-TiO2. First, amino acids and Ti4+ form coordination compounds, which then aggregate through hydrogen bond interactions. In the process of coordination compound aggregation, amino acids and sugars are used as the capping ligands for the encapsulation. Finally, rod-shaped particles with regular morphology are formed. The component and surface information on RLA-TiO2 were further analyzed by XPS spectra, which are shown in Fig. S7. Firstly, in the survey spectra of XPS, Ti, O, C, and N signals were detected (Fig. S7A). Secondly, for Ti 2p, two peaks were observed at 458.6 and 464.4 eV, which can be assigned to Ti 2p3/2 and Ti 2p1/2, respectively [37]. In the spectrum of O 1s, three peaks at 529.8, 531.5, and 532.9 eV were detected, which corresponds to O2−, −OH, and chemically adsorbed water as well as the ether group (COe), respectively [37]. For C 1s, three peaks can be fitted at 284.8, 286.3, 288.5 eV, corresponding to CeC and CH bonds, CO, and COee], respectively [37–39]. Note that, N signal was also detected, and the peak at 399.6 eV was represented the NHe bond. We speculate that it comes from an amino acid in aloe extract [40]. The other two lower peaks are corresponded to chemically modified –N2 (400–402 eV) [41]. By analyzing the XPS results, we speculated that amino acids and sugars existed on the surface of RLATiO2, which once again demonstrated that amino acids and sugars could play an important role in encapsulating the synthesis of rodshaped titanium dioxide. These results combined with above investigations of STEM-EDS and FT-IR solidly proved that amino, carboxyl and hydroxyl groups were detected on the surface of RLA-TiO2, which play the key factors for the formation of rod-like morphology. Generally, the amorphous TiO2 has negligible photocatalytic activity owing to its poor crystallization. To obtain the well-crystallization of TiO2, calcination and hydrothermal methods were used, respectively. Firstly, FT-IR spectra of samples were studied after calcination and hydrothermal methods. Fig. 2A shows a broad band around 3400 cm−1, which is caused by the stretching vibration of the hydroxyl group on the surface of TiO2 [42]. Besides, peaks at 471 and 464 cm−1 are corresponded to the stretching vibration of TiO2. Compare to RLA-TiO2 (Fig. S6b), organic groups of amino, carboxyl and hydroxyl groups were removed from the surface of samples after calcination and hydrothermal process. XRD pattern is an important characterization to evaluate the well crystallization of samples. As shown in Fig. 2B, very poor crystal phase was observed in the RLA-TiO2 precursor. After calcination and hydrothermal process, well crystallized peaks were observed in both samples of RL-TiO2 and TP-TiO2. The distinct peaks at 2θ = 25.10°, 37.84°, 47.70°, 53.91°, 54.58° and 62.55°, were correspond well to the anatase phase of TiO2 of (101), (004), (200), (105), (211), and (204) planes, respectively [43]. There are no other impurity peaks were detected, suggesting that the purity of the anatase TiO2 were obtained after calcination and hydrothermal process. Note that, compared with the TPTiO2, the half width of peak (101) in RL-TiO2 is narrower, indicating that its grain size is larger.
ΔG = ΔGS − C
(1)
Herein, ΔGS-C refers to in situ crystallization energy of RLA-TiO2 particles, which is less than zero and means the process is crystallization direction. RLA-TiO2 contains a certain proportion of amino acids and sugars, which will bind some TiO2 units in the crystallization process, 4
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Fig. 2. (A) FT-IR spectra of TP-TiO2 (a) and RL-TiO2 (b); (B) XRD patterns of the samples of RLA-TiO2 (a), RL-TiO2 (b), and TP-TiO2 (c). (C and D) XPS spectra of TPTiO2 and RL-TiO2 of O 1s (C) and Ti 2p (D).
Fig. 3. SEM (A and E) images of RL-TiO2 (A) and TP-TiO2 (E), TEM (B, C, F, and G) and HRTEM (D and H) images of RL-TiO2 (B–D) and TP-TiO2 (E–H), STEM image of TP-TiO2 (I), EDS images of Ti (J) and O (K) elements in the area of image I.
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Fig. 4. (A) Nitrogen adsorption–desorption isotherm of TP-TiO2, (B) UV–vis DRS spectra of (a) RL-TiO2 and (b) TP-TiO2.
where C is concentration of the CIP molecules, t is reaction time, and k is the rate constant. It can be found that the rate constants of TP-TiO2 (4.03 × 10−2 min−1) is 3.4 times than that of RL-TiO2 (1.19 × 10−2 min−1), which further confirm that TP-TiO2 presents superior photocatalytic performance for the elimination of CIP pollutants. In the practical application, the stability of photocatalyst is a very important factor. To evaluate the stability of as-prepared TP-TiO2, cyclic experiments for the photodegradation of CIP were performed. As shown in Fig. 6A, TP-TiO2 showed negligibility decrease of photocatalytic activity with five cyclic experiments. Moreover, the SEM image shows that 3D tripyramid structures also same as the morphology of the original TP-TiO2, which is shown in Fig. 6B. These results indicate the as-prepared TP-TiO2 kept stable photocatalytic during long time photo-irradiation reactions. Moreover, by detecting the XRD pattern before and after the photocatalytic degradation of CIP of TP-TiO2, it was found that the position of the characteristic peak did not change, further indicating that the crystallization state of catalyst remains stable during photocatalysis (Fig. S12). Generally, in the photocatalytic system of TiO2, the oxidation ability of photogenic holes and the reduction ability of photogenic electrons are very strong. The photogenic hole can oxidize OH-/ H2O to form % OH, and the photogenic electron can reduce O2 to form %O2−, as shown in the following equation (3)-(5).
allowing them to crystallize in tiny areas. At the same time, convection occurs in the liquid center during the hydrothermal process, generating additional driving forces in the liquid reaction system inside the hydrothermal reactor. Due to the strong impact force generated by convection and the relatively large stressed area in the longitudinal position of the rod-like particles, RLA-TiO2 gradually fractured from the middle in the crystallization process and finally constructs 3D tripyramid structures. 3.2. Optical absorption properties of photocatalysts The 3D TiO2 structure constitutes the TiO2 interconnection network, and the existence of pores in the particles significantly increases the surface area of the samples. Fig. 4A shows the nitrogen adsorptiondesorption isotherm of TP-TiO2. The result shows the sample with hysteresis loop, indicating the distribution of pores in the TP-TiO2. This non-dense particle arrangement increases the number of active sites involved in the reaction and improves the catalytic efficiency. The specific surface area of TP-TiO2 is 84 m² g−1, while the specific surface area of RL-TiO2 is only 65 m² g−1. The optical properties of samples were studied by UV–Vis diffuse reflectance spectroscopy (DRS). In general, the absorption degree of photocatalyst is an important factor to reflect its photocatalytic performance. Fig. 4B shows the UV–Vis DRS spectra of TP-TiO2 and RLTiO2 with a wavelength range of 200–800 nm. Both the absorbance of two catalysts showed obvious absorption edge at 387 nm, showing the typical TiO2 absorption and with the band gap of 3.2 eV. 3.3. Photocatalytic activity and stability under ultraviolet-visible light
(3)
e− + O2 → •O2−
(4)
h+ + H2 O→ •O H+ H+
(5)
In order to explore the reaction mechanism in the photocatalytic degradation process of TP-TiO2, and to find the active species which played the decisive role in the degradation process of CIP, a series of active species trap experiments were conducted, which is shown in Fig. 7A. The effects of different kinds of trapping agents on CIP degradation were studied, so as to judge the contribution of each reactive oxygen group in the reaction process. It can be seen that when superoxide dismutase (SOD) was added, the degradation rate of CIP was significantly reduced due to the scavenging of superoxide radicals. Similarly, when potassium iodide (KI) is added, the holes are cleared and the degradation rate of CIP is greatly slowed down. These results indicate that superoxide radicals and photogenic holes are the key factors for CIP degradation. However, when isopropanol was added, the degradation rate of CIP was almost constant with without scavenger, indicating that hydroxyl radical had little contribution to the photodegradation of CIP. In order to further understand the generation of active radicals during irradiation, DOMO was used to detect %O2− radical. As shown in Fig. 7B, 11 characteristic peaks of DMPO-%O2− adducts could be observed after ultraviolet light irradiation, but the related characteristic peaks did not appear under dark conditions. This result suggests that superoxide radicals do exist in the photocatalytic system with TP-TiO2 as catalyst.
Based on the above optical properties, we evaluated the photocatalytic performance of TP-TiO2 and RL-TiO2 by photocatalytic degradation of antibiotic CIP. The photodegradation of CIP over different photocatalysts was monitored by measuring the real-time UV–vis absorption of CIP at 275 nm, as shown in Fig. S11. Firstly, pure CIP without any photocatalyst kept stable under UV–vis light irradiation. By using as-prepared TiO2 as photocatalysts, the absorptions of CIP molecules in the solution were decrease with the increase of photo-irradiation time, indicating the degradation of CIP molecules. The ratios of photocatalytic degradation of CIP were summarized in Fig. 5A. By using RL-TiO2 as photocatalyst, 52% CIP molecules were removed under 60 min UV–vis light irradiation. However, when TPTiO2 was used as the photocatalyst, 90% CIP pollutants were degraded under the same conditions. To show the detail photocatalytic degradation rate, the correlation between ln(C/C0) and the reaction time t are shown as plotted in Fig. 5B. The nice linear correlation between ln (C/C0) and t indicates that the decomposition reaction of CIP photocatalyzed by as-prepared TiO2-based catalysts follows the first-order kinetics:
− dC /dt = kC
TiO2 + hv → e− + h+
(2) 6
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Fig. 5. (A) Photocatalytic degradation CIP curves and (B) kinetic curves of CIP degradation with different photocatalysts and without catalyst.
intermediate CIP-5 (m/z 245) [49] and finally with completely degradation to CO2, H2O, and other small molecules products.
Based on the above analysis, the basic mechanism for the degradation of CIP was proposed (Fig. 8). Firstly, after excitation of TiO2 under UV–vis light irradiation, generated electrons from the valance band (VB) to conduction band (CB). Secondly, adsorbed oxygen molecules on the surface of TP-TiO2 will be reduced by these electrons to form %O2− species. On one hand, the generation of %O2− inhibits electron-hole recombination, thus improving the quantum yield of photocatalytic reactions. On the other hand, these %O2− is an important active oxidative group, CIP molecules will be degraded to smaller molecules and finally to CO2, H2O, and others. At the same time, the generated holes also as the main oxidative components, it also participates in the degradation of CIP. The transformation products formed during CIP degradation were studied by LC–MS. Fig. S13 show the representative chromatograms of the solution after reaction for 0, 30 and 60 min. At 0 min, a typical ion peak for pure CIP at retention time of 8.79 min was observed in the original solution. Different ion peaks were detected, indicating the formation of different intermediates after 30 min of light irradiation with TP-TiO2. Lastly, the intensities of these peaks decrease after 60 min of light irradiation with TP-TiO2, suggesting the mineralization of CIP molecules. The representative intermediates after 30 min light irradiation together with the corresponding mass spectra, were shown in Table S1. The possible photocatalytic degradation pathway of CIP was shown in Fig. 8, by referencing relevant reports and identifying intermediate products [48]. The oxidative degradation of the piperazine moiety is the main degradation pathway. Firstly, Piperazine was oxidized and lost a formaldehyde group, generating two isomers with m/z of 334 (CIP-1). Secondly, CIP-1 offs formaldehyde to produce CIP-2 (m/z 306). Thirdly, CIP-2 underwent secondary amine nitrogen loss, oxidation and formaldehyde loss from the amine side chain, and successively formed CIP-3 (m/z 291) and CIP-4 (m/z 263). This part of the transformation process was consistent with the results of CIP degradation in the literature [48]. Lastly, CIP-4 reduced and defluorinated to form
4. Conclusions In conclusion, a green and eco-friendliness technology for the synthesis of rod-shaped amorphous TiO2 with high yield in aloe extract environment was designed and then 3D anatase TP-TiO2 structures were obtained by further hydrothermal crystallization. The form of small particle accumulation in 3D TP-TiO2 makes each other to form an interactive network with special configurations and high specific surface area, which can provide more active sites for catalytic reactions. Under UV–vis light irradiation, the as-prepared 3D TP-TiO2 architectures for the photocatalytic degradation rate of CIP reached to 4.03 × 10−2 min−1. Compare to rod-like TiO2, the 3D tripyramid architectures greatly improved the efficiency of photocatalytic operation of TP-TiO2. Moreover, such architectures also displayed well stability for the degradation of CIP molecules. According to the analysis of photocatalytic mechanism, %O2− and holes were the main driving agent for CIP degradation under UV–vis light irradiation. The present studies showed a green and facile method to construct advanced architectures with high photocatalytic performance, which have responded to the current concept of economic and environmental protection and has great potential for further research. Declaration of Competing Interest 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. Acknowledgment This work was sponsored by the National Natural Science
Fig. 6. (A) Regeneration photodegradation of CIP with TP-TiO2 as catalyst. (B) SEM image of TP-TiO2 after photocatalytic reactions. 7
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Fig. 7. (A) Photocatalytic disinfection of CIP by TP-TiO2 respectively with different scavengers: superoxide dismutase (SOD, 0.16 g L−1), isopropyl alcohol (0.87 mM) and potassium iodide (KI, 6.67 mM). (B) The production of •O2- in TP-TiO2 system under different light and dark conditions.
Fig. 8. Schematic diagram of photocatalytic process and the degradation pathway of CIP molecules.
Foundation of China (21603111 and 21563009).
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