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In-situ N-doped mesoporous black TiO2 with enhanced visible-light-driven photocatalytic performance
Chemical Physics 513 (2018) 86–93
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Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
In-situ N-doped mesoporous black TiO2 with enhanced visible-light-driven photocatalytic performance ⁎
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Hang Zhanga,1, Yan Zhanga,1, Junwei Yina, Zhenzi Lib, , Qi Zhua, , Zipeng Xinga, a b
T
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Department of Environmental Science, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous TiO2 Surface N doping Visible-light-driven photocatalysis Photocatalytic degradation
In-situ N-doped mesoporous black TiO2 photocatalyst is fabricated by a facile sol-gel method combined with an in-situ solid-state chemical reduction technique, using urea as nitrogen resource. The N-doped mesoporous black TiO2 possesses high surface area of 96 m2 g−1, large pore size of 5.4 nm and narrow band gap of 2.05 eV. Meanwhile, the optical absorption extends to visible light region because of Ti3+ and N co-doping. The N-doped mesoporous black TiO2 exhibits higher photocatalytic activity than that of others under visible light irradiation for photocatalytic degradation of methyl orange (MO) and rhodamine B (RhB), which is up to 93.27% and 93.64%, respectively. The enhancement is ascribed to the synergistic effect of the N and Ti3+ co-doping extending the visible light absorption, the separation efficiency of photogenerated charge carriers, and the mesoporous structure with large surface area offering more surface active sites, which is beneficial for improving the photocatalytic activity.
1. Introduction As one of the most promising photocatalysts for the degradation of organic pollutants, the production of hydrogen and the supercapacitor, titanium oxide (TiO2) has caused wide concern because of its nontoxicity, low cost, high stability and excellent photochemical activities [1,2]. However, there exist several disadvantages of TiO2 which influence the photocatalytic performance: the large band gap of TiO2 (∼3.2 eV) deeply impedes its practical applications, and the fast recombination of the photogenerated electron-hole pairs greatly reduces the photocatalytic activities [3,4]. Some researchers have tried to overcome these defects. Doping TiO2 with nonmetal such as nitrogen (N) [5], carbon (C) [6], sulphur (S) [7], and fluorine (F) [8] can improve the performance of TiO2, since suitable dopants can narrow the TiO2 bandgap and accelerate the separation of photo-induced electrons and holes [9]. Among different systems, N doping is considered to be an ideal alternative because N 2p states can effectively mix with O 2p states [10,11]. It has been predicted by theory and demonstrated in experiments that N-doped TiO2 [12–14] exhibited excellent photocatalytic activity under visible light excitation for the degradation of organic compounds. The existence of nitrogen doping extended the optical absorption to the visible light region and narrowed the bandgap of TiO2 by mixing N 2p and O 2p states on the top of the ⁎
valence band [15–17]. Recently, Ti3+ doped TiO2 has attracted much attention. It is reported that Ti3+ doped TiO2 could enhance visible light absorption and improve the photocatalytic performance [18–20]. The hydrogenation of TiO2 is known to dramatically reduce the bandgap energy. It is shown that when TiO2 is treated with high pressure in H2 atmosphere, the black anatase TiO2 is obtained. Therefore, the formation of midgap states above the valence band is likely responsible for the creation of Ti3+ centers within the TiO2 lattice, thus modifying the band gap, which is important for photocatalytic process [21–23]. So far black TiO2 containing Ti3+ centers has been often obtained by creating surface defects on the TiO2 nanoparticles under high pressure or extended reaction periods [24–26]. It is believed that the interaction between the N-doping and Ti3+ is the key to the enhanced visible light absorption and superior photocatalytic activities [27–29]. In this work, a facile sol-gel and in-situ solid-state chemical reduction methods are conducted to synthesize a Ti3+ and N co-doped mesoporous black TiO2. The band gap of the as-prepared sample has been narrowed. Photodegradation testing demonstrates that Ti3+ and N codoped TiO2 shows significant improvement for decomposing MO and RhB under visible light irradiation. The interaction between N-dopant and Ti3+ is demonstrated to be the key of enhanced photo-sensitive prosperities and excellent photocatalytic activities.
Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (Q. Zhu), [email protected] (Z. Xing). 1 Note: H. Zhang and Y. Zhang contributed equally. https://doi.org/10.1016/j.chemphys.2018.07.021 Received 7 March 2018; Accepted 17 July 2018 Available online 18 July 2018 0301-0104/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental materials
measurements were employed with a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. The laser beam was using a 50 × objective lens to a ca. 1 μm spot on the surface of the sample. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI-5700 ESCA equipped with Al-Kα X-ray source. All binding energies were calibrated using the C 1 s peak at 284.6 eV of the surface adventitious carbon as a reference. The Fourier transform infrared spectra (FI-IR) of the samples were taken with a PerkinElmer spectrum one system, using KBr as diluents. Transmission electron microscopy (TEM) was examined with a JEM2100 electron microscope (JEOL, Japan). Nitrogen adsorption-desorption isotherms at 77 K were performed with an AUTOSOR-1 (Quantachrome Instruments) nitrogen adsorption apparatus. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area. The pore volumes and pore diameter distributions were calculated from the adsorption-desorption isotherms using the Barrett-Joyner-Halenda (BJH) model. The UV–vis diffuse reflectance spectroscopy (UV–vis DRS) were recorded on a UV–vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment using BaSO4 as the reflectance standard.
All reagents, i.e., tetrabutyl titanate (TiOBu)4, absolute ethanol (EtOH), nitric acid (HNO3), and carbamide, were purchased from Tianjin Kermel Chemical Reagent Co. LTD, China. Sodium boron hydride (NaBH4, 98%) was purchased from Aladdin Reagent Company, China. All reagents used in the experiments were analytical grade and employed without further purification, and the deionized (DI) water was used throughout this study. 2.1. Preparation of N doped TiO2 photocatalysts The N doped TiO2 powders were prepared using the sol-gel method, as described below. In a typical preparation process, 20 mL of tetrabutyl titanate was dissolved in 80 mL of absolute ethanol to produce homogeneous Ti(OBu)4-EtOH solution. Meanwhile, volume ratio of DI water/ ethanol/nitric acid was kept at 4/5/1 under magnetic force stirring to form EtOH-HNO3-water solution. Later, water based solution was added dropwise into Ti precursor solution under constant stirring for 1 h. To get the desired concentration of N in TiO2, the required stoichometric amount of aqueous solution of carbamide was then added to the EtOHHNO3-water solution and stirred vigorously for 0.5 h to form the light yellow solution. Next, the transparent solution was slowly added dropwise to the Ti(OBu)4-EtOH solution under magnetic force stirring to carry out a hydrolysis and stirred vigorously for 4 h. After aging for 18 h, doped solution was dried at 60 °C to form the gel and then heated with 2 °C min−1 for elevating temperature till 450 °C, and the N doped TiO2 powders were eventually obtained after calcination at 450 °C for 4 h. Finally, the as-synthesized samples were successfully obtained. The color of these N doped TiO2 nanoparticles was light yellow, which were denoted as N-TiO2.
2.4. Photocatalytic test The photocatalytic activity of the samples was tested by investigating the degradation performance of methyl orange (MO) and rhodamine B (RhB) under visible-light irradiation. A 350 W xenon lamp was mounted with 420 nm cut-off glass filter was used as a visible light source. In a typical process, 25 mg as-prepared photocatalyst was added into 30 mL of 10 mg L−1 dye solution to perform the reaction suspension. Then, the suspension was magnetically stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. Afterward, the mixture was illuminated with visible light under magnetic stirring. At the given time intervals, 5 mL of suspension was collected and centrifuged to obtain a clear solution. The filtrates were analyzed by a Shimadzu Model UV2550 spectrophotometer at the characteristics wavelength of RhB and MO solution under the 553 and 464 nm. Furthermore, using the same procedure, the photocatalytic activity of the samples for the decomposition of MO was also investigated.
2.2. Preparation of black N doped TiO2 2 g of the as-prepared N-TiO2 powders were mixed with 5 g of NaBH4, and the mixture was stirred for 1 h. Then the mixture was transferred into a tubular furnace, heated from room temperature to 350 °C for 1 h under N2 atmosphere with a heating rate of 5 °C min−1. After naturally cooling down to room temperature, the mixture was washed with DI water and absolute ethanol for several times to remove the unreacted NaBH4, and then dried at 60 °C. Finally, the black N doped TiO2 was obtained, which was denoted as b-N-TiO2.
3. Results and discussion The crystal phase compositions of the samples are characterized by X-ray diffraction measurements at room temperature [30]. The XRD patterns of various samples are shown in Fig. 1a. The diffraction peaks at 25.3, 37.8, 48.1, 54.1, 54.9, 62.7, 68.9, and 70.2° correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), and (2 2 0) for anatase TiO2 lattice plane, respectively. It shows that all diffraction peaks can be fully indexed to anatase crystalline phase of TiO2 [31]. It is
2.3. Characterization X-ray diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer with a Cu Kα radiation (λ = 1.5406 Å). Raman
Fig. 1. XRD patterns (a) and Raman spectra (b) of TiO2 (a), N-TiO2 (b), and b-N-TiO2 (c), respectively. The inset of (b) is the magnified spectra between 100 and 200 cm−1. 87
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these results are consistent with the result of XRD. The XPS measurements are carried out to further investigate the surface composition and chemical state of the samples. Fig. 3a shows that the Ti 2p peaks of b-N-TiO2 can be deconvoluted into four peaks as Ti3+ 2p3/2 at 457.84 eV, Ti4+ 2p3/2 at 458.43 eV, Ti3+ 2p1/2 at 463.54 eV, and Ti4+ 2p1/2 at 464.11 eV, respectively [38]. These Ti3+ species are formed due to the Ti4+ reduction of TiO2 by the NaBH4. The presence of Ti3+ species can suppress the recombination of photogenerated electron-hole pairs, and thus promote the photocatalytic activity. Fig. 3b shows O 1 s configuration of the b-N-TiO2. The peaks at 529.6 and 531.1 eV should be attributed to Ti-O bonds and the surface –OH groups, respectively [39]. However, the peak of the oxygen in the TiO2 lattice shifts to lower energy due to the existence of oxygen vacancy on the TiO2 surface. Oxygen vacancies (Ov) should be produced by releasing the O from N doped TiO2 during the reduction process [40]. As shown in Fig. 3c, the dominant N 1s XPS peaks for b-N-TiO2 samples located at 401.2 eV and 399.6 eV which are consistent with the binding energies of Ti-O-N and N-Ti-O, respectively. The results demonstrate that the nitrogen is successfully doped into the lattice of TiO2. Therefore, the obtained XPS results can be in good agreement with the above analysis. The valence band (VB) positions of TiO2, N-TiO2 and b-N-TiO2, can be observed in Fig. 3d. The VB edge is studied by linear extrapolation of the peaks to the baselines. The VB edge of TiO2, N-TiO2 and b-N-TiO2 is 2.30, 1.86 and 1.86 eV, respectively. The VB edge of N-TiO2 is below Fermi energy, indicating N is forming in TiO2 [41]. The N doped TiO2 with N up-shifts the VB edge induces a remarkable narrowing of bandgap, and then improving the visible-light photocatalytic activity. Evidently, the VB edge of b-N-TiO2 is similar to N-TiO2, suggesting that Ti3+ self-doped can not influence the VB edge. However, according to the above UV–vis DRS results, the bandgap of b-N-TiO2 is narrower than N-TiO2, suggesting that the doped Ti3+ reduces the conduction
rather remarkable that no peaks related to the doping species could be observed in the XRD patterns. One of the reasons might be that the concentration of dopants is too low to be detected by XRD [32]. Compared with pure TiO2, the increase of intensity peak of the N-doped TiO2 indicates higher crystallinity. For the b-N-TiO2 sample, the intensity of (1 0 1) diffraction peak becomes weaker due to the Ti3+ doping. The particle size is calculated from the anatase peak (1 0 1) with the Scherrer equation, and is found to be 11.2 nm. The result is corresponded with the TEM observation. As a further confirmation, the anatase crystal structure is also investigated by the Raman technique, which is displayed in Fig. 1b. The Raman spectra show four obvious peaks at 158, 402, 520 and 643 cm−1, which are ascribed to the Raman-active modes of anatase phase with Eg, B1g, A1g and Eg symmetries, respectively [33]. A slightly shift in Raman peaks is also observed as the replacement by N dopant in the TiO2 lattice [34]. So the N-doped TiO2 samples are confirmed to be successfully prepared. Due to the addition of NaBH4, the intensity of Raman peak shows apparently decreased due to the formation of Ti3+ and oxygen vacancy in TiO2 lattice [35], which is corresponded to the XRD results. TEM images are taken to analyze the morphology and crystal structure of the samples. From Fig. 2a and b, we can find that the prepared samples are composed of small particles with good dispersion. Fig. 2c and d exhibit the HRTEM images of TiO2 and N-doped TiO2, the crystal lattice fringes with interplanar spacing of 0.35 nm, which are corresponded to the (1 0 1) crystal facet of anatase TiO2, implying that the doping and solid-state reduction cannot change the crystal phase of the samples [36]. As shown in Fig. 2d, the b-N-TiO2 sample shows about 1–2 nm thick disordered surface layer. The disordered layer arises from Ti3+ and oxygen vacancy, which is beneficial for the separation of photogenerated carriers and inhibit the rapid recombination, thus improving electron transfer and photocatalytic activation [37]. Therefore,
Fig. 2. TEM and HRTEM images of N-TiO2 (a and c) and b-N-TiO2 (b and d), respectively. 88
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Fig. 3. The XPS spectra of b-N-TiO2, (a) Ti 2p, (b) O 1s, (c) N 1s, (d) VB XPS spectra of TiO2, N-TiO2 and b-N-TiO2 samples.
Additionally, the surface area and the BJH pore size of the N-TiO2 and b-N-TiO2 are all similar to each other. It can be considered that the visible-light photocatalytic activity may be excellent due to the existence of mesoporous structure [47]. The photocatalytic activity of the prepared TiO2 samples is evaluated by the degradation of MO and RhB under visible light irradiation. It can be seen in Fig. 5a and b, the photolysis studies prove that the MO and RhB are stable under visible light irradiation with significantly low self-degradation percentage of 1.8%. In the concentration reduction of MO and RhB are very low with degradation percent of 1.35% and 1.52% under visible light irradiation. These results generally show that the self-degradation of MO and RhB under visible light irradiation can be neglected. It is found that the pure TiO2, N-TiO2, b-TiO2 and b-NTiO2 show that the concentration of MO are reduced by 19.76, 80.28, and 93.27%, respectively, the concentration of RhB are reduced by 18.34, 76.66, and 93.64%, respectively, after visible light irradiation for 3 h. Moreover, the photocatalytic activity of N-TiO2 is higher than that of pure TiO2. The huge different in degradation ratio between TiO2 and N-TiO2 sample is due to the existence of nitrogen dopants in TiO2 lattice structure, as confirmed by XPS analysis. It has been widely reported that the nitrogen doping in the TiO2 lattice structure can improve photocatalytic activity under visible light irradiation [48]. When the N species and Sn species were codoped into TiO2, its photocatalytic activity has an obvious enhancement. The main reason for the improvement of photocatalytic activity should be attributed to the synergistic effect of the introduction of N and Sn co-doped TiO2 [49]. The degradation ratio by b-N-TiO2 is much higher than that of pure TiO2, NTiO2 and b-TiO2, indicating the b-N-TiO2 possesses excellent visible light photocatalytic activity.
band (CB) position of b-N-TiO2 [42]. The optical properties of as-synthesized TiO2 samples are investigated by UV–vis diffuse reflectance spectra and shown in Fig. 4a. For pure TiO2 sample, a strong absorbance in UV region is found for anatase TiO2. After doping with nitrogen, the absorption of N-TiO2 and b-N-TiO2 in visible region is greatly enhanced. It is expected that nitrogen doping contributed to the red-shift because of narrowing the band gap [43]. There occurs a broad absorption shoulder centered at about 430 nm and tailing absorption extends out to approximately 700 nm, which is the typical absorption feature reported for N doped TiO2 materials [44]. On this basis, for the b-N-TiO2 sample, the visiblelight absorption drastically increases when the wavelength is longer than 400 nm, superior to N-TiO2. Furthermore, estimations of the bandgap energies are obtained from the diffuse reflectance spectra of three samples. As shown in Fig. 4b, the band gap of TiO2, N-TiO2 and b-NTiO2 is 3.0, 2.48, and 2.05 eV, respectively. These results suggest that the b-N-TiO2 with a narrower band gap is more active than TiO2 and NTiO2 in the visible-light region. The strong absorbance in the visible region of b-N-TiO2 is attributed to the existence of oxygen vacancies, Ti3+ and N species. The N2 adsorption-desorption isotherms and BJH pore size distribution curves are employed to investigate the pore structure of the prepared samples. As shown in Fig. 4c, the samples exhibit typical IV nitrogen adsorption isotherm with a H2 hysteresis loop at a relative pressure between 0.4 and 0.9, implying the presence of mesoporous structure [45]. From Fig. 4d, the pore size distribution of N-TiO2 and bN-TiO2 calculated by the BJH method are around 5.4 nm, indicating the prepared TiO2 are mesoporous materials [46]. The BET specific surface areas of N-TiO2 and b-N-TiO2 are 94 and 96 m2 g−1, respectively. 89
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Fig. 4. UV-vis diffuse reflectance absorption spectra (a) and determination of the indirect interband transition energies (b) for TiO2, N-TiO2 and b-N-TiO2 samples, respectively, (c) N2 adsorption-desorption isotherm curves (c) and the corresponding pore size distribution plots (d) of N-TiO2 and b-N-TiO2.
achieving high photocatalytic performance of TiO2. The electrons can react with O2 to superoxide radical anions, which can further create the reactive hydroxyl radicals. The holes of VB react with H2O molecules to generate •OH radicals, which favors the photocatalysis. As is shown in Fig. S2a and b, the result reveals that no degradation is detected in the 60 min, and thus it can be considered that the selfdegradation is negligible. Compared with pure TiO2, the photocatalytic activity of other samples is all improved under visible light. In particular, the b-N-TiO2 exhibits the best photodegration ability, which reachs 91.5% of phenol removal within 150 min under visible light irradiation. In addition, the first-order rate constants k of TiO2, N-TiO2, bN-TiO2 have shown in Fig. S2b are 0.0164, 0.0056, 0.0002 min−1, respectively. Similarly, the b-N-TiO2 shows the highest value at 82 times higher than that of pure TiO2, which can be ascribed to the synergistic effect between N and Ti3+ co-doped. Cycling degradation of phenol is also provided to test the stability of the catalyst, which is shown in Fig. S2c. After six cycling degradation, the photocatalytic activity of b-NTiO2 displays neglect decrease. This result demonstrates the high stability of Ti3+ and N co-doped TiO2 photocatalyst prepared by this method. The mineralization of RhB and MO are inspected by TOC analysis. Fig. 5f shows the TOC removal rate on the initial 10 mg/L of RhB and MO photocatalytic degradation process with 150 min. In Fig. 5f, 5.4, 54.6 and 83.2% of TOC removal rate can be obtained for the initial 10 mg/L of RhB, 8.7, 62.4, 86.5% of TOC removal rate can be obtained for the initial 10 mg/L of MO. It indicates the RhB and MO are easier to be degraded by photocatalytic degradation. These results indicate the bN-TiO2 photocatalyst can mineralize the pollutants efficiently. The formation of hydroxyl radical and superoxide radical anions are
As shown in Fig. 5c and d, to quantitatively evaluate the photocatalytic efficiencies of these catalysts, the corresponding apparent reaction rate constants k of the MO degradation calculated by pseudofirst-order kinetic model are 0.0014, 0.0109, and 0.0188 min−1 for pure TiO2, N-TiO2, and b-N-TiO2, and the rate constants k of RhB degradation are 0.0013, 0.0101 and 0.0186 min−1 for pure TiO2, N-TiO2, and b-N-TiO2. Evidently, the kinetic constants k of the b-N-TiO2 for MO degradation is 1.7 and 1.4 times higher than that of b-TiO2. The kinetic constants k of the b-N-TiO2 for MB degradation is 1.5 times higher than that of N-TiO2. The enhancement of the photocatalytic activity of the sample is caused by the doping of Ti3+ and N [50]. Cycling degradation of MO and RhB is also provided to test the stability of the catalyst, which is shown in Fig. 5e. After six cycling degradation, the photocatalytic activity of b-N-TiO2 displays neglect decrease. This result demonstrates the high stability of Ti3+ and N co-doped TiO2 photocatalyst prepared by this method. Based on the results and discussions above, it can be concluded that b-N-TiO2 can generate much active radicals during the photocatalytic process, resulting in better photocatalytic activity. Therefore, a degradation mechanism is illustrated in Fig. 5f. Under visible light irradiation, the electrons can be excited from N 2p impurity energy level to the conduction band (CB), because N atoms are doped into the lattice of TiO2, which locate above the valence band (VB) [51]. As a consequence, the doping of nitrogen narrows the bandgap of TiO2. When the photocatalyst is illuminated by the visible light, the introduction of Ti3+ can produce impurity level below the CB, which further narrowing the band gap [52]. According to literature reports, part Ti4+ is reduced to Ti3+ and Ov can significantly prolong the lifetime of photo-induced electrons and improve the separation of the electron and hole pairs, 90
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Fig. 5. Photocatalytic activity with different samples (a and b), and variations of ln (C0/C) versus with different samples under visible-light irradiation on MO and RhB removal (c and d), six cycles of MO and RhB photocatalytic degradation with b-N-TiO2 (e), schematic diagram of the photocatalytic degradation of organic contaminates by b-N-TiO2 under visible-light irradiation (f), TOC removal on the 10 mg/L of RhB and MO photocatalytic degradation process with 150 min (g).
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Fig. 6. DMPO-trapping ESR results of the formation of hydroxyl radical and superoxide radical anions.
References
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4. Conclusions In summary, we demonstrated that in-situ N-doped mesporous black TiO2 photocatalyst were prepared through a facile sol-gel method and an efficient in-situ solid-state reduction method. The large specific surface area of b-N-TiO2, which indicated the porous structure existed in the sample, in this study, the mesopore was the aggregation of interparticles, which was confirmed by the N2 absorption and TEM observation. Compared with the as-prepared TiO2 and N-TiO2, the b-NTiO2 showed wonderful photocatalytic performance in the degradation of MO and RhB under visible light irradiation. The enhanced photocatalytic activity was ascribed to the narrowed band gap of TiO2 by N and Ti3+ doping, which inhibited rapid recombination of photoinduced carriers. Therefore, it provides a new method to enhance the photocatalytic performance of Ti3+and N doped TiO2, which provides a new thought to design and prepare other semiconductors for various applications.
Acknowledgements We gratefully acknowledge the support of this research by the Natural Science Foundation of Heilongjiang Province (B2018010 and H2018012), the Postdoctoral Science Foundation of China (2017M611399), the Heilongjiang Postdoctoral Startup Fund (LBHQ14135), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016018), and Postdoctoral Science Foundation of Heilongjiang Province (LBHZ16150).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.chemphys.2018.07.021. 92
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In-situ N-doped mesoporous black TiO2 with enhanced visible-light-driven photocatalytic performance ⁎
⁎
Hang Zhanga,1, Yan Zhanga,1, Junwei Yina, Zhenzi Lib, , Qi Zhua, , Zipeng Xinga, a b
T
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Department of Environmental Science, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous TiO2 Surface N doping Visible-light-driven photocatalysis Photocatalytic degradation
In-situ N-doped mesoporous black TiO2 photocatalyst is fabricated by a facile sol-gel method combined with an in-situ solid-state chemical reduction technique, using urea as nitrogen resource. The N-doped mesoporous black TiO2 possesses high surface area of 96 m2 g−1, large pore size of 5.4 nm and narrow band gap of 2.05 eV. Meanwhile, the optical absorption extends to visible light region because of Ti3+ and N co-doping. The N-doped mesoporous black TiO2 exhibits higher photocatalytic activity than that of others under visible light irradiation for photocatalytic degradation of methyl orange (MO) and rhodamine B (RhB), which is up to 93.27% and 93.64%, respectively. The enhancement is ascribed to the synergistic effect of the N and Ti3+ co-doping extending the visible light absorption, the separation efficiency of photogenerated charge carriers, and the mesoporous structure with large surface area offering more surface active sites, which is beneficial for improving the photocatalytic activity.
1. Introduction As one of the most promising photocatalysts for the degradation of organic pollutants, the production of hydrogen and the supercapacitor, titanium oxide (TiO2) has caused wide concern because of its nontoxicity, low cost, high stability and excellent photochemical activities [1,2]. However, there exist several disadvantages of TiO2 which influence the photocatalytic performance: the large band gap of TiO2 (∼3.2 eV) deeply impedes its practical applications, and the fast recombination of the photogenerated electron-hole pairs greatly reduces the photocatalytic activities [3,4]. Some researchers have tried to overcome these defects. Doping TiO2 with nonmetal such as nitrogen (N) [5], carbon (C) [6], sulphur (S) [7], and fluorine (F) [8] can improve the performance of TiO2, since suitable dopants can narrow the TiO2 bandgap and accelerate the separation of photo-induced electrons and holes [9]. Among different systems, N doping is considered to be an ideal alternative because N 2p states can effectively mix with O 2p states [10,11]. It has been predicted by theory and demonstrated in experiments that N-doped TiO2 [12–14] exhibited excellent photocatalytic activity under visible light excitation for the degradation of organic compounds. The existence of nitrogen doping extended the optical absorption to the visible light region and narrowed the bandgap of TiO2 by mixing N 2p and O 2p states on the top of the ⁎
valence band [15–17]. Recently, Ti3+ doped TiO2 has attracted much attention. It is reported that Ti3+ doped TiO2 could enhance visible light absorption and improve the photocatalytic performance [18–20]. The hydrogenation of TiO2 is known to dramatically reduce the bandgap energy. It is shown that when TiO2 is treated with high pressure in H2 atmosphere, the black anatase TiO2 is obtained. Therefore, the formation of midgap states above the valence band is likely responsible for the creation of Ti3+ centers within the TiO2 lattice, thus modifying the band gap, which is important for photocatalytic process [21–23]. So far black TiO2 containing Ti3+ centers has been often obtained by creating surface defects on the TiO2 nanoparticles under high pressure or extended reaction periods [24–26]. It is believed that the interaction between the N-doping and Ti3+ is the key to the enhanced visible light absorption and superior photocatalytic activities [27–29]. In this work, a facile sol-gel and in-situ solid-state chemical reduction methods are conducted to synthesize a Ti3+ and N co-doped mesoporous black TiO2. The band gap of the as-prepared sample has been narrowed. Photodegradation testing demonstrates that Ti3+ and N codoped TiO2 shows significant improvement for decomposing MO and RhB under visible light irradiation. The interaction between N-dopant and Ti3+ is demonstrated to be the key of enhanced photo-sensitive prosperities and excellent photocatalytic activities.
Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (Q. Zhu), [email protected] (Z. Xing). 1 Note: H. Zhang and Y. Zhang contributed equally. https://doi.org/10.1016/j.chemphys.2018.07.021 Received 7 March 2018; Accepted 17 July 2018 Available online 18 July 2018 0301-0104/ © 2018 Elsevier B.V. All rights reserved.
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2. Experimental materials
measurements were employed with a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. The laser beam was using a 50 × objective lens to a ca. 1 μm spot on the surface of the sample. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI-5700 ESCA equipped with Al-Kα X-ray source. All binding energies were calibrated using the C 1 s peak at 284.6 eV of the surface adventitious carbon as a reference. The Fourier transform infrared spectra (FI-IR) of the samples were taken with a PerkinElmer spectrum one system, using KBr as diluents. Transmission electron microscopy (TEM) was examined with a JEM2100 electron microscope (JEOL, Japan). Nitrogen adsorption-desorption isotherms at 77 K were performed with an AUTOSOR-1 (Quantachrome Instruments) nitrogen adsorption apparatus. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area. The pore volumes and pore diameter distributions were calculated from the adsorption-desorption isotherms using the Barrett-Joyner-Halenda (BJH) model. The UV–vis diffuse reflectance spectroscopy (UV–vis DRS) were recorded on a UV–vis spectrophotometer (UV-2550, Shimadzu) with an integrating sphere attachment using BaSO4 as the reflectance standard.
All reagents, i.e., tetrabutyl titanate (TiOBu)4, absolute ethanol (EtOH), nitric acid (HNO3), and carbamide, were purchased from Tianjin Kermel Chemical Reagent Co. LTD, China. Sodium boron hydride (NaBH4, 98%) was purchased from Aladdin Reagent Company, China. All reagents used in the experiments were analytical grade and employed without further purification, and the deionized (DI) water was used throughout this study. 2.1. Preparation of N doped TiO2 photocatalysts The N doped TiO2 powders were prepared using the sol-gel method, as described below. In a typical preparation process, 20 mL of tetrabutyl titanate was dissolved in 80 mL of absolute ethanol to produce homogeneous Ti(OBu)4-EtOH solution. Meanwhile, volume ratio of DI water/ ethanol/nitric acid was kept at 4/5/1 under magnetic force stirring to form EtOH-HNO3-water solution. Later, water based solution was added dropwise into Ti precursor solution under constant stirring for 1 h. To get the desired concentration of N in TiO2, the required stoichometric amount of aqueous solution of carbamide was then added to the EtOHHNO3-water solution and stirred vigorously for 0.5 h to form the light yellow solution. Next, the transparent solution was slowly added dropwise to the Ti(OBu)4-EtOH solution under magnetic force stirring to carry out a hydrolysis and stirred vigorously for 4 h. After aging for 18 h, doped solution was dried at 60 °C to form the gel and then heated with 2 °C min−1 for elevating temperature till 450 °C, and the N doped TiO2 powders were eventually obtained after calcination at 450 °C for 4 h. Finally, the as-synthesized samples were successfully obtained. The color of these N doped TiO2 nanoparticles was light yellow, which were denoted as N-TiO2.
2.4. Photocatalytic test The photocatalytic activity of the samples was tested by investigating the degradation performance of methyl orange (MO) and rhodamine B (RhB) under visible-light irradiation. A 350 W xenon lamp was mounted with 420 nm cut-off glass filter was used as a visible light source. In a typical process, 25 mg as-prepared photocatalyst was added into 30 mL of 10 mg L−1 dye solution to perform the reaction suspension. Then, the suspension was magnetically stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. Afterward, the mixture was illuminated with visible light under magnetic stirring. At the given time intervals, 5 mL of suspension was collected and centrifuged to obtain a clear solution. The filtrates were analyzed by a Shimadzu Model UV2550 spectrophotometer at the characteristics wavelength of RhB and MO solution under the 553 and 464 nm. Furthermore, using the same procedure, the photocatalytic activity of the samples for the decomposition of MO was also investigated.
2.2. Preparation of black N doped TiO2 2 g of the as-prepared N-TiO2 powders were mixed with 5 g of NaBH4, and the mixture was stirred for 1 h. Then the mixture was transferred into a tubular furnace, heated from room temperature to 350 °C for 1 h under N2 atmosphere with a heating rate of 5 °C min−1. After naturally cooling down to room temperature, the mixture was washed with DI water and absolute ethanol for several times to remove the unreacted NaBH4, and then dried at 60 °C. Finally, the black N doped TiO2 was obtained, which was denoted as b-N-TiO2.
3. Results and discussion The crystal phase compositions of the samples are characterized by X-ray diffraction measurements at room temperature [30]. The XRD patterns of various samples are shown in Fig. 1a. The diffraction peaks at 25.3, 37.8, 48.1, 54.1, 54.9, 62.7, 68.9, and 70.2° correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), and (2 2 0) for anatase TiO2 lattice plane, respectively. It shows that all diffraction peaks can be fully indexed to anatase crystalline phase of TiO2 [31]. It is
2.3. Characterization X-ray diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer with a Cu Kα radiation (λ = 1.5406 Å). Raman
Fig. 1. XRD patterns (a) and Raman spectra (b) of TiO2 (a), N-TiO2 (b), and b-N-TiO2 (c), respectively. The inset of (b) is the magnified spectra between 100 and 200 cm−1. 87
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these results are consistent with the result of XRD. The XPS measurements are carried out to further investigate the surface composition and chemical state of the samples. Fig. 3a shows that the Ti 2p peaks of b-N-TiO2 can be deconvoluted into four peaks as Ti3+ 2p3/2 at 457.84 eV, Ti4+ 2p3/2 at 458.43 eV, Ti3+ 2p1/2 at 463.54 eV, and Ti4+ 2p1/2 at 464.11 eV, respectively [38]. These Ti3+ species are formed due to the Ti4+ reduction of TiO2 by the NaBH4. The presence of Ti3+ species can suppress the recombination of photogenerated electron-hole pairs, and thus promote the photocatalytic activity. Fig. 3b shows O 1 s configuration of the b-N-TiO2. The peaks at 529.6 and 531.1 eV should be attributed to Ti-O bonds and the surface –OH groups, respectively [39]. However, the peak of the oxygen in the TiO2 lattice shifts to lower energy due to the existence of oxygen vacancy on the TiO2 surface. Oxygen vacancies (Ov) should be produced by releasing the O from N doped TiO2 during the reduction process [40]. As shown in Fig. 3c, the dominant N 1s XPS peaks for b-N-TiO2 samples located at 401.2 eV and 399.6 eV which are consistent with the binding energies of Ti-O-N and N-Ti-O, respectively. The results demonstrate that the nitrogen is successfully doped into the lattice of TiO2. Therefore, the obtained XPS results can be in good agreement with the above analysis. The valence band (VB) positions of TiO2, N-TiO2 and b-N-TiO2, can be observed in Fig. 3d. The VB edge is studied by linear extrapolation of the peaks to the baselines. The VB edge of TiO2, N-TiO2 and b-N-TiO2 is 2.30, 1.86 and 1.86 eV, respectively. The VB edge of N-TiO2 is below Fermi energy, indicating N is forming in TiO2 [41]. The N doped TiO2 with N up-shifts the VB edge induces a remarkable narrowing of bandgap, and then improving the visible-light photocatalytic activity. Evidently, the VB edge of b-N-TiO2 is similar to N-TiO2, suggesting that Ti3+ self-doped can not influence the VB edge. However, according to the above UV–vis DRS results, the bandgap of b-N-TiO2 is narrower than N-TiO2, suggesting that the doped Ti3+ reduces the conduction
rather remarkable that no peaks related to the doping species could be observed in the XRD patterns. One of the reasons might be that the concentration of dopants is too low to be detected by XRD [32]. Compared with pure TiO2, the increase of intensity peak of the N-doped TiO2 indicates higher crystallinity. For the b-N-TiO2 sample, the intensity of (1 0 1) diffraction peak becomes weaker due to the Ti3+ doping. The particle size is calculated from the anatase peak (1 0 1) with the Scherrer equation, and is found to be 11.2 nm. The result is corresponded with the TEM observation. As a further confirmation, the anatase crystal structure is also investigated by the Raman technique, which is displayed in Fig. 1b. The Raman spectra show four obvious peaks at 158, 402, 520 and 643 cm−1, which are ascribed to the Raman-active modes of anatase phase with Eg, B1g, A1g and Eg symmetries, respectively [33]. A slightly shift in Raman peaks is also observed as the replacement by N dopant in the TiO2 lattice [34]. So the N-doped TiO2 samples are confirmed to be successfully prepared. Due to the addition of NaBH4, the intensity of Raman peak shows apparently decreased due to the formation of Ti3+ and oxygen vacancy in TiO2 lattice [35], which is corresponded to the XRD results. TEM images are taken to analyze the morphology and crystal structure of the samples. From Fig. 2a and b, we can find that the prepared samples are composed of small particles with good dispersion. Fig. 2c and d exhibit the HRTEM images of TiO2 and N-doped TiO2, the crystal lattice fringes with interplanar spacing of 0.35 nm, which are corresponded to the (1 0 1) crystal facet of anatase TiO2, implying that the doping and solid-state reduction cannot change the crystal phase of the samples [36]. As shown in Fig. 2d, the b-N-TiO2 sample shows about 1–2 nm thick disordered surface layer. The disordered layer arises from Ti3+ and oxygen vacancy, which is beneficial for the separation of photogenerated carriers and inhibit the rapid recombination, thus improving electron transfer and photocatalytic activation [37]. Therefore,
Fig. 2. TEM and HRTEM images of N-TiO2 (a and c) and b-N-TiO2 (b and d), respectively. 88
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Fig. 3. The XPS spectra of b-N-TiO2, (a) Ti 2p, (b) O 1s, (c) N 1s, (d) VB XPS spectra of TiO2, N-TiO2 and b-N-TiO2 samples.
Additionally, the surface area and the BJH pore size of the N-TiO2 and b-N-TiO2 are all similar to each other. It can be considered that the visible-light photocatalytic activity may be excellent due to the existence of mesoporous structure [47]. The photocatalytic activity of the prepared TiO2 samples is evaluated by the degradation of MO and RhB under visible light irradiation. It can be seen in Fig. 5a and b, the photolysis studies prove that the MO and RhB are stable under visible light irradiation with significantly low self-degradation percentage of 1.8%. In the concentration reduction of MO and RhB are very low with degradation percent of 1.35% and 1.52% under visible light irradiation. These results generally show that the self-degradation of MO and RhB under visible light irradiation can be neglected. It is found that the pure TiO2, N-TiO2, b-TiO2 and b-NTiO2 show that the concentration of MO are reduced by 19.76, 80.28, and 93.27%, respectively, the concentration of RhB are reduced by 18.34, 76.66, and 93.64%, respectively, after visible light irradiation for 3 h. Moreover, the photocatalytic activity of N-TiO2 is higher than that of pure TiO2. The huge different in degradation ratio between TiO2 and N-TiO2 sample is due to the existence of nitrogen dopants in TiO2 lattice structure, as confirmed by XPS analysis. It has been widely reported that the nitrogen doping in the TiO2 lattice structure can improve photocatalytic activity under visible light irradiation [48]. When the N species and Sn species were codoped into TiO2, its photocatalytic activity has an obvious enhancement. The main reason for the improvement of photocatalytic activity should be attributed to the synergistic effect of the introduction of N and Sn co-doped TiO2 [49]. The degradation ratio by b-N-TiO2 is much higher than that of pure TiO2, NTiO2 and b-TiO2, indicating the b-N-TiO2 possesses excellent visible light photocatalytic activity.
band (CB) position of b-N-TiO2 [42]. The optical properties of as-synthesized TiO2 samples are investigated by UV–vis diffuse reflectance spectra and shown in Fig. 4a. For pure TiO2 sample, a strong absorbance in UV region is found for anatase TiO2. After doping with nitrogen, the absorption of N-TiO2 and b-N-TiO2 in visible region is greatly enhanced. It is expected that nitrogen doping contributed to the red-shift because of narrowing the band gap [43]. There occurs a broad absorption shoulder centered at about 430 nm and tailing absorption extends out to approximately 700 nm, which is the typical absorption feature reported for N doped TiO2 materials [44]. On this basis, for the b-N-TiO2 sample, the visiblelight absorption drastically increases when the wavelength is longer than 400 nm, superior to N-TiO2. Furthermore, estimations of the bandgap energies are obtained from the diffuse reflectance spectra of three samples. As shown in Fig. 4b, the band gap of TiO2, N-TiO2 and b-NTiO2 is 3.0, 2.48, and 2.05 eV, respectively. These results suggest that the b-N-TiO2 with a narrower band gap is more active than TiO2 and NTiO2 in the visible-light region. The strong absorbance in the visible region of b-N-TiO2 is attributed to the existence of oxygen vacancies, Ti3+ and N species. The N2 adsorption-desorption isotherms and BJH pore size distribution curves are employed to investigate the pore structure of the prepared samples. As shown in Fig. 4c, the samples exhibit typical IV nitrogen adsorption isotherm with a H2 hysteresis loop at a relative pressure between 0.4 and 0.9, implying the presence of mesoporous structure [45]. From Fig. 4d, the pore size distribution of N-TiO2 and bN-TiO2 calculated by the BJH method are around 5.4 nm, indicating the prepared TiO2 are mesoporous materials [46]. The BET specific surface areas of N-TiO2 and b-N-TiO2 are 94 and 96 m2 g−1, respectively. 89
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Fig. 4. UV-vis diffuse reflectance absorption spectra (a) and determination of the indirect interband transition energies (b) for TiO2, N-TiO2 and b-N-TiO2 samples, respectively, (c) N2 adsorption-desorption isotherm curves (c) and the corresponding pore size distribution plots (d) of N-TiO2 and b-N-TiO2.
achieving high photocatalytic performance of TiO2. The electrons can react with O2 to superoxide radical anions, which can further create the reactive hydroxyl radicals. The holes of VB react with H2O molecules to generate •OH radicals, which favors the photocatalysis. As is shown in Fig. S2a and b, the result reveals that no degradation is detected in the 60 min, and thus it can be considered that the selfdegradation is negligible. Compared with pure TiO2, the photocatalytic activity of other samples is all improved under visible light. In particular, the b-N-TiO2 exhibits the best photodegration ability, which reachs 91.5% of phenol removal within 150 min under visible light irradiation. In addition, the first-order rate constants k of TiO2, N-TiO2, bN-TiO2 have shown in Fig. S2b are 0.0164, 0.0056, 0.0002 min−1, respectively. Similarly, the b-N-TiO2 shows the highest value at 82 times higher than that of pure TiO2, which can be ascribed to the synergistic effect between N and Ti3+ co-doped. Cycling degradation of phenol is also provided to test the stability of the catalyst, which is shown in Fig. S2c. After six cycling degradation, the photocatalytic activity of b-NTiO2 displays neglect decrease. This result demonstrates the high stability of Ti3+ and N co-doped TiO2 photocatalyst prepared by this method. The mineralization of RhB and MO are inspected by TOC analysis. Fig. 5f shows the TOC removal rate on the initial 10 mg/L of RhB and MO photocatalytic degradation process with 150 min. In Fig. 5f, 5.4, 54.6 and 83.2% of TOC removal rate can be obtained for the initial 10 mg/L of RhB, 8.7, 62.4, 86.5% of TOC removal rate can be obtained for the initial 10 mg/L of MO. It indicates the RhB and MO are easier to be degraded by photocatalytic degradation. These results indicate the bN-TiO2 photocatalyst can mineralize the pollutants efficiently. The formation of hydroxyl radical and superoxide radical anions are
As shown in Fig. 5c and d, to quantitatively evaluate the photocatalytic efficiencies of these catalysts, the corresponding apparent reaction rate constants k of the MO degradation calculated by pseudofirst-order kinetic model are 0.0014, 0.0109, and 0.0188 min−1 for pure TiO2, N-TiO2, and b-N-TiO2, and the rate constants k of RhB degradation are 0.0013, 0.0101 and 0.0186 min−1 for pure TiO2, N-TiO2, and b-N-TiO2. Evidently, the kinetic constants k of the b-N-TiO2 for MO degradation is 1.7 and 1.4 times higher than that of b-TiO2. The kinetic constants k of the b-N-TiO2 for MB degradation is 1.5 times higher than that of N-TiO2. The enhancement of the photocatalytic activity of the sample is caused by the doping of Ti3+ and N [50]. Cycling degradation of MO and RhB is also provided to test the stability of the catalyst, which is shown in Fig. 5e. After six cycling degradation, the photocatalytic activity of b-N-TiO2 displays neglect decrease. This result demonstrates the high stability of Ti3+ and N co-doped TiO2 photocatalyst prepared by this method. Based on the results and discussions above, it can be concluded that b-N-TiO2 can generate much active radicals during the photocatalytic process, resulting in better photocatalytic activity. Therefore, a degradation mechanism is illustrated in Fig. 5f. Under visible light irradiation, the electrons can be excited from N 2p impurity energy level to the conduction band (CB), because N atoms are doped into the lattice of TiO2, which locate above the valence band (VB) [51]. As a consequence, the doping of nitrogen narrows the bandgap of TiO2. When the photocatalyst is illuminated by the visible light, the introduction of Ti3+ can produce impurity level below the CB, which further narrowing the band gap [52]. According to literature reports, part Ti4+ is reduced to Ti3+ and Ov can significantly prolong the lifetime of photo-induced electrons and improve the separation of the electron and hole pairs, 90
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Fig. 5. Photocatalytic activity with different samples (a and b), and variations of ln (C0/C) versus with different samples under visible-light irradiation on MO and RhB removal (c and d), six cycles of MO and RhB photocatalytic degradation with b-N-TiO2 (e), schematic diagram of the photocatalytic degradation of organic contaminates by b-N-TiO2 under visible-light irradiation (f), TOC removal on the 10 mg/L of RhB and MO photocatalytic degradation process with 150 min (g).
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Fig. 6. DMPO-trapping ESR results of the formation of hydroxyl radical and superoxide radical anions.
References
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4. Conclusions In summary, we demonstrated that in-situ N-doped mesporous black TiO2 photocatalyst were prepared through a facile sol-gel method and an efficient in-situ solid-state reduction method. The large specific surface area of b-N-TiO2, which indicated the porous structure existed in the sample, in this study, the mesopore was the aggregation of interparticles, which was confirmed by the N2 absorption and TEM observation. Compared with the as-prepared TiO2 and N-TiO2, the b-NTiO2 showed wonderful photocatalytic performance in the degradation of MO and RhB under visible light irradiation. The enhanced photocatalytic activity was ascribed to the narrowed band gap of TiO2 by N and Ti3+ doping, which inhibited rapid recombination of photoinduced carriers. Therefore, it provides a new method to enhance the photocatalytic performance of Ti3+and N doped TiO2, which provides a new thought to design and prepare other semiconductors for various applications.
Acknowledgements We gratefully acknowledge the support of this research by the Natural Science Foundation of Heilongjiang Province (B2018010 and H2018012), the Postdoctoral Science Foundation of China (2017M611399), the Heilongjiang Postdoctoral Startup Fund (LBHQ14135), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016018), and Postdoctoral Science Foundation of Heilongjiang Province (LBHZ16150).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.chemphys.2018.07.021. 92
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