Journal of Colloid and Interface Science 528 (2018) 109–115
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Regular Article
Synthesis of hierarchical hollow sodium titanate microspheres and their application for selective removal of organic dyes Ye Zhang a, Gongyi Li a, Junming Liu b, Tao Wang b, Xue Wang b, Bin Liu b, Yunling Liu b, Qisheng Huo b,⇑, Zengyong Chu a,⇑ a b
College of Liberal Arts and Sciences, National University of Defense Technology, Changsha 410073, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
g r a p h i c a l a b s t r a c t This work demonstrated the synthesis of hierarchical hollow sodium titanate microspheres through a template-assisted alkaline hydrothermal process and investigated their adsorption properties toward organic dyes in water.
a r t i c l e
i n f o
Article history: Received 15 March 2018 Revised 21 May 2018 Accepted 21 May 2018 Available online 22 May 2018 Keywords: Sodium titanate Hierarchical hollow microspheres Selective adsorption Organic dyes
a b s t r a c t Titanate-based materials are attractive inorganic adsorbents for wastewater treatment. In this study, hierarchical hollow sodium titanate microspheres (HHSTMs) were successfully synthesized via a template-assisted method. Silica microspheres were selected as hard templates, with a uniformly smooth TiO2 shell first grown onto the surface of the SiO2 cores. Then, through an alkaline hydrothermal process, the silica core was removed and the TiO2 shell gradually converted into a sodium titanate shell with a preserved morphology. The as-synthesized HHSTMs are constructed from twined nanobelts, with a high surface area of 308 m2 g1. A typical organic dye, methylene blue, was employed to investigate the adsorption properties of the HHSTMs. The adsorption process matched well with the Langmuir isothermal model, with the maximum adsorption capacity of methylene blue reaching 443 mg g1. Moreover, the resulting HHSTMs can be used to selectively capture of methylene blue from a cationic-anionic dye binary system due to their negatively charged surface. All adsorption processes were very fast and could complete in ten minutes. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding authors. E-mail address:
[email protected] (Z. Chu). https://doi.org/10.1016/j.jcis.2018.05.069 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.
Today, water pollution threatens human health and has become a matter of public concern [1,2]. Organic dyes are one of the main
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hazards in water, and various methods such as adsorption, ozonation, ion exchange and photocatalytic degradation have been used to remove dyes from polluted water [3–6]. Among such methods, adsorption technology has been widely used because it is simple, efficient and low cost. During the past few decades, many adsorbents have been developed, and it is found that the adsorption activities of these materials are closely related to their structures [7–9]. Many studies have indicated that hierarchical hollow structured materials constructed from nanosized building blocks exhibit enhanced adsorption properties due to their large void space, porous structure and high surface area [10–12]. For example, Lou and coworkers found that the adsorption capacity of Congo red by hierarchical urchin-like a-FeOOH hollow spheres can reach 275 mg g1, mostly higher than that reported for similar materials [13]. Thus, it is of great significance to design and synthesize novel adsorbents with hierarchical hollow structures. Titanate materials are inorganic materials that are widely used in the energy and environmental fields with the advantages of low cost, wide-ranging sources for the raw materials and environmentally friendly [14–16]. Recently, titanate-based absorbents have attracted much attention and have been widely investigated [17,18]. Lin and coworkers synthesized calcined titanate nanotubes with a maximum adsorption capacity of 133.33 mg g1 toward methylene blue [19]. Lu and coworkers found the methylene blue adsorption capacity of layered protonated titanate nanosheets can reach 184 mg g1 [20]. Hierarchical hollow structures can improve the adsorption performance of materials, but reports for the synthesis of this type of titanate material are very limited [21]. It is still a great challenge to design and synthesize hierarchical hollow titanate materials with enhanced performance for organic dye adsorption. It is known that different physical and chemical properties of starting materials will significantly affect the morphology and property of the final product. In this paper, we developed a simple template-assisted synthesis method to obtain hierarchical hollow sodium titanate microspheres (HHSTMs) via an alkaline hydrothermal process. Silica microspheres were used as hard templates and core-shell structured SiO2@TiO2 microspheres (the term ‘‘@” is usually used to denote the core-shell structures, with A @ B indicating that A is coated with B) were utilized as a precursor. Moreover, the adsorption performance of these HHSTMs toward organic dyes was discussed, and the max adsorption capacity and the adsorption property for encapsulation of the target dye molecules in a mixture of solutions were also investigated. 2. Experimental 2.1. Chemicals Tetraethyl orthosilicate (99.0%), sodium hydroxide (96.0%), ethanol (99.7%), 2-propanol (99.7%) and ammonia (28.0 wt% aqueous solution) were purchased from Beijing chemical works, China. Tetrabutyl titanate (98.0%) was purchased from Aladdin. Acid-red 14 (98%) was obtained from Energy Chemical. Methylene blue (99.0%) and methyl orange (99.0%) were purchased from Sinopharm Chemical Reagent Co.. Ltd. Rhodamine B (98%) was obtained from Tianjin Guangfu Fine Chemical Industry Research Institute, China. All chemicals were used as-received. 2.2. Synthesis of SiO2 microspheres SiO2 microspheres were synthesized by a modified Stöber method based on a previous report [22]. In a typical process, 23.5 mL of water, 63.3 mL of 2-propanol and 13 mL of ammonia (28% aqueous solution) were mixed together and heated in a water
bath at 35 °C. Then, 0.6 mL of tetraethyl orthosilicate was added dropwise to this mixed solution and reacted for 0.5 h under stirring. Then, 5 mL of tetraethyl orthosilicate was added dropwise to the above-mixed solution again and further reacted for 2 h at 35 °C. The SiO2 microspheres were collected by centrifugation and then washed with ethanol and water three times. 2.3. Synthesis of SiO2@TiO2 core shell structure microspheres SiO2@TiO2 core shell structure microspheres were fabricated according to an extended classic Stöber method [23]. Then, 0.1 g of SiO2 microspheres were dispersed in 80 mL of ethanol by ultrasonication, followed by the addition of 0.4 mL of ammonia and stirring at 45 °C for 30 min to form a homogeneous dispersion. Then, 0.8 mL of tetrabutyl titanate was added dropwise to this mixed solution and reacted for 8 h under stirring conditions. The resultant product was collected by centrifugation, followed by washing with deionized water and ethanol three times. 2.4. Synthesis of hierarchical hollow sodium titanate microspheres (HHSTMs) In a typical synthesis, 0.03 g of SiO2@TiO2 core shell structure microspheres were dispersed in 5 mL of sodium hydroxide solution (10 M). Then, the mixture was transferred into a 23 mL Teflonlined autoclave and heated at 170 °C for 2 h under static conditions. The white product was collected by centrifugation and washed several times with deionized water. 2.5. Material characterization Transmission electron microscope (TEM) images were obtained using an FEI Tecnai G2 F20 S-twin D573 field-emission transmission electron microscope operating at 200 kV. Scanning electron microscope (SEM) images were obtained using a JEOL JSM-6700F field-emission scanning electron microscope operating at 5 kV. The zeta potential was detected using a Nano ZS90 laser particle analyzer (Malvern Instruments, UK) at 25 °C. Powder X-ray diffraction (XRD) data were collected by a Rigaku D/Max 2550 X-ray diffractometer using CuKa radiation (k = 1.5418 Å) operating at 200 mA and 50 kV. N2 adsorption-desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2010 sorptometer. UV–Vis spectral data were recorded with a Shimadzu UV-2450 spectrometer. X-ray photoelectron spectroscopy (XPS) was recorded using a Thermo ESCALAB 250. 2.6. Dye adsorption experiments Dye adsorption experiments were conducted at room temperature in a neutral aqueous solution using a thermostating shaker in a dark environment to avoid the decolorization of organic dyes in light. Adsorption tests for methylene blue (MB) were carried out using HHSTMs as adsorbents first, with the MB concentration set at 20 mg L1 and the adsorbent concentration fixed at 167 mg L1. Furthermore, to investigate the adsorption capacity of the HHSTMs toward MB, the adsorption isotherm for MB with the HHSTMs was obtained via a stepwise variation of the initial MB concentration from 10 mg L1 to 160 mg L1. At appropriate time intervals, the adsorbent was separated from the stock solution by filtration and the concentration of the clear solution was analyzed by a UV–Visible spectrophotometer at the maximum absorption wavelength of the dye solution. The adsorption capacity Qe (mg g1) of the dye was calculated using the mass balance equation:
Qe ¼
ðC 0 C e ÞV m
ð1Þ
Y. Zhang et al. / Journal of Colloid and Interface Science 528 (2018) 109–115
where C0 and Ce (mg L1) are the initial and equilibrium concentrations for the methylene blue solution, V (L) is the volume of the solution and m (g) is the mass of the HHSTM adsorbent. To measure the selective adsorption property of the HHSTMs toward different organic dyes, the HHSTM adsorbent (5 mg) was added to 30 mL of a binary system of methylene blue and Acid Red 14, methylene blue and Rhodamine B and methylene blue and methyl orange, respectively, with a concentration of 10 mg L1 for all the dyes. UV–Vis adsorption spectra were recorded to determine the residual concentration of the different dye in the solution according to the maximum absorption wavelength at a given adsorption time. The adsorption efficiency (R%) for each dye was calculated using the following equation:
R¼
C0 Ct 100 C0
ð2Þ
where C0 represents the initial concentration of the dyes and Ct represents the concentration of the dyes at any given time.
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Six repeat experiments underlying error estimates were carried out in this part to accurately investigate the adsorption properties of the HHSTMs.
3. Results and discussion 3.1. Synthesis and characterization of hierarchical hollow sodium titanate microspheres (HHSTMs) The template-assisted synthesis process for realizing hierarchical hollow titanate microspheres (HHSTMs) is illustrated in Fig. 1. First, monodispersed SiO2 microspheres were synthesized and a TiO2 shell coated onto the SiO2 template based on the modified Stöber method [23]. Then, these SiO2@TiO2 core-shell microspheres were immersed in a 10 M sodium hydroxide solution at 170 °C. The silica core was dissolved and HHSTMs with spinous surface nanostructures were obtained following the alkaline
Fig. 1. Schematic illustration of the template-assisted synthesis process for HHSTMs. (I) Deposition of TiO2 shell on SiO2 microspheres; (II) Formation of HHSTMs in 10 M sodium hydroxide solution at 170 °C for 2 h.
Fig. 2. SEM and TEM images of the SiO2@TiO2 core-shell microspheres (a and b) and HHSTMs (c and d).
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hydrothermal treatment for 2 h. Sodium hydroxide solution plays an important dual role, which can be used to not only remove the silica core to obtain the hollow structures but also to react with the TiO2 shell to obtain the final surface morphology. The as-synthesized SiO2 microspheres showed a uniform spherical shape and a smooth surface (Fig. S1). The morphologies of the SiO2@TiO2 core-shell microspheres and HHSTMs are shown in Fig. 2. It can be clearly observed that the SiO2@TiO2 core shell structures fabricated after the coating process also possessed a relatively smooth surface, with a diameter ranging from 500 to 600 nm (Fig. 2a). TEM images reveal that the thickness of the TiO2 layer is approximately 100 nm (Fig. 2b). HHSTMs with a preserved original sphere morphology were observed following the alkaline hydrothermal process, with the broken microspheres observed in the SEM image demonstrating the existence of hollow interiors, which can also be verified by TEM analysis (Fig. 2c and d). The rough surfaces of the HHSTMs were constructed of twined nanobelts with a width of 5–10 nm, as shown in Fig. 2d (inset). We have randomly selected 200 entities to analyze the average particle diameters and diameter distribution for each of the colloidal particles. Histograms representing the diameter population for the colloidal particles are shown in Fig. S2. The average particle diameters of SiO2 microspheres, SiO2@TiO2 and HHSTMs are approximately 350 nm, 550 nm and 580 nm, respectively. The formation of the nanobelt structure is caused by the high concentration of sodium hydroxide solution, with the growth of these titanate nanostructures following a 3D-2D-1D mechanism [24]. At a low concentration (such as 5 M sodium hydroxide solution), the lamellar structure cannot roll into a 1D structure, thus, only nanosheets were obtained (Fig. S3). Fig. 3 shows the XRD patterns measured for the SiO2@TiO2 coreshell microspheres and HHSTMs. The SiO2@TiO2 core-shell microspheres showed no obvious diffraction peaks, indicating an amorphous state. The diffraction peaks for HHSTMs at approximately 2h = 10°, 24°, 28°, 48° were all assigned to Na2Ti3O7 (JCPDS card no. 31-1329), with the intense peak at approximately 10° ascribed to the typical interlayer distance in Na2Ti3O7 corresponding to (1 0 0) lattice planes, which is approximately 0.863 nm based on XRD analysis [25]. The high-resolution XPS spectrum for Ti 2p is shown in Fig. S4. The binding energy value of 458.2 eV for the Ti 2p doublet revealed that the oxidation state for the titanium ions in the HHSTMs is Ti4+ [26]. The binding energy values of 1071.05 eV for the Na 1 s peak and 529.85 eV for the O 1s peak are consistent with the literature [27]. The binding energy values and atom percent-
Fig. 3. XRD patterns for SiO2@TiO2 core-shell microspheres and HHSTMs (c and d).
ages for the HHSTMs are summarized in Table S1, with the molar ratio of Na to Ti measured to be approximately 2:3. N2 adsorption-desorption isothermal were measured for the HHSTMs to analyze their Brunner-Emmet-Teller (BET) specific surface area and porosity. The HHSTMs showed a large BET area of 308 m2 g1. As shown in Fig. 4, HHSTMs exhibited a characteristic type IV isotherm with an obvious H3 hysteresis loop, which should be relative to the mesopores between the nanobelts and the large void space in hollow interiors. The mesopore size distribution for HHSTMs based on the Barret-Joyner-Halenda (BJH) method from N2 adsorption was in a narrow range of 2–15 nm centered at approximately 7.5 nm (Fig. 4, inset).
3.2. Adsorption of MB The HHSTMs show a large BET specific surface area and hollow and porous structure, which are all favorable for adsorption. Methylene blue (MB) is a typical toxic dye, with a large amount of MB (>7.0 mg L1) leading to high blood pressure, mental disorders, nausea and abdominal pain [28]. As shown in Fig. 5, the HHSTMs were capable of completely removing almost all the MB rapidly, with the MB concentration set at 20 mg L1. The adsorption process can substantially complete in ten minutes, and the
Fig. 4. N2 adsorption-desorption isothermal for the HHSTMs.
Fig. 5. Adsorption rates for methylene blue removal as a function of contact time for an HHSTM sample in a neutral solution at room temperature, with optical images of the methylene blue before (left) and after (right) treatment with the adsorbent (adsorbent dose 167 mg L1).
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MB concentration decreased below 0.1 mg L1, with the color of the solution changing from blue to colorless. The adsorption isotherm for MB with the HHSTMs is presented in Fig. 6. The equilibrium adsorption amount for the HHSTMs toward MB increased rapidly at first and then slowed down. Finally, the adsorption amount showed no obvious change. A Lang-
muir and Freundlich isotherm model was used to describe the relationship between the adsorbent HHSTMs and adsorbate MB. The nonlinear Langmuir equation can be expressed as
Qe ¼
bQ m C e 1 þ bC e
ð3Þ
The Freundlich equation is given as
Q e ¼ kC e1=n
ð4Þ
where Qe is the equilibrium adsorption amount for MB on the HHSTMs (mg g1), and Ce is the equilibrium concentration in the solution phase (mg L1); Qm (mg g1) is the maximum adsorption of MB. The adsorption isotherm parameters are shown in Table 1. It is found that the Langmuir model with higher R2 values (R2 = 0.972) is more suitable for describing the adsorption process, suggesting that the adsorption of MB is limited to monolayer coverage with-
Table 1 The isotherm parameters for MB adsorption onto the HHSTMs.
Fig. 6. The adsorption isotherms for methylene blue onto the HHSTM sample (adsorbent dose 167 mg L1).
Isotherms
Parameters
Values
Langmuir model
Qm (mg g1) b (L g1) R2
443 ± 11 6.94 ± 1.22 0.972
Freundlich model
k (mg g1) n R2
264 ± 27 6.88 ± 1.36 0.846
Fig. 7. Adsorption efficiency curves for different organic dyes onto a HHSTM sample at various time points: (a) MB &AR 14; (b) MB & MO; (c) MB & RhB. (d) The relative adsorption efficiency of the organic dyes in the three different types of mixed-dye solutions. The photographs inset show the colors of the dye solutions before (left) and after (right) treatment with adsorbent (adsorbent dose was 167 mg L1).
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size of Rhodamine B is much bigger than MB, which may cause the ineffective adsorption of RhB. 4. Conclusions
Fig. 8. Zeta potential for HHSTMs dispersed in water.
out following further adsorption. The maximum adsorption capacity (Qm) for MB according to the Langmuir model is 443 mg g1, higher than that reported previously [19,29,30]. Compared to dye capture, it is also very important and attractive to investigate the adsorption property of HHSTMs for encapsulation of the target dye molecules in a mixture of solutions. Therefore, an additional three common organic dyes, acid-red 14 (AR 14), methyl orange (MO) and Rhodamine B (RhB) were utilized to investigate the selective adsorption properties of the HHSTMs. The HHSTMs were immersed in aqueous solutions containing two dyes (MB and AR 14, MB and MO, MB and RhB), with each dye having a concentration of 10 mg L1. As shown in Fig. 7, the adsorption efficiency of the HHSTMs toward these dyes showed no obvious change after ten minutes, which indicates that the adsorption process is substantially complete in ten minutes and that the adsorption rate is also very fast in the dye mixtures. Interestingly, it is found that the color of the dye solutions changed greatly after being treated with the adsorbents and that the HHSTMs exhibited different adsorption properties toward different dyes. Moreover, the adsorption experiment results revealed that the adsorption efficiency of the HHSTMs in the mixture dye solution for MB was nearly 100%, while for AR 14, MO and RhB the adsorption efficiency was 8.3%, 11.9% and 33.6%, respectively. Fig. 7d shows the relative adsorption efficiency of the HHSTMs toward the organic dyes in the mixed-dye solutions. Based on the above results, it can be found that the HHSTMs exhibited good selective capture activities toward organic dyes. Usually, the encapsulation of organic dyes by adsorbents in the liquid phase is influenced by factors such as electrostatic attraction, pore size, acid-base interactions and interactions/stacking [9,31–33]. A zeta potential analysis was carried out to investigate the surface charge state for the HHSTMs. As shown in Fig. 8, HHSTMs show a negative zeta potential, with an average value of 27.7 mV obtained after six tests, indicating that the HHSTMs possess a negatively charged surface. The chemical structures of the different organic dyes and the corresponding adsorption efficiency are shown in Table S2. We can see that MB and RhB are positively charged, but AR 14 and MO are negatively charged. Thus, HHSTMs are more likely to capture cationic MB and RhB molecules dyes compared to the anionic dye molecules. In this work, the HHSTMs comprise a typical inorganic salt without organic functional groups, thus, electrostatic attraction can be the main factor that determines the adsorption properties toward different dyes [26]. Structurally, the molecule
In summary, hierarchical hollow sodium titanate microspheres (HHSTMs) composed of nanobelts were successfully fabricated by a novel template-assisted alkaline hydrothermal process using SiO2@TiO2 core-shell microspheres as a precursor, which has been rarely reported in the literature [19,21,34]. The as-prepared HHSTMs show a high surface area of 308 m2 g1 and excellent adsorption properties toward a typical organic dye: methylene blue. The adsorption processes are very fast and complete in ten minutes, with the maximum adsorption capacity for the HHSTMs toward methylene blue (MB) reaching 443 mg g1, which is higher than that reported for similar materials [19,29,30]. In addition, the high negative zeta potential charged surface of the material can contribute to the selective capture ability for MB in a cationicanionic dye binary system. Thus, HHSTMs are attractive inorganic adsorbents with many advantages, and can serve as a novel potential functional material for wastewater treatment. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21671074) and the Open project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (No. 2018-24). Appendix A. Supplementary material Electronic Supplementary Information (ESI) is available: Figures show the SEM images, TEM images and XPS results for the resultant samples. The structures for the organic dyes can also be found therein. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018. 05.069. References [1] M. Shao, X. Tang, Y. Zhang, W. Li, Front. Ecol. Environ. 4 (2006) 353–361. [2] R.P. Schwarzenbach, T. Egli, T.B. Hofstetter, U.V. Gunten, B. Wehrli, Annu. Rev. Environ. Resour. 35 (2010) 109–136. [3] Z. Wu, X. Yuan, J. Zhang, H. Wang, L. Jiang, G. Zeng, ChemCatChem 9 (2017) 41– 64. [4] H. Wang, Y. Huang, J. Hazard. Mater. 191 (2011) 163–169. [5] Y. He, Y. Tan, J. Zhang, Acta Chim. Sinica 72 (2014) 1228–1232. [6] H. Zubair, J. Sung Hwa, J. Hazard. Mater. 283 (2015) 329–339. [7] Y. Shao, B. Ren, H. Jiang, B. Zhou, L. Lv, J. Ren, L. Dong, J. Li, Z. Liu, J. Hazard. Mater. 333 (2017) 222–231. [8] L. Li, G. Qi, B. Wang, D. Yue, Y. Wang, T. Sato, J. Hazard. Mater. 343 (2017) 19– 28. [9] G. Ersan, O.G. Apul, F. Perreault, T. Karanfil, Water Res. 126 (2017) 385–398. [10] Y. Zhang, Y. Ye, X. Zhou, Z.L. Liu, P. Zhu, D. Li, X. Li, J. Mater. Chem. A 4 (2016) 838–846. [11] S. Yang, P. Huang, L. Peng, C. Cao, Y. Zhu, F. Wei, Y. Sun, W. Song, J. Mater. Chem. A 4 (2016) 400–406. [12] J.B. Fei, Y. Cui, X.H. Yan, W. Qi, Y. Yang, K.W. Wang, Q. He, J.B. Li, Adv. Mater. 20 (2008) 452–456. [13] B. Wang, H. Wu, L. Yu, R. Xu, T.-T. Lim, X.W. Lou, Adv. Mater. 24 (2012) 1111– 1116. [14] Y. Zhang, Z. Jiang, J. Huang, L.Y. Lim, W. Li, J. Deng, D. Gong, Y. Tang, Y. Lai, Z. Chen, RSC Adv. 5 (2015) 79479–79510. [15] X. Wang, Y. Li, Y. Gao, Z. Wang, L. Chen, Nano Energy 13 (2015) 687–692. [16] L. Wen, Z. Xiao, T. Wang, D. Zhao, J. Ni, Chem. Eng. J. 286 (2016) 427–435. [17] D. Yang, H. Liu, Z. Zheng, S. Sarina, H. Zhu, Nanoscale 5 (2013) 2232–2242. [18] F. Miao, Y. Wen, Z. Wu, Q. Chen, H. Zhan, A.C.S. Appl, Mater. Interfaces 5 (2013) 12654–12662. [19] Y. Tang, D. Gong, Y. Lai, Y. Shen, Y. Zhang, Y. Huang, J. Tao, C. Lin, Z. Dong, Z. Chen, J. Mater. Chem. 20 (2010) 10169–10178. [20] C.H. Lin, S.H. Wong, S.Y. Lu, A.C.S. Appl, Mater. Interfaces 6 (2014) 16669– 16678. [21] Y. Qi, L. Yi, M. Yang, G. Wang, T. Li, J. Li, Appl. Surf. Sci. 293 (2014) 359–365.
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