Ethylene glycol-assisted fabrication and superb adsorption capacity of hierarchical porous flower-like magnesium oxide microspheres for phosphate

Saeed Ahmed a, Jingsong Pan b, Muhammad Naeem Ashiq c, Dianqing Li a, Pinggui Tang a and Yongjun Feng *a
aState Key Laboratory of Chemical Resource Engineering, Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Chaoyang district, Beijing, 100029, China. E-mail: yjfeng@mail.buct.edu.cn
bShandong Institute of Industry and Information Technology, No. 134, Jiefang Road, Jinan, Shandong Province 250013, China
cInstitute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan

Received 26th March 2019 , Accepted 10th May 2019

First published on 10th May 2019


A series of hierarchical flower-like magnesium oxide (MgO) microspheres were prepared with an ethylene glycol (EG)-assisted route at room temperature using ammonia as a precipitating agent. Effects of the ethylene glycol (EG) ratio on the structure, morphology and pore properties were carefully investigated. The hierarchical porous MgO microsphere exhibited a surface area of 75 m2 g−1 and total pore area of 47.37 m2 g−1 at the highest ratio of EG (EG/Mg2+ = 10) in the reaction system. The prepared MgO microspheres exhibited an outstanding removal capacity of 574.71 mg g−1 for phosphate following the pseudo second order kinetic model (R2 = 0.99) and Langmuir isotherm model with the endothermic nature of phosphate adsorption which resulted from the high surface area and suitable pore size. Both the isothermal parameter RL between 0 and 1 and the negative the Gibbs free energy value suggested that phosphate adsorption on a MgO microsphere was a favorable process. Undoubtedly, this template-free mild synthesis method effectively promotes wide practical applications and mass scale production of porous MgO microsphere adsorption material.


Introduction

Explosive growth of the human population and rapid increases in food requirements led to excessive usage of phosphate fertilizer, and furthermore this phosphorous overloading in ecosystems is threatening the sustainable development of our living system.1–3 For example, a blue-green algae accident occurred in Lake Taihu in 2007,4 and in Lake Erie between Canada and the USA in 2011.5 Therefore, it is of great interest and importance to decrease the discharge of phosphorous-containing chemicals and to develop high-efficiency technologies for removal of them from a polluted environment.6–9

Among the developed technologies to date, such as physical,10–12 chemical,13–17 and biological methods,18,19 adsorption technologies have been attracting the most attention because of their low cost, high efficiency, environmental friendliness, and easy operations.20–23 Their adsorption capacities and removal efficiencies mainly depend on the adsorption materials used which involve their chemical compositions, crystalline structures, morphologies, and pore structures.24–26 As for the same chemical composition, particularly a large surface area and appropriate pore size as well as pore size distribution play key roles to increase adsorption capacity and accelerate the corresponding removal rate of adsorption materials.3,25 It is very important to expose the adsorption active site to as many surface areas as possible, and provide transport passages as large as possible.27,28 That is to say, a suitable transport passage favours enhancing the utilization efficiency of surface areas of the absorbents.29,30 In comparison, self-assembled flower-like microspheres have attracted a special interest due to high their surface areas, large transport passages, full exposure of active sites, and good stability.31–33

Recently, a porous MgO microsphere as one kind of low-cost and high-performance adsorbent, has shown widely practical applications for removing various pollutants including heavy metal ions,34,35 organic dyes,36,37 and phosphate anions,38etc. Most of them were prepared using template-assisted hydrothermal routes at high temperatures and high pressures in pressure-proof reactors with complicated removals post-treatment of templates.39 Undoubtedly, these methods are not suitable for mass scale production using large scale applications. Therefore, it is of increasing interest to explore high efficiency methods for controllable fabrication of hierarchical porous flower-like MgO microspheres.

In this work, we developed an ethylene glycol-assisted method to fabricate hierarchical porous flower-like MgO microspheres under mild conditions and investigated adsorption performance towards phosphate anions as well as the possible adsorption mechanism. Fig. 1 demonstrates the formation process of hierarchal macroporous MgO by calcining the sphere-like flower precursors at a certain temperature, which resulted from the electrostatic interaction between magnesium ions in the solution and the hydroxyl groups of ethylene glycol. These hierarchical flower-like MgO microspheres exhibit splendid adsorption capacity for phosphate anions in aqueous solution owing to their large surface areas and pore sizes.


image file: c9qi00331b-f1.tif
Fig. 1 Systematic growth of hierarchical flower-like MgO microsphere in an ethylene glycol-assisted route.

Experimental section

Materials

Magnesium sulphate (MgSO4), ethylene glycol (C2H6O2, 98 wt%), ammonia solution (NH3·H2O, 25–28 wt%), and sodium dihydrogen phosphate (NaH2PO4·2H2O) were of analytical grade. All chemicals were utilized as received without additional purification.

Fabrication of hierarchical flower-like magnesium oxide microspheres

Three hierarchical flower-like magnesium oxide samples were prepared in an ethylene glycol-assisted precipitation route at room temperature using anhydrous MgSO4 as the magnesium source, NH3·H2O as a precipitating agent, and ethylene glycol (EG) as a pore forming and structure directing agent with different ratios between EG and Mg2+ from 0–10. For EG/Mg = 10, for example, 6.02 g (0.05 mol) MgSO4 was dissolved into 100 mL water by stirring for few minutes, and 31.04 g (0.50 mol) ethylene glycol was added into a magnesium sulphate solution. The magnesium ions were precipitated by addition of 80 mL (1.2 mol) ammonia solution with a rate of 120 mL h−1 by a pump. With another two samples, a similar procedure was carried out using 0.0 g (0.0 mol) and 15.52 g (0.25 mol) of ethylene glycol for EG/Mg = 0.0, and 5, respectively. The resulting white precipitates were washed ten times with distilled water and dried in an oven at 80 °C overnight. Finally, three MgO powders were obtained by annealing the precursors at 500 °C for 3 h with a ramp rate of 1 °C min−1, and individually marked as EG/Mg = 0, 5 and 10 based on the feeding ratio between ethylene glycol and magnesium in the reaction system. The prepared MgO samples were characterized for crystalline structure with an X-ray diffractometer (SHIMADZU XRD-6000) using a Cu Kα radiation source, and a scanning electron microscope (SEM, ZEISS SUPRA 55) for morphologies. A low-temperature N2 adsorption–desorption isotherm was determined on a Micrometrics (ASAP 2460 2.01) while the surface area was estimated based on a BET method, and pore size distribution was accomplished based on a DFT method. The macroporous nature of MgO microspheres was estimated with an automatic mercury porosimeter (Micrometrics for AutoPore 9520) in the pressure range of 0.2–30[thin space (1/6-em)]000 psi by a mercury intrusion method, and macropore size evaluation was made with the Washburn-Laplace equation. A Fourier transform infrared spectrophotometer (FT-IR, Bruker Vector 22) was used for functional group identification and phosphate concentration was estimated spectrophotometrically on a UV visible spectrophotometer (SHIMADZU UV-2501PC) following the Molybdenum blue method.40

Adsorption behavior of phosphate

For the adsorption kinetic test, 0.01 g MgO powder was dispersed into 0.10 L (0.05 g L−1) phosphate solution in a conical beaker with pH0 of 5 after an adjustment using 0.01 mol L−1 HCl. The conical beakers were retained in a thermostated shaker at 303 K with a shaking speed of 180 rpm. Approximately 1.0 mL of the suspension was extracted after desired intervals of time, separated by a microfiltration membrane (pore size, Φ 0.45 μm) and the remaining phosphate amount was measured using the Molybdenum blue method.40 The removal capacity for different time intervals was calculated using eqn (1).
 
image file: c9qi00331b-t1.tif(1)
where Co (mg L−1) is the initial concentration, Ct (mg L−1) is the concentration at the contact time t (h), V (L) is the volume of solution and m (g) is mass of MgO. The obtained kinetic data were fitted for pseudo first order and second order (eqn (2) and (3)),41 where k1 (h−1) is the first order rate constant, and k2 (mg g−1 h−1) is the second order rate constant, qt (mg g−1) is the adsorption capacity at time t (h) and qe (mg g−1) is the equilibrium adsorption capacity.
 
ln(qeqt) = ln[thin space (1/6-em)]qek1t(2)
 
image file: c9qi00331b-t2.tif(3)

For the adsorption isotherm test, 0.01 g MgO was dispersed into 0.05 L in a concentration range of 0.025–0.25 g L−1 phosphate solution with pH0 of 5 after adjustment. The suspensions were shaken for 8 h at 303 K with a shaking speed of 180 rpm. The equilibrium adsorption capacity was calculated using eqn (4).

 
image file: c9qi00331b-t3.tif(4)

The obtained isotherm data were fitted for the Langmuir (eqn (5))42 and Freundlich (eqn (6))43 isotherm models, where KL (L mg−1) is the Langmuir constant, KF is the Freundlich constant and RL (eqn (7))44 is the equilibrium parameter.

 
image file: c9qi00331b-t4.tif(5)
 
image file: c9qi00331b-t5.tif(6)
 
image file: c9qi00331b-t6.tif(7)

For the adsorption thermodynamic determination, 0.01 g of MgO sample was dispersed into 0.05 L (0.05 mg L−1) solution with pH0 of 5 after adjustment, and placed in a thermostat shaker at 303, 313 and 323 K for an equilibrium time of 8 h with a shaking speed of 180 rpm. The equilibrium adsorption capacities at different thermodynamic temperatures were calculated using eqn (4). The Kd and ΔG° values were evaluated using eqn (8) and (9), respectively.38,45 The ΔS° and ΔH° values were extracted from linear fitting between ln[thin space (1/6-em)]Kd and 1/T (eqn (10)).

 
image file: c9qi00331b-t7.tif(8)
 
ΔG° = −RT[thin space (1/6-em)]ln[thin space (1/6-em)]Kd(9)
 
image file: c9qi00331b-t8.tif(10)
where Kd (mL g−1) is the distribution coefficient, ΔG° (kJ mol−1) is free energy change, R (J mol−1 K−1) is the universal gas constant, T (K) is the thermodynamic temperature, ΔS° (kJ mol−1 K−1) is the entropy and ΔH° (kJ mol−1) is the enthalpy of adsorption reaction.

Results and discussion

Structure and morphology

Fig. 2 displays powder X-ray diffraction (P-XRD) patterns for three MgO samples produced after calcination under an air atmosphere at 500 °C with three different feeding ratios (0, 5, and 10) of EG over Mg2+ in the reaction system. These matched well with the standard ICDD PDF 2004 card for MgO (PDF # 45-0946) as mentioned in the graph with the corresponding hkl. For all these samples, different diffraction peaks appear at 37.1°, 43.1°, 62.5°, 74.7° and 78.6°/2θ, which individually correspond to (111), (200), (220), (311) and (222) for a cubic close packed MgO phase. These results suggest the ethylene glycol-assisted room temperature method is an available route for the precursor of a hierarchical porous flower-like MgO.
image file: c9qi00331b-f2.tif
Fig. 2 X-ray diffraction pattern of three hierarchical flower-like MgO microsphere samples, individually prepared from the precursors with different molar ratios of EG over Mg2+ from 0 to 10.

Fig. 3 shows the morphology of three MgO samples prepared with different feeding ratios between EG and Mg2+ from 0 to 10 in the reaction system. Fig. 3A–C present MgO formed in the absence of EG, which looks like a flower bud composed of lamellar sheets with a highly stacked density. With an increase in the feeding ratio to 5 and 10, as shown in Fig. 3D–F, and G–I, the petals of the flowers expand to produce larger pores, which helps expose more active sites for the adsorption process. Furthermore, the diameter of the flower-like MgO microsphere was increased from 12.69 μm to 13.21 μm. Interestingly, the additional amount of EG strongly influenced the open degree of the hierarchical MgO microspheres. Therefore, EG plays a key role to finely control pore size of MgO, probably resulting from electrostatic interaction between EG and Mg2+ as reported in the literature.46–50 The Mg2+ coordinates the hydroxyl groups of EG to form an alkoxide coordination complex in a basic medium with different divalent cations to form sheets arranged in an ordered way to construct a microsphere.51,52


image file: c9qi00331b-f3.tif
Fig. 3 SEM images of EG/Mg = 0.0 (A, B, C), EG/Mg = 5 (D, E, F), EG/Mg = 10 (G, H, I) for hierarchical flower-like MgO microspheres individually prepared with different molar ratios of EG over Mg2+ from 0 to 10.

Pore properties

Fig. 4A demonstrates the N2 adsorption–desorption isotherm estimated by the BET method. All the samples exhibit a II type adsorption isotherm with a H3 hysteresis loop. Generally, a type II isotherm represents macropores in a material and unrestricted monolayer adsorption while a H3 hysteresis loop indicates slit-like pores with unlimited monolayer adsorption53 and they are in good agreement with results observed from the SEM in Fig. 3.
image file: c9qi00331b-f4.tif
Fig. 4 Nitrogen adsorption–desorption isotherms. (A) Pore size distribution. (B) Cumulative intrusion variation. (C) Macropores distribution. (D) Hierarchical flower-like MgO samples individually prepared with the different ratios of EG over Mg2+ from 0–10.

Fig. 4B further elaborates pore size distribution in a range of ca. 0.07–137 nm, further divided into three regions: 1.3–2.5 nm, 2.5–7.4 nm and 7.4–137 nm separated by dotted lines in the graph. Comparatively, there are significant differences of pore size distribution in each region, for example, EG/Mg = 0 has more pores in the first region and EG/Mg = 10 has more pores in the second and third regions (2.5–136.7 nm).

From Table 1, one can observe that all the pore parameters increased with an increase in the ratio between EG and Mg2+ in the reaction system and similar results were observed from the SEM images. Among the three samples, Mg/EG = 10 has the highest surface area (75 m2 g−1), total pore volume (0.53 cm3 g−1) and total pore area (47.37 m2 g−1). As observed in SEM images (Fig. 3), various macropores were present in the flower-like MgO microspheres and can't be estimated by the nitrogen adsorption desorption isotherm. Thus, a mercury intrusion method was used for the macropore properties of flower-like MgO microspheres and the corresponding results are shown in Fig. 4C, D and Table 2. From EG/Mg = 0 to 10, the pore parameters e.g., mercury intrusion pore volume, total pore area and average pore diameter increased as observed in Fig. 3. For example, the total pore area increased ca. 11.09% from 42.08 m2 g−1 for EG/mg = 0 to 47.33 m2 g−1 for EG/Mg = 10; the mercury intrusion volume was improved ca. 19.16% from 2.11 mg L−1 for EG/Mg = 0 to 2.61 mg L−1 for EG/Mg = 10; average pore diameter was also enhanced ca. 9.09% from 0.20 μm for EG/Mg = 0 to 0.22 μm for EG/Mg = 10.

Table 1 Pore parameters for three flower-like MgO microspheres
Samples Surface area (m2 g−1) Total pore volume (cm3 g−1) Pore diameter (nm) Total pore area (m2 g−1)
EG/Mg = 0 53 0.30 22.85 32.56
EG/Mg = 5 56 0.40 29.05 35.40
EG/Mg = 10 75 0.53 28.74 47.37


Table 2 Calculated texture parameters obtained from mercury intrusion porosimeter
Sample Total pore area (m2 g−1) Hg intrusion pore volume (cm3 g−1) Average pore diameter (μm)
EG/Mg = 0 42.08 2.11 0.20
EG/Mg = 5 45.62 2.30 0.21
EG/Mg = 10 47.33 2.61 0.22


Fig. 4D further elaborates the pore size distribution for a flower-like microsphere from ca. 0.005 μm to 9.32 μm which was established based on the Washburn Laplace equation, and pore distribution was divided into four regions: 0.005–0.015 μm, 0.015–0.28 μm, 0.28–5.25 μm and 5.25–9.32 μm as separated by dotted lines. For instance, EG/Mg = 0 has more pores in the range of 0.28–5.25 μm while EG/Mg = 5 and 10 have more pores in the ranges of 0.015–0.28 μm and 5.25–9.32 μm, respectively. Probably, the large pore sizes and high surface areas are favourable for more exposure of active adsorption sites and faster mass transfer to the inner material surface.29,30,54 Undoubtedly, both of them help to enhance the adsorption capacity and hasten the adsorption rate.

Adsorption kinetic and isothermal results

Fig. 5A presents the adsorption kinetic for phosphate anions with a hierarchical flower-like MgO microsphere. Here, one observes that the short equilibrium time of ca. 6 h occurs for both EG/Mg = 5 and 10 as related to that of ca. 8 h for EG/Mg = 0, and the equilibrium removal capacity of 287 mg g−1 for EG/Mg = 10 is three times higher than that of 94 mg g−1 for EG/Mg = 0, probably resulting from a relatively higher surface area and larger pores. The obtained kinetic data were linearly fitted for a pseudo first order (Fig. 5B) and pseudo second order (Fig. 5C) models, respectively. Furthermore, based on the fitting parameters listed in Table 3, the phosphate adsorption behaviours of three MgO samples obey the pseudo second kinetic model well, suggesting a mixed process of physical and chemical adsorption of phosphate.
image file: c9qi00331b-f5.tif
Fig. 5 (A) Variation of removal capacity with time when phosphate = 0.05 g L−1, volume = 0.1 L, MgO = 0.01 g, pH0 = 5. (B) Pseudo first order. (C) Pseudo second order. (D) Effect of equilibrium concentration on removal capacity when phosphate = 0.025–0.25 g L−1, volume = 0.05 L, pH0 = 5, MgO = 0.01 g. (E) Langmuir isotherm model. (F) Freundlich isotherm model for flower-like MgO microspheres.
Table 3 Kinetic parameters for three MgO samples
Samples q e,exp (mg g−1) Pseudo first order Pseudo second order
q e,cal (mg g−1) k 1 (h−1) R 2 q e,cal (mg g−1) k 2 (g mg−1 h−1) R 2
EG/Mg = 0 93.75 68.95 0.40 0.93 102.46 0.001 0.99
EG/Mg = 5 185.71 119.34 0.51 0.85 206.61 0.005 0.99
EG/Mg = 10 287.88 180.50 0.50 0.92 310.56 0.004 0.99


Besides, Fig. 5D displays the adsorption isotherm of three MgO samples in a concentration range of 0.025 to 0.25 g L−1. The obtained isotherm data were individually fitted for Langmuir (Fig. 5E) and Freundlich (Fig. 5F) models. The obtained parameters in Table 4 suggest that the adsorption isotherm data are better fitted in the Langmuir isotherm model based on the correlation coefficient values. Generally, the Langmuir model indicates a monolayer surface adsorption of phosphate on MgO. Furthermore, the equilibrium isotherm parameter (RL) is between 0 and 1, suggesting that phosphate adsorption by the three MgO samples is favourable (Table 4).55 The maximum Langmuir removal capacities for EG/Mg = 0, 5 and 10 are 396.83 mg g−1, 515.46 mg g−1 and 574.71 mg g−1, respectively. In comparison, the qmax of 574.71 mg g−1 for the present flower-like MgO microsphere lists a relatively high removal capacity in Table 5.

Table 4 Isotherm parameters for flower-like MgO microspheres
Samples Langmuir isotherm parameters Freundlich isotherm parameters
q max (mg g−1) K L (L mg−1) R 2 R L K F (mg g−1) (L mg−1)1/n n R 2
EG/Mg = 0 396.83 0.04 0.99 0.33 75.21 3.28 0.94
EG/Mg = 5 515.46 0.05 0.98 0.31 118.97 3.74 0.96
EG/Mg = 10 574.71 0.06 0.99 0.29 132.26 3.57 0.97


Table 5 Removal capacity comparison for phosphate with different adsorbent materials
Adsorbent pH Removal capacity (mg g−1) Ref.
MgO 5 574.71 This study
MgO 5 478.5 42
MgO 5 236 43
MgO 4.03 398 56
MgO 5 75.13 57
Mg@CaCO3 10 190 58
Mg(OH)2 5 52.08 59
Fe@SiO2 7 0.693 60
G@La2O3 6.2 82.6 61
ZrO2@SiO2 43.8 62
Fe@MnO2 7 112.36 63
ZnAl LDHs 7 232 64
1Fe HNT 4 5.46 65
NaLa(CO3)2/Fe3O4 7 77.85 66
La-Zeolite 6 17.2 67
APTCMS-CMK-3 7 38.09 68
Hydrous ferric oxide 8.5 78.5 69


Adsorption thermodynamic

Fig. 6A displays the effect of temperature on the removal capacity for phosphate by a flower-like MgO microsphere. The equilibrium removal capacity was increased following thermodynamic temperatures from 303 K to 323 K, suggesting an endothermic nature of phosphate adsorption on the flower-like MgO microspheres. All the Gibbs free energy values were negative at all the investigated temperatures, which suggests a favourable and spontaneous process of the adsorption (Table 6) as suggested by RL in Table 4.45Fig. 6B further presents the linear fitting plot between 1/T and ln[thin space (1/6-em)]Kd, and Table 6 lists the corresponding parameters including enthalpy and entropy. The positive value of enthalpy confirms the endothermic nature of phosphate adsorption by MgO as reported in the literature.70 A positive values of entropy suggests that randomness increased at the interface between phosphate and MgO with the rise of thermodynamic temperature during the process of phosphate uptake.71
image file: c9qi00331b-f6.tif
Fig. 6 (A) Effect of thermodynamics temperature on removal capacity when phosphate = 0.05 g L−1, volume = 0.05 L, MgO = 0.01 g, pH0 = 5. (B) Linear plot between ln[thin space (1/6-em)]Kd and 1/T for three flower-like MgO microspheres.
Table 6 Thermodynamic parameters for three flower-like MgO microspheres
Samples T (K) K d (mg L−1) ΔG° (KJ mol−1) ΔS° (KJ mol−1 K−1) ΔH° (KJ mol−1)
EG/Mg = 0 303 1.94 −4882.00 158.85 43144.90
313 2.51 −6518.25
323 2.65 −7118.85
EG/Mg = 5 303 2.61 −6577.92 174.93 46300.41
313 2.92 −7589.23
323 3.22 −8644.04
EG/Mg = 10 303 3.00 −7547.66 137.56 35400.70
313 3.38 −8784.69
323 3.79 −10176.10


Adsorption mechanism

In order to examine the adsorption mechanism of hierarchical flower-like MgO microspheres, the EG/Mg = 10 sample after phosphate adsorption was further characterized by XRD, FT-IR, SEM, and EDS. Here, the PXRD pattern of MgO after adsorption in Fig. 7A exhibits a series of new diffraction peaks compared with that of MgO in Fig. 1, which matches those for Mg(OH)2 (PDF # 44-1482) well, indicating that MgO absorbs a proton from water to form reactive specie MgOH2+.59 Also, the FT-IR spectra for MgO after adsorption in Fig. 7B display a new band occurring at 1074 cm−1, attributed to the bending vibration of adsorbed phosphate.72 Along with phosphate, the two bands (1640 cm−1 and 3440 cm−1) are recognised as the bending and stretching vibrations of a hydroxyl group and the band at 2362 cm−1 belongs to carbon dioxide adsorbed from the atmosphere. Besides, XPS spectra for the phosphorous species in Fig. 7C confirm existence of the PO43− and HPO42−.59 resulting from the reaction between Mg(OH)2 and H2PO4.73 That is to say, the adsorption process involves both a chemical pathway and physical process, which is in agreement with the pseudo second order kinetic model. The EDS results of MgO after adsorption in Fig. 7D further show ca. 11.32 wt% phosphorous was adsorbed into the porous MgO.
image file: c9qi00331b-f7.tif
Fig. 7 (A) X-ray diffraction pattern after adsorption. (B) FT-IR spectrum before and after adsorption of phosphate. (C) SEM image. (D ) EDS after adsorption of phosphate for hierarchical spherical flower-like macroporous MgO (Mg/EG = 10).

Conclusions

In summary, a hierarchical flower-like MgO microsphere was successfully prepared at room temperature by adjusting the molar ratios between ethylene glycol and magnesium cations in the reaction system. The result reveals that EG plays a key role in the morphologies, pore structure and corresponding removal capacity of MgO for phosphate due to an interface effect between EG and water. The highest removal capacity of 574.71 mg g−1 was related to the unique flower-like spherical morphology, which has more available adsorption sites with appropriate surface areas. Phosphate adsorption follows a pseudo second order kinetic, Langmuir adsorption isotherm model with endothermic properties and spontaneous phosphate uptake by the flower-like MgO microspheres. These hierarchical flower-like MgO microspheres are a promising candidate to overcome sustainable problems due to their superb removal capacity and facile preparation at room temperature.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by National Key R & D Program of China (No. 2016YFB0301600), the National Natural Science Foundation of China, and the Fundamental Research Funds for the Central Universities (12060093063). S. Ahmed specially thanks the financial support from Chinese Scholarship Council.

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