The fabrication and adsorption mechanism of flower-like MoS2/g-C3N4 nanocomposites as new adsorbent materials were investigated by batch sorption experiments in this paper. The influence of factors such as pH, adsorbent dosage, concentration and temperature on the adsorption properties of flower-like MoS2/g-C3N4 nanocomposites were studied in detailed. The adsorption isotherm data was fitted with the Langmuir model, and the adsorption kinetic characteristics conform to the quasi second-order kinetic equation. Thermodynamic data showed that the adsorption process of methylene blue (MB) was feasible, endothermic and spontaneous. At 45 ℃, the maximum adsorption capacity was 278.4 mg/g. The adsorbed MB solution was used to water the wheat and chickpea plants within 15 days. Compared with MB solution, the treated MB solution made the plants grow much more better.
Research Article
Controllable synthesis and adsorption mechanism of flower-like MoS2/g-C3N4 nanocomposites for removal of methylene blue in water
https://doi.org/10.21203/rs.3.rs-924254/v1
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published 31 Mar, 2022
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Flower-like MoS2/g-C3N4
Adsorption
Methylene blue (MB)
Mechanism
Ecological assessment
With remarkable industrial development and the continuous growth of the global population, people's demand for textile and clothing is increasing, which resulted in the continuous discharge of distinctive toxic pollutants into the water streams that raises serious environmental problems (Pan et al., 2016). Considering that in the textile, leather and paper industries, about 80% of the dye wastewater is discharged into the water without any treatment every year, which has brought very serious damage to the water supply environment, and has become a major problem that seriously threatens the quality of the biosphere and drinking water supply around the world (Carolin et al., 2017; Katheresan et al., 2018; Oladipo & Gazi, 2014).
Dye wastewater mainly comes from the production industry of dyes and dye intermediates, which is composed of mother liquor of various products and intermediates crystallization, materials lost in the production process and sewage washing the ground, among which organic dye wastewater accounts for a large proportion. Most organic dyes have certain biological toxicity, which can threaten the health of organisms in the water and even cause their death. At present, the commonly used methods for the treatment of printing and dyeing wastewater include photocatalytic degradation (Pang et al., 2018), adsorption (Yu et al., 2018), biological treatment (Wang et al., 2013), filtration (Zanacic et al., 2016) and membrane technology (Farhat et al., 2016). Because the adsorption method is low cost, effective, easy to operate and not easy to cause secondary pollution, it has gradually become an indispensable solution for printing and dyeing wastewater treatment (Gao et al., 2016; Wang et al., 2019). The effect of dye wastewater treatment by adsorption method mainly depends on the adsorbent structure and surface morphology, so the preparation of adsorbent with good adsorption performance is the key to achieve efficient treatment of dye wastewater.
There has been extensive research on the preparation and modification of adsorption materials, among which carbon materials is the most representative. It has a large specific surface area and good adsorption performance, and is the most widely used adsorbent (L. Chen et al., 2017; Manilo et al., 2016; Murray & Ormeci, 2018). More and more attention has been paid to obtaining cheap and easily available materials with large specific surface area, unique structural properties and excellent chemical stability. Graphite phase carbon nitride is a new type of non-metallic graphite semiconductor material, which has the advantages of good stability, metal-free “earth abundant nature” and many adsorption sites(Wu et al., 2018). It showed excellent application prospects in the field of adsorption (Hu et al., 2015; Zhang et al., 2016). However, the bulk g-C3N4 synthesized directly from organic precursors often has a smaller specific surface area (less than 20 m2 · g− 1), and meagre mass/diffusion transfer rate. From a practical perspective, particularly in adsorption, the establishment of controlled porosity and morphology at the nanoscale in bulk g-C3N4 is essential to maximize its efficiency. In 2013, Galen D. Stucky and co-workers synthesized carbon nitride mesoporous hollow spheres by changing the precursor materials, which greatly improved the specific surface area and adsorption sites (Jun et al., 2013). MoS2, indicating that a large number of sulfides (S2−) can absorb various heavy metal and organic pollutants through electrostatic, hydrophobic or chemical complex interactions (Li et al., 2018; Liu et al., 2019). Ibrahim M. Alarifi and his colleagues synthesized and used MoS2 adsorption to adsorb Congo red, at 50°C, the maximum adsorption capacity was 80.64 mg/g (Alarifi et al., 2021). Therefore, we assembled MoS2 onto g-C3N4 to prepare MoS2/g-C3N4 nanocomposites which can combine their excellent properties.
As a common organic dye, methylene blue (MB) is widely used in production, life and scientific research. Studying the adsorption of adsorbent on MB waste liquid can provide an economic and effective method for water pollution treatment. In order to obtain a new adsorbent with good adsorption effect on methylene blue, we synthesized flower-like MoS2/g-C3N4 nanocomposites via hydrothermal reaction (Scheme 1). According to the experimental results, the flower-like MoS2/g-C3N4 nanocomposites have obvious adsorption for MB. Especially at 45 ℃, the maximum adsorption capacity of the flower-like MoS2/g-C3N4 nanocomposites is 278.4 mg/g. This adsorption capacity is better than many adsorbents reported previously. In addition, the adsorption of MB on the nanocomposites has good recyclability, and it still has good stability after five cycles. We studied the factors affecting MB adsorption (pH, adsorbent dose and original MB concentration), the adsorption isotherm model, adsorption kinetics and thermodynamics. Additionally, we introduced the applicability of the report study of the treated solution. The effluent from the MB removal process is then used to water wheat and chickpeas to examine their effects on plant growth.
2.1 Materials
Melamine, cyanuric acid and Na2MoO4·2H2O were from Aladdin Chemical Reagent Co., Ltd (Shanghai, China), dimethyl sulfoxide and thiourea were from Guangdong Guanghua Sci-Tech Co., Ltd (Guangdong, China). All chemical substances were analytically pure.
2.2 Preparation of g-C3N4 hollow sphere
The nanocomposite was synthesized on the basis of Refs (Jun et al., 2013; S. Zhao et al., 2020). First, added 1 g melamine to 50 ml dimethyl sulfoxide (DMSO), added 1 g cyanuric acid to 25 ml DMSO, stirred for 30 minutes to form a solution, and labeled as A and B. Then, in the process of stirring, dropped solution B into solution A and continued stirring for 30 minutes to form white precipitate. After the reaction, the supernatant was discarded, the precipitate washed and centrifuged, dried at 50 ℃ for 6 h, and ground into powder, named as MCA. Finally, the MCA was heated to 550 ℃ for 3 h in muffle furnace to obtain spherical g-C3N4.
2.3 Preparation of MoS2/g-C3N4 nanocomposites
First, 0.29 g of Na2MoO4 · 2H2O was added in 40 ml water, and form a solution. Subsequently, 0.005 g of polyvinyl pyrrolidone (PVP) was added in the solution and magnetically stirred for 10 min. After that, hydrochloric acid was added to adjust the solution pH to 1. Then, 1.44 g of thiourea was added, and ultrasonic dispersion was carried out for 10 min to form a solution. 0.192 g g-C3N4 was put into the solution, and ultrasonic oscillation was carried out for 2 h. Finally, the reaction mixture was transferred to a high-temperature reactor, and reacted at 220 ℃ for 18 h. After the reaction, the product was washed, centrifuged and dried at 50 ℃ for 6 h to obtain MoS2/g-C3N4. Pure MoS2 can be obtained without adding g-C3N4.
2.4 The study of plant growth.
First, 10 seeds of wheat and chickpea of the same shape and size were selected and sprouted in wet cotton day and night. And then planted the seeds in the same amount on the test tube with the same amount of MB solution (40 mg/L) and the treated solution (MB solution adsorbed by MoS2/g-C3N4 nanocomposites). After that, these plants' root and bud growth was analyzed in the next 15 days (Gunture, Aggarwal, et al., 2020; Gunture, Kaushik, et al., 2020; Gunture et al., 2019).
2.5 Characterization of materials
The prepared adsorbents were characterized by XRD (Bruker D2 PHASER X) using Cu kα as the irradiation source (λ = 1.5418 Å). The chemical composition and electronic state of the adsorbents were studied by XPS (Kratos Axis Ultra DLD). The adsorbents’ appearance was recorded by FESEM (FEI Verios G4). And the structural details of the adsorbents were confirmed by HRTEM (FEI Talos F200X TEM). BET (3H-2000PS2) of Beishide Instrument Technology Co., Ltd. (Beijing, China) was used to carry out a nitrogen adsorption-desorption experiment on the adsorbent. The UV-Vis spectrophotometer made by Shanghai Jinghua Instrument was used to analyze the absorbance of the solution.
2.6 Adsorption procedure
Different mass of MoS2/g-C3N4 nanocomposites were added into 50 ml MB solution. The different original concentrations were C0 (mg/L). After that, the solution in the round bottom flask was heated at different temperatures (15°C, 25°C, 35°C, 45°C) with stirring at 300 rpm for about 60 minutes (until adsorption equilibrium was reached). Adjust the solution pH to 1–11 by HCl or NaOH. After adsorption, centrifugation, take the supernatant. Then, the absorbance of MB at the maximum absorption wavelength (664nm) was measured by UV spectrophotometer, and after the adsorption equilibrium, the equilibrium concentrations were Ce (mg/L), the removal efficiency (η%) and equilibrium adsorption capacity (qe) of MB solution were counted by these formula:
$$\eta \left(%\right)=\frac{{C}{0}-{C}{e}}{{C}_{0}}\times 100%$$
1
$${q}{e}=\frac{\left({C}{0}-{C}_{e}\right)\cdot V}{m}$$
2
where V (L) was the volume of MB solution, m (g) was the weight of MoS2/g-C3N4 nanocomposites and qe (mg/g) was the adsorption capacity at equilibrium.
3.1 Structure characterization
The powder XRD patterns of the g-C3N4, MoS2 and MoS2/g-C3N4 nanocomposites were shown in Fig. 1. In the typical XRD pattern, all diffraction peaks of g-C3N4 correspond to the standard card (JCPDS 87-1526). At 13.2 °, the peak value corresponds to the (100) crystal planes, caused by the periodic arrangement of stacking units between layers of g-C3N4. The peak value at 27.5 ° corresponds to the (002) crystal planes, which is the characteristic peak of the accumulation of graphite like layered structure of g-C3N4 (Lin et al., 2019). The diffraction peaks of pure MoS2 at 15.1 °, 32.7 °, 35.2 °, 38.8 °, 44.2 ° and 57.3 ° belongs to the (002), (100), (102), (103), (006) and (110) crystal planes of hexagonal MoS2 (JCPDS 37-1492), respectively (Ali et al., 2019). From this typical XRD patterns of MoS2/g-C3N4 nanocomposites, we can see the peak of MoS2 clearly, but the peak value of g-C3N4 is very small, which may be due to the nanocomposites contain the relatively low content of g-C3N4. The other possible reason of weak intensity of (002) peak of g-C3N4 may coincides or interfere with the (100) crystal planes of MoS2 peak. Nevertheless, the diffraction peak intensity of MoS2 decreases with the addition of g-C3N4, especially at 15.1 ° peak. This indicates that the addition of g-C3N4 further limits the accumulation of the molybdenum disulfide layer (Y. X. Chen et al., 2017).
XPS analysis further determined the chemical state and element composition of the MoS2/g-C3N4 nanocomposites. Figure 2a illustrated the XPS survey spectrum confirmed C, N, Mo, and S elements exist in MoS2/g-C3N4 nanocomposites. Figure 2b represents the spectra of C1s, where 284.1 eV corresponds to the C-C in aromatic rings, 285.8 eV corresponds to N-C = N and 288.0 eV corresponds to C-NH2 (Yi et al., 2017). Figure 2c suggests that the peaks of N 1s spectra at 398.6 eV, 399.1 eV and 403.8 eV belong to C = N-C, N-(C)3 and N–H structures, respectively (Cao et al., 2017). In addition, the two peaks in the Mo 3d spectrum at around 228.95 eV and 232.13 eV belong to Mo 3d5/2 and Mo 3d3/2 in Fig. 2d, respectively. There was another peak of 226.01 eV in this area which is part of S 2s. The spectrum of S 2p in Fig. 2e shows two peaks: 161.87 eV and 163.20 eV were due to s 2p3/2 and s 2p1/2, respectively. These studies confirmed the successful preparation of MoS2/g-C3N4 nanocomposites.
The morphology and structures of pure g-C3N4, pure MoS2 and MoS2/g-C3N4 nanocomposites were directly analyzed by SEM and TEM. Figure 3 (a-b) FESEM analysis confirmed that the as prepared g-C3N4 possessed the hollow sphere morphology, and Fig. 3a shows that the precursor MCA of g-C3N4 is spherical-like. Figure 3b shows the g-C3N4 formed after calcination of MCA, which is hollow and spherical. Figure 3c shows pure MoS2, which is flower-like. Figure 3d indicates that the morphology of the MoS2/g-C3N4 nanocomposites basically keeps the appearance of MoS2. Figure 3e shows the HRTEM micrograph of MoS2/g-C3N4 nanocomposites. The figure shows that MoS2 is flower like structure, and g-C3N4 is coated by MoS2. Figure 3f shows the presence of well-defined lattice fringes of 0.62 nm, this can be attributed to the (002) crystal plane of MoS2 (Monga et al., 2020).
Figure S1 showed the N2 adsorption desorption isotherm. Since there was almost no adsorption limitation at the high-pressure stage, the adsorption isotherm of MoS2/g-C3N4 nanocomposites was determined to be type-IV isotherm with H3 type hysteresis loops, were closely related to the capillary condensation phenomenon of slit pores generated by the stacking of lamellar particles (Liu et al., 2020; Zhu et al., 2020). The pore size of MoS2/g-C3N4 nanocomposites was mainly between 2–17 nm, this indicates the composite was mesoporous. It can be seen in Table S1, the BET surface area, average pore size and pore volume of MoS2/g-C3N4 nanocomposites were 70.656 m2/g, 15.4494 nm and 0.2729 cm3/g, respectively. These sufficient parameters indicated that mesoporous MoS2/g-C3N4 nanocomposites have excellent adsorption properties for MB.
3.2 Experiment majorization
3.2.1 Effect of pH value
The pH value has great influence on the ionization degree of MB molecule and the surface charge of adsorbent in the solution (I et al., 2020). For the sake of studying the effect of pH value on MB adsorption, we set the pH value from 1 to 11. In Fig. S2, the MB adsorption efficiency of MoS2/g-C3N4 nanocomposites increased with the pH value in the range of 1 to 11. And the surface charge of MoS2/g-C3N4 nanocomposites was shown in Fig. S3. At relatively low pH, the proton ions in the solution may compete with the active sites of cationic dye MB and MoS2/g-C3N4 nanocomposites, which inhibits the adsorption of MB on synthetic adsorbent. With the increase of pH, the surface charge of MoS2/g-C3N4 nanocomposites became lower, resulting in stronger electrostatic attraction between MB and adsorbents. The results showed that the higher the pH value was, the higher the adsorption efficiency was. Fig. S4 showed that the MoS2/g-C3N4 nanocomposites have higher adsorption efficiency when pH = 7, compared with g-C3N4 and MoS2.
3.2.2 Effect of adsorbent dosage
By adding different amounts of MoS2/g-C3N4 nanocomposites into MB solution, the effects of adsorption dose on adsorption efficiency and adsorption capacity were tested. Fig. S5 illustrated when the adsorbent Dosage increases, the adsorption efficiency of MB first increases, and then almost remains unchanged, the equilibrium adsorption capacity of MB decreased from 251.2mg/g to 99.7mg/g. This was due to the equilibrium state of the adsorption process, the adsorbent can no longer adsorb MB. It is obvious that a high MB adsorption rate can be achieved even with a small amount of adsorbent. Considering the removal efficiency and practicability, we selected 10mg adsorbent for the following experiments.
3.2.3 Effect of MB concentration
The adsorption capacity and adsorption efficiency of the nanocomposites were tested by adding different concentrations of MB solution. Fig. S6 showed that when MB concentration increased from 20mg/L to 100mg/L, the equilibrium adsorption capacity increased from 98.8mg/g to 227.4mg/g. It may be that with the increase of MB concentration, the adsorbate in solution has stronger driving force to overcome the mass transfer resistance between solution and adsorbent (L. Zhao et al., 2020). But with the increase of MB concentration, the adsorption rate of MB reduced from 98.80–45.48%, because at a certain concentration, the active center of the adsorbent reached saturation.
3.2.4 Effect of adsorption temperature
In this work, the temperatures were set at 15 ℃, 25 ℃, 35 ℃ and 45 ℃, respectively. Fig. S7 showed that temperature had obvious influence on MB adsorption. With the increase of temperature from 15 ℃ to 45 ℃, the adsorption efficiency rose from 86.84–98.17%. Because when the temperature increased, the molecular Brownian motion will accelerate, and low temperature will usually reduce the diffusion rate of MB in the nanocomposites (Fu et al., 2017). Considering the adsorption efficiency and practicability, the following experiments were carried out at 25 ℃.
3.2.5 Recyclability of MoS2/g-C3N4 for the MB adsorption
The recyclability and stability of adsorbents can greatly improve efficiency and reduce cost. In this study, MB was desorbed with 0.1mol/L NaOH. Fig. S8 showed the MB adsorption efficiency of MoS2/g-C3N4 nanocomposites for five consecutive cycles. The results illustrated that the adsorption efficiency of MB decreased gradually because the adsorbate could not be completely desorbed and occupied some active adsorption sites. The adsorption efficiency of the MoS2/g-C3N4 nanocomposite for MB was still 73.58% after five cycles, which showed that the adsorbent has good reusability.
3.3 Adsorption kinetics
Figure 4a showed the effect of contact time on adsorption capacity. With the increase of contact time, the adsorption capacity increased rapidly from the beginning to slowly until the adsorption equilibrium reached 60 minutes. In this study, pseudo-first-order, pseudo-second-order kinetics and intra-particle diffusion models were used to analyze the adsorption process (Tehrani & Zare-Dorabei, 2016). The calculation formula of the pseudo-first-order kinetics model was as follows:
The calculation formula of intra-particle diffusion model was as follows:
Where qe (mg/g) is described above, qt (mg/g) represent the adsorption capacity at any contact time, and t (min) is the adsorption time. k1, k2 and k3 are the pseudo-first-order, pseudo-second-order and intra-particle diffusion model rate constants respectively. C (mg/g) is the intercept obtained by fitting the model of intra-particle diffusion.
The linear fitting results of the two kinetic models were shown in Fig. 4. It illustrated that the pseudo-second-order model (Fig. 4c) is better than the pseudo-first-order model (Fig. 4b) in describing the process of adsorption kinetics, which is proved by the higher R2 (R2 = 0.9999) value. In addition, the calculated results of the two kinetic models were shown in the Table S2. Theoretical equilibrium adsorption capacity qe (cal) of the pseudo-second-order model is more consistent with the experimental data qe (exp), which indicates that the adsorption of MB on MoS2/g-C3N4 nanocomposites conforms to the pseudo-second-order model, and the adsorption process is mainly controlled by chemical adsorption (Fang et al., 2018; Guan et al., 2017).
The model of intra-particle diffusion after fitting is shown in (Fig. 4d). The graph was nonlinear, which proved that intra-particle diffusion was not the only factor limiting particle diffusion. There are three processes for MB adsorption. The first stage was from the solution to the surface of MoS2/g-C3N4 nanocomposites. The second stage was the diffusion of MB in MoS2/g-C3N4 nanocomposites. The third stage was the final balance stage. The above analysis showed that the intra-particle diffusion and surface adsorption occur simultaneously, which affects the adsorption of MB significantly.
3.4 Adsorption isotherm
Figure S9 showed the MB adsorption isotherms of MoS2/g-C3N4 nanocomposite at different temperatures. The experimental data were fitted into Langmuir model for monolayer adsorption (Eq. (6)) and Friedrich model of multilayer adsorption (Eq. (7)) (Gunture, Kaushik, et al., 2020). They can be expressed as:
where Ce (mg/L) was the equilibrium concentration of MB, qm (mg/g) was the theoretical maximum adsorption capacity, KL (L/mg) and KF were Langmuir constant and Friedrich constant, respectively. n was the constant related to the surface inhomogeneity of adsorbent (L. Zhao et al., 2020).
The fitting results of the two isotherm models were shown in Fig. 5. The datas in Table 1 showed that R2 values in the Langmuir model (Fig. 5a) were bigger than those in the Friedrich model (Fig. 5b). Therefore, the adsorption of MB on MoS2/g-C3N4 nanocomposites followed the Langmuir isotherm model. At 45 ℃, qmax is 278.4 mg/g, which indicates that MoS2/g-C3N4 nanocomposites has higher MB adsorption effect. The comparison of qmax of different adsorbents is given in Table S3. Therefore, the adsorption capacity of MoS2/g-C3N4 nanocomposites for MB was similar or higher than that of other commonly used adsorbents.
We use the separation coefficient RL to evaluate the feasibility of adsorption by the following equation:
Where C0 (mg/L) and KL (L/mg) had explained above. As shown in Table S4, we observed that 0 < RL < 1, it indicated that the process of MB adsorption was favorable (Mohammadnejad et al., 2018).
Table 1. Modeling parameters of adsorption isotherm calculated by Langmuir model and Freundlich model.
3.5 Adsorption thermodynamics
At different temperatures, we used various thermodynamic parameters to understand the adsorption process. The thermodynamic parameters can be obtained by the following expression:
Where KL is the equilibrium distribution coefficient, R is the molar gas constant (8.314 J/mol − 1·K − 1), T (K) is the temperature, ΔGθ is the Gibbs free energy change, ΔHθ is the enthalpy change, ΔSθ is entropy change. Figure 6 showed the plot of ln KL against 1/T. From Table 2 we can see that the value of ΔGθ was negative, indicating that the adsorption process was a spontaneous process at different temperatures, and the increase in temperature was conducive to the adsorption of MB. The value of ΔHθ was positive, indicating that the adsorption was an endothermic process. In addition, the value of ΔSθ was positive which indicated that the disorder of the solid-liquid interface increases during the adsorption process.
Table 2. Thermodynamic parameters for adsorption of MB by MoS2/g-C3N4 nanocomposites.
3.6 Ecological assessment of treated wastewater
In order to evaluate the ecological characteristics of the treated MB solution, the untreated MB solution and the treated solution were used for the culture of wheat and chickpea seeds, respectively [24]. The growth of germinated wheat and chickpea seeds was analyzed in the next 15 days. As shown in Fig. 7a, the average length of wheat seedlings grown with MB solution is 14.7 cm, while that is 17.2 cm under treated solution. As shown in Fig. 7b, the average length of chickpea grown with MB solution is 21.4 cm, and that with treated solution is 43.6 cm. The results showed that MB solution had strong inhibition on root and bud germination of both plants. Compared with untreated MB solution, the treated solution shows a great improvement role for growth of root and bud parts of wheat and chickpea.
3.7 Adsorption mechanism
Figure S10 showed the possible adsorption mechanism of MoS2/g-C3N4. From the above that the adsorption of MB by MoS2/g-C3N4 belonged to Langmuir monolayer adsorption model. The adsorption rate decreased with the increase of adsorption capacity. From the molecular structure of MB and MoS2/g-C3N4, there was an electrostatic interaction between MB and MoS2/g-C3N4 due to that MB is a cationic dye, while MoS2/g-C3N4 is negatively charged in the solution (Fig. S3). As shown in Figure S10, MB and MoS2/g-C3N4 both contain aromatic rings and possible stronger π-π interaction. In addition, both MoS2/g-C3N4 and MB contain nitrogen atoms, and hydrogen bonds may be formed between them. The above analysis indicated that the adsorption of MB was mainly driven by electrostatic interactions, together with π-π interaction and hydrogen bond.
We designed and synthesized MoS2/g-C3N4 nanocomposites for MB adsorption in this study. The experimental results indicated that it is feasible to remove MB from water by adsorption. The factors affecting the adsorption performance of MB (pH of solution, adsorption dose, MB concentration and adsorption temperature) were studied. The adsorption process followed the Langmuir monolayer adsorption isotherm model and the pseudo second-order kinetic model. Thermodynamic data showed that adsorption is a spontaneous process, and the increase of temperature is conducive to the adsorption of MB. MB can reach the adsorption equilibrium within 60 min, and the maximum adsorption capacity of MB was 278.4 mg/g at 45 ℃. On the other hand, after the treated wastewater was used for watering the seeds of wheat and chickpea, we found that the treated wastewater was more conducive to the growth of wheat and chickpea than MB solution. The present study indicated that the synthesized MoS2/g-C3N4 nanocomposites have good application prospects for waste-water treatment and environmental remediation.
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Declaration of interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding: This research received no external funding
Author Contributions: Conceptualization, C.Q.; methodology, Z.W., J.H.; validation, C.Q., Z.W., J.H., N.C.; investigation, Z.W., J.H., J.W., C.Z, C.Q., N.C.; resources, C.Q., N.C.; data curation, Z.W., C.Q., J.H., C.Z., J.W., W.I., N.C.; writing-original draft preparation, Z.W., J.H., C.Q.; writing-review and editing, Z.W., J.H., C.Q., W.I.; supervision, C.Q., N.C.; project administration, C.Q., N.C.; funding acquisition, C.Q., N.C.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (21572180, 21602174), the Intergovernmental Science and Technology Cooperation and Exchange Program between China and Romania (43-24-20180510), the Natural Science Basic Research Plan in Shaanxi Province of China (2012JZ2002), and Academic Funding of Northwestern Polytechnical University (NPU, 17GH0402, W212002). We gratefully acknowledge the Analytical and Testing Center (ATC) of Northwestern Polytechnical University for providing well-support as SEM, TEM and XRD throughout the research work.
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