Controlled Synthesis of Hollow α-Fe2O3 Microspheres Assembled With Ionic Liquid for Enhanced Visible-Light Photocatalytic Activity

Introduction

Nanostructured oxides with a variety of useful functionalities are widely applied in photocatalysis, energy storage, etc. (Guo et al., 2017; Li et al., 2017; Zhao et al., 2017, 2018). In particular, hematite (α-Fe₂O₃), an environmentally-friendly magnetic photocatalyst (E_g = 2.2 eV), has been identified as an important material because of its potential for a wide range of practical applications (Brezesinski et al., 2011; Yang et al., 2014; Zhou et al., 2014; Ma et al., 2018). Recently, self-assembled hematite with highly specific hollow nano/micro-structures and unique properties has emerged as being of great interest to material scientists. To date, various self-assembled hollow α-Fe₂O₃ nano/micro-structures have been prepared by different synthetic techniques, including the two-step reaction process (Yu J. et al., 2009), hydrothermal (Xu et al., 2012), and solvothermal approaches (Song et al., 2012; Zhu et al., 2012), thermal oxidation at high temperature (Xie et al., 2010), etc. For example, Yu and co-workers successfully fabricated cage-like Fe₂O₃ hollow spheres and carbonaceous polysaccharide spheres were used as porous crystalline shell templates, followed by calcination at 500°C for 4 h. Song and co-workers proposed a hydrolysis route to synthesis Fe₂O₃ hierarchical hollow spheres, via a sodium dodecyl benzenesulfonate (SDBS)-assisted hydrolysis process. However, these template methods often suffer from a problem: it is difficult to remove the template and surface-active agent completely. Furthermore, template removal always leads to additional problems in sample morphology. Therefore, it is still necessary to develop a simple route to synthesizing assembled hollow α-Fe₂O₃ structures.

Ionic liquids (ILs), or “designer liquids,” are known for their superior properties such as recyclability and stable chemical properties. They are used extensively in catalysis, separations, electrochemistry, and especially nanochemistry (Welton, 1999; Wasserscheid and Keim, 2000; Seddon, 2003). The most important advantage of ILs is that they can form extended hydrogen bond systems in the liquid, thereby enabling highly-structured nanostructures (Mele et al., 2003). This special performance can be used as the “entropic driver” for synthesis of self-assembled nanostructures. It has been proved that ILs can be used not only as solvents, but also as templates for preparing nanomaterials with improved properties (Endres et al., 2003; Cooper et al., 2004; Liu et al., 2006). For instance, a variety of nano/micro-structures have been synthesized via ILs, especially assembled hollow structures such as Bi₄O₅Br₂ and BiOBr spheres (Xia et al., 2011a; Mao et al., 2017) and α-Fe₂O₃ hollow polyhedral (Xu et al., 2013). These hollow structures may also exhibit unique properties for photocatalysis.

In this work, we report a simple and feasible ionic liquid-assisted solvothermal method to synthesize self-assembled α-Fe₂O₃ hollow microspheres. The ionic liquid [C₄Mim]BF₄ acts as a soft template can be easily removed by annealing in air. Compared with α-Fe₂O₃ particles, the as-synthesized α-Fe₂O₃ hollow spheres exhibit enhanced photocatalytic activity due to their porous self-assembly structure and higher surface area. As far as we known, α-Fe₂O₃ porous hollow microspheres with high surface area (above 200 m²/g) prepared via this ionic liquid-assisted solvothermal synthesis route is reported for the first time. Additionally, the possible formation mechanisms have also been proved. The as-prepared α-Fe₂O₃ samples exhibit ferromagnetic properties and can be recycled easily.

Experimental

Materials and Methods

Materials

Ferric chloride (FeCl₃·6H₂O, 99%) and ethylenediamine (EDA) were purchased from Tianjin Damao Chemical Co. The ionic liquid ([C₄mim]BF₄) was obtained from Beijing Solarbio Technology Co. Ethylene glycol (EG) was purchased from Tianjin Chemical Reagent Co. and distilled water was used throughout the experiment.

Synthesis

In a typical experimental process, FeCl₃·6H₂O (0.81 g) was dissolved into 60.0 mL of ethylene glycol (EG). Then, 0, 0.1, 0.2, 0.3, 0.4, and 0.5 mL of [C₄Mim]BF₄ were added to the above solution. After 2 h, 3 mL of ethylenediamine (EDA) was added into the mixture solution to form a well-distributed emulsion. After 1 h, the emulsion was transferred into a Teflon-sealed autoclave with a capacity of 100 mL, sealed and heated at 200°C for 20 h. After cooling to room temperature, the resultant precipitate was washed with deionized water and ethanol several times until the solution was neutral. After drying in a vacuum oven at 60°C for 6 h, the dried products were calcined in air at 250°C for 6 h. The reaction time was adjusted to study the mechanism.

Materials Characterization

X-ray diffraction (XRD) patterns were obtained with a Rigaku RINT 2500 diffractometer using a Cu-Kα radiation source (λ = 1.5406 Å). The morphology and microstructure of the as-prepared samples were examined with scanning electron microscopy (SEM, JEOL 6500F). TEM and HRTEM images were recorded with a Tecnai accelerating voltage of 120 and 200 kV, respectively. The nitrogen adsorption/desorption isotherms at 77.35 K were collected on an AUTOSORB iQ-MP instrument after heating the samples at 150°C for 2 h. The surface areas were estimated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05–0.3 Pore size distributions were analyzed using nitrogen adsorption data in a Barrett-Joyner-Halenda (BJH) model. Magnetic properties of the as-synthesized samples were measured with a physical property measurement system (PPMS-9T, ever cool II, USA).

Photocatalytic Reaction

20.0 mg of α-Fe₂O₃ photocatalyst catalyst and 50.0 mL of RhB dye aqueous solution (10.0 mg/L) were mixed in a Py”prex reactor and stirred in the dark for 30 min to reach a complete absorption/desorption equilibrium. Afterwards, the suspension was exposed to a 300 W Xenon lamp and stirred simultaneously. Subsequently, 3.0 mL of suspension was centrifuged at 5,000 rpm for 4 min to remove the photocatalyst. Finally, the concentration of RHB was monitored with a TU-1901 UV-vis spectrophotometer by monitoring the absorbance at 553 nm.

Results and Discussion

Structure and Morphology Characterization

The purity and crystallinity of the as-prepared powder samples are measured by XRD (Figure 1). The XRD shows that all diffraction peaks can be indexed in α-Fe₂O₃ (JCPDS 33–0664) with structural parameters of a = b = 5.038 Å, c = 13.749 Å, α = β = 90°, and γ = 120°. No other peaks are observed, demonstrating that the α-Fe₂O₃ products have high-purity and a single phase. Therefore, we can infer that anions and cations of the [C₄Mim]BF₄ are not doped into the α-Fe₂O₃ lattice. As the [C₄Mim]BF₄ addition increases, the peak intensities of α-Fe₂O₃ also gradually increase, indicating that sample (D) has the highest degree of crystallization. This result proves that the addition of ionic liquid can enhance the crystallization of as-synthesized materials. A similar phenomenon is also observed in the previous report (Xu et al., 2013).

FIGURE 1

Figure 1. XRD patterns of the α-Fe₂O₃ with different amounts of [C₄Mim]BF₄: (A) 0 mL, (B) 0.1 mL, (C) 0.2 mL, and (D) 0.3 mL.

According to the full width at half-maximum (fwhm) of the diffraction peaks, the average crystallite size of the α-Fe₂O₃ nanoparticles and microspheres ([C₄Mim]BF₄ = 0.1, 0.2, 0.3 mL) can be estimated from the Scherrer equation to be about 40.2, 84.6, 101.2, and 198.5 nm, respectively.

$\begin{array}{l}{D}_{hkl}=K\lambda /({\beta }_{hkl}cos{\theta }_{hkl})\end{array}$

where D_hkl is the particle size perpendicular to the normal line of (hkl) plane, K is a constant (it is 0.9), β_hkl is the full width at half-maximum of the (hkl) diffraction peak, θ_hkl is the Bragg angle of (hkl) peak, and λ is the wavelength of X-ray.

The morphology and structure of the as-prepared samples are studied by SEM and TEM Figure 2 shows the typical SEM micrographs of the α-Fe₂O₃ samples prepared with varying [C₄Mim]BF₄ additions at 200°C for 20 h. In the absence of [C₄Mim]BF₄ (Figures 2A–C), only α-Fe₂O₃ pseudo cubic particles are formed. The morphology and size of α-Fe₂O₃ particles are non-uniform and the alignment is disordered. As the amount of [C₄Mim]BF₄ increased from 0.1 to 0.3 mL, the as-prepared α-Fe₂O₃ changed from irregular particles to the uniform microspheres.

FIGURE 2

Figure 2. SEM images of α-Fe₂O₃ samples prepared with varying addition of [C₄Mim]BF₄ at 200°C for 20 h: (A–C) 0 mL, (D–F) 0.1 mL, (G–I) 0.2 mL, (J–L) 0.3 mL.

Figures 2D–F show the SEM image of [C₄Mim]BF₄ = 0.1 mL, which is composed of nanoparticles. When the addition of [C₄Mim]BF₄ is up to 0.2 mL, perfectly spherical α-Fe₂O₃ is formed. As shown in Figure 2G, some broken α-Fe₂O₃ microspheres reveal that the as-obtained α-Fe₂O₃ microspheres are of a hollow structure. Therefore, we can infer that [C₄Mim]BF₄ is used as a template in the process of α-Fe₂O₃ synthesis. The three-tiered organization of crystallites for these hollow dandelions is shown in the magnified image (Figure 2H). Numerous nanosheets are present on the surface of the dandelion spheres, with a puffy appearance and the average diameter being about 6.5 μm. From Figure 2I, we can see that individual nanosheets have an average size of about 100 nm, which is in agreement with the result calculated from Scherrer's formula. When the [C₄Mim]BF₄ amount increases to 0.3 mL (Figures 2J,H,L), the similar nanosheets stack up in three-dimensional ordered microspheres (5.0–7.0 μm) with a larger thickness (about 200 nm). Figure 3A shows the TEM image of an α-Fe₂O₃ microsphere. However, the structure in [C₄Mim]BF₄ = 0.3 mL demonstrates no internal void space. A high-resolution TEM (HRTEM) image (Figure 3B) exhibited lattice spacing of 0.252 nm, corresponding to the (110) lattice plane. Additionally, some examples of synthesizing nanostructured α-Fe₂O₃ using different kinds of ILs are given in Table 1.

FIGURE 3

Figure 3. (A) TEM image of single sphere obtained at [C₄Mim]BF₄ = 0.3 mL, (B) HRTEM image. The inset shows the corresponding SAED pattern (up-right).

TABLE 1

Table 1. Comparison of different α-Fe₂O₃ samples synthase by this work and others reported in previous references.

Additionally, the self-assembly micron spheres are made up of a bunch of nanosheets, indicating that the spheres are porous in structure. It is striking that when the addition of [C₄Mim]BF₄ reaches to 0.4 mL, precipitation is very sparse. Until 0.5 mL, no precipitation synthesized after reaction. This phenomenon can be explained as follows: EG and ionic liquids are not mutually soluble, so the two-phase interface is formed. With an increase in the addition of ionic liquids, the disorder of atoms at the two-phase interface increases, resulting in a growing of interfacial energy, which lifts the nucleation resistance. Owing to large resistance, crystal cannot nucleate.

Nitrogen Sorption

Figure 4 shows the nitrogen adsorption-desorption isotherms and pore size distributions of the as-prepared α-Fe₂O₃. The isotherms of α-Fe₂O₃ samples shown in Figure 4A are of type IV with a hysteresis loop from 0.5 to 1.0 (P/P₀). The BET-specific surface areas of α-Fe₂O₃ hollow microspheres are calculated to be 183, 208, and 221 m²g⁻¹, all larger than that of α-Fe₂O₃ synthesized without [C₄Mim]BF₄ (only 44 m²g⁻¹). Correspondingly, the BJH pore size of the as-synthesized α-Fe₂O₃ ranges from 5.0 to 10.0 nm (Figure 4B). The smaller pore structures may arise from the crystal growth, and the larger pores ascribe to the stacking of the α-Fe₂O₃ nano-structures and the hollow structure. Additionally, the peaks of the pore sizes of the α-Fe₂O₃ move to the right after adding [C₄Mim]BF₄, showing that average pore size increases. From this result, we can infer that the crystallinity of the as-prepared α-Fe₂O₃ improved (Zhu et al., 2012), which is in agreement with the XRD analysis. Table 2 summarizes the mean pore size and S_BET of the as-obtained α-Fe₂O₃. It shows that with increasing additions of [C₄Mim]BF₄, S_BET increases dramatically. This result may be attributed to two factors: first, compared with irregular α-Fe₂O₃ nanoparticles or nanosheets, hollow assembly structures have more opportunities to participate in nitrogen adsorption. Without the [C₄Mim]BF₄, the specific surface area of α-Fe₂O₃ is only 44.0 m²g⁻¹ (Xia et al., 2011b). Second, the presence of a large number of mesoporous structures (6–7 nm) leads to a large specific surface area.

FIGURE 4

Figure 4. (A) Nitrogen adsorption-desorption isothermal, (B) the pore size distribution curve for the α-Fe₂O₃.

TABLE 2

Table 2. Brumaire–Emmett–Teller (BET) surface area and mean pore diameters of the as-prepared samples.

Possible Formation Mechanism of α-Fe₂O₃ Hollow Spheres

To better understand the formation process of the α-Fe₂O₃ porous hollow spheres, time-dependent experiments were carried out. As shown in Figures 5A–D, representative SEM images at different time intervals are displayed. First, a hollow core-shell spherical structure was generated at 10 h. The mean diameter of the core is about 2.0 μm, which was further confirmed by the TEM image shown in Figure S1. Second, some microparticles were deposited on its surface at 14 h. Then, as the reaction time reached 16 h, some nanosheets began to cover the surface of the nuclei, indicating that deposition was still in progress. When the duration reached 20 h, a hollow sphere covered with a great number of nanosheets was obtained.

FIGURE 5

Figure 5. SEM images of α-Fe₂O₃ hollow microsphere structures synthesized in the presence of ionic liquid [C₄Mim]BF₄ = 0.2 mL at 200°C. (A)10 h, (B) 14 h, (C) 16 h, (D) 20 h. (E) XRD patterns of the α-Fe₂O₃ at different time.

Based on the above experimental results, the nucleation and growth mechanism for the microsphere in the two-phase system was proposed, as shown in Figure 6. First, Fe³⁺ reacted with EDA in EG-[C₄Mim]BF₄ solution to form a relatively stable complex ion of [Fe (EDA)³]³⁺ (Zhang et al., 2006). High temperature caused complex ions [Fe (EDA)³]³⁺ to decompose into FeOOH. Thus, FeOOH was first formed. Then, the FeOOH nanoparticles dissolved and reacted with the obtained Fe²⁺ in a [C₄Mim]BF₄ assisted system to form α-Fe₂O₃ nanoparticles. From Figure 5E, the peaks of FeOOH and α-Fe₂O₃ can be observed at 10 h. The formed α-Fe₂O₃ nanoparticles were unstable and had a tendency to form larger congeries, which may have been driven by the minimization of interfacial energy. That is why after 14 h, the sample only showed the crystal image of α-Fe₂O₃. However, EG solution has greater viscosity and fewer surface hydroxyls than the aqueous solution, resulting in kinetically slower nucleation and aggregation of nanocrystals, which lead to the formation of perfectly oriented assemblies by the adequate rotation to find the low-energy configuration interface (Zhu et al., 2013). A longer reaction time leads to directional alignment to generate spherical nuclei at 10 h. During the subsequent process, nanoparticles gradually assemble into nanosheets and stack on the surface of the template. The reason for the formation of nanosheets is that small particles gradually adhere to large particles to form thin sheets, which is commonly referred as Ostwald ripening. Finally, a specified morphology forms. Such a process is also observed in several other reports (Lou et al., 2006; Zhu et al., 2008; Yu X. et al., 2009). In this study, the adjacent primary ferric alkoxide nanocrystals have high activity due to their high surface energy. They further grow into nanosheets by directional aggregation, which greatly reduces the interface energy of small primary nanocrystals. Then, by directional attachment and self-assembly, the nanosheets gradually evolve into 3D flower-like superstructures. At the same time, it is combined with the mature process of Ostwald to form a favorable hollow structure.

FIGURE 6

Figure 6. Schematic illustration of the formation of α-Fe₂O₃ hollow spheres.

In this formation process, the reaction time is one of the most important controlling factors. Apart from the reaction time, EDA is also a critical factor. EDA is used as a precipitant during this process. When a part of an EG molecule loses protons and coordinates with FeCl₃ to form ferric alkoxide, H⁺ is produced in the reaction of EG with metal chloride. If H⁺ cannot be removed, the accumulation of H⁺ will inhibit the formation of further iron oxides. In this paper, EDA takes the lead to react with Fe³⁺ to form a stable complex and inhibit the formation of H⁺ in the reaction system.

Enhancement of Photocatalytic Activity

The photocatalytic degradation of RhB is monitored by measuring the absorption behavior of the solution at 553 nm. The evolutions of the spectrums are shown in Figure 7A for α-Fe₂O₃ synthesized without [C₄Mim]BF₄ and in Figure 7B for that synthesized with 0.3 mL [C₄Mim]BF₄. When the addition amount reaches 0.3 mL, 90% of RhB is decomposed after 90 min irradiation, while it is no more than 50% for the sample synthesized without [C₄Mim]BF₄. Thus, the use of [C₄Mim]BF₄ in the synthesis process effectively improves the photocatalytic performance. To clarify the effect of [C₄mim]BF₄, we figure out the rate constants of the degradation processes for the α-Fe₂O₃ synthesized with various concentrations of [C₄Mim]BF₄. To evaluate the reactivity, the apparent reaction rate constant (k) is calculated. Figure 7C shows ln (C/C₀)—t plots for the α-Fe₂O₃ samples synthesized with different [C₄Mim]BF₄ amounts as well as the blank test. The rate constant k is 0.00756, 0.00927, 0.0183, 0.0212 min⁻¹ for [C₄Mim]BF₄ = 0.0, 0.1, 0.2, and 0.3 mL, respectively, shown in Figure 7D. Thus, the photocatalytic performance is found to improve with increasing [C₄Mim]BF₄ addition amounts. As shown in Figure 7E, we carried out a coarse comparison of degradation efficiencies between this study and other studies based on the photodegradation of RhB by α-Fe₂O₃.

FIGURE 7

Figure 7. UV–vis absorption spectra during photocatalytic reaction of RhB over α-Fe₂O₃ with different [C₄Mim]BF₄ additions at different time intervals: (A) 0 mL, (B) 0.3 mL, (C) plots of ln(C/C₀) against time t of various samples: without catalyst, the α-Fe₂O₃ hollow spheres prepared with addition = 0, 0.1, 0.2, and 0.3 mL, respectively. (D) Reaction kinetics of RhB degradation under visible light irradiation by α-Fe₂O_3. (E) Comparison of degradation efficiencies of different α-Fe₂O₃ samples during the RhB photodegradation driven by visible light. Red balls refer to the α-Fe₂O₃ samples prepared in this work and blue balls refer to the α-Fe₂O₃ samples reported in previous references. ([1] Xu et al., 2012; [2] (Pawar and Choi, 2015); [3] (Cai et al., 2014); [4] (Xu et al., 2012); [5] (Liang et al., 2014); [6] (Zhou et al., 2011); [7] (Wu et al., 2013)).

In addition to the high catalytic efficiency, the catalyst possessed robust stability. In our case, α-Fe₂O₃ samples with the amount of 0.3 mL [C₄Mim]BF₄ also have stable photoactivity. After 3 months, the degradation rate of RHB has little change in k value (Figure 7D). An SEM image of the sample after placement for 3 months is shown in Figure S2. It shows that the morphology and structure of the as-synthesized α-Fe₂O₃ have few changes, which is very important for maintaining the photocatalytic activity of samples.

The photocatalytic activities of the α-Fe₂O₃ are closely related to its energy band. Figure 8A shows UV-vis diffuse reflectance spectra of α-Fe₂O₃ with additions of [C₄Mim]BF₄ = 0 and 0.3 mL in the wavelengths of 200–800 nm. Although the α-Fe₂O₃ particles and spheres show similar optical properties, the microsphere with a [C₄Mim]BF₄ addition of 0.3 mL shows stronger absorption, from 600 to 800 nm, resulting in a larger surface area that can absorb more light (Wang et al., 2011). Therefore, the as-prepared α-Fe₂O₃ microspheres exhibit better photocatalytic performance. Figure 8B is the plot of (αhv)² vs. the energy of the absorbed light for α-Fe₂O₃. From Figure 8B, the band gap is determined to be 2.05 eV for the α-Fe₂O₃, indicating that the oxygen vacancies do not change the band gaps of α-Fe₂O₃ microstructures significantly.

FIGURE 8

Figure 8. (A) UV-vis diffuse reflectance spectra and (B) plots of (αhv)² vs. photon energy hv of the samples with addition = 0, 0.3 mL.

Furthermore, in the process of photocatalytic reaction, ·OH is considered to be another main reaction species leading to the oxidative decomposition of organic pollutants. However, it was revealed that ·OH could not be generated during irradiation for Fe₂O₃ (Xiang et al., 2011). Because the photo-generated holes and electrons on Fe₂O₃ could not react with OH⁻/H₂O and O₂ to form ·OH and O²⁻, respectively, no ·OH can be generated in Fe₂O₃ (Xiang et al., 2011). This indicates that reactive species other than ·OH are present. However, several research results suggest that photo-generated holes and electrons can directly give rise to photocatalytic oxidation. As shown in this study, the photocatalytic performances of these α-Fe₂O₃ samples strongly depend on the [C₄Mim]BF₄ concentrations in the synthesis process. Therefore, it is likely that the high photocatalytic activity results from the novel structure, large surface area and strong absorption of visible light. Generally, large specific surface area provides more unsaturated coordination, which helps to improve the efficiency of electron hole separation (Tang et al., 2004). Furthermore, the increase of unsaturated coordination sites may improve the surface electron transfer rate. The increase of the surface electron transfer rate leads to the reduction of the probability of recombination and, therefore, the photo-generated charge carriers could more easily transfer to the surface to degrade the adsorbed RhB.

Magnetic Properties

It is well-known that α-Fe₂O₃ exhibits ferromagnetism (Sun et al., 2010). Figure 9 and Figure S3 shows the room temperature magnetic hysteresis loops and the magnetic field sweeping from −10.0 to 10.0 k Oe. Table 3 collects the values of remnant magnetization (Mr) and coercivity (Hc) of the as-synthesized α-Fe₂O₃ samples. It can be determined from the shape of the hysteresis loop that the synthesized α-Fe₂O₃ sample shows ferromagnetism. Additionally, the hysteresis loop did not reach saturation up to the maximum applied magnetic field, because of the presence of large-shape anisotropy (Bharathi et al., 2010). It is also shown in Table 3 that the residual magnetization and coercivity of assembled microspheres of α-Fe₂O₃ are large than α-Fe₂O₃ nanoparticles. It is widely understood that the morphology and structure of the as-synthesized samples will greatly affect the magnetization of ferromagnetic materials (Sorescu et al., 1999). Therefore, the assembly of the nano-sized and oriented particles and sheets into non-random structure results in the change of the single domain to the multidomain, leading to higher remnant magnetization and coercivity (Park et al., 2000). In summary, the difference can be attributed to the high crystallization, single domain size, surface (Tong et al., 2015), structure, and shape.

FIGURE 9

Figure 9. (A) Magnetization loops for α-Fe₂O₃ particles synthesized with different additions of [C₄Mim]BF₄. (B) A magnified view of curve.

TABLE 3

Table 3. The values of remnant magnetization (M_r) and coercivity (H_c) of the as-synthesized samples.

Conclusion

In summary, porous self-assembled α-Fe₂O₃ hollow microspheres were successfully prepared via a facile ionic liquid assistant synthesis approach. A nucleation–aggregation evacuation mechanism was the main formation of the porous hollow structures. EDA and [C₄Mim]BF₄ play significant roles on the formation of porous α-Fe₂O₃ hollow spheres. The increase in [C₄Mim]BF₄ promotes photocatalytic activities and α-Fe₂O₃ microspheres show higher photocatalytic activities. We believe that high specific surface area and porous hollow structure play an important role in improving the photocatalytic performance of as-prepared α-Fe₂O₃. Additionally, when the addition amount reaches 0.3 mL, the synthesized samples show ferromagnetism and can be easily recycled. The α-Fe₂O₃ hollow porous spheres prepared by our method have high photocatalytic activity, ideal ferromagnetic properties, and high specific surface area. They are expected to exhibit use as applications in sensors, catalysis, separation technology, environmental engineering, controlled drug delivery, and more.

Data Availability

All datasets generated for this study are included in the manuscript and/or the Supplementary Files.

Author Contributions

HY and YulZ partly designed the experiments and wrote the manuscript. QH, JZ, YuaZ, XX, and YL assisted in the analysis and interpretation of the data. JT and FW proposed the project and revised the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors acknowledge financial support from the Taishan Outstanding Overseas Scholar Program of Shandong Province, China. The work was financially supported by the Natural Science Foundation of Shandong Province, China (Grant number: ZR2018JL021 and ZR2014EMQ011), the National Natural Science Foundation of China (Grant number: 51402160 and 51673103) and the Qingdao Postdoctoral Application Research Project. The work was also supported by the Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, and the National Demonstration Center for Experimental Applied Physics Education (Qingdao University).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2019.00058/full#supplementary-material

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