High yield synthesis and optical properties of MgF₂ nanowires with high aspect ratios

Abstract

Ultrapure highly crystalline magnesium fluoride (MgF₂) nanowires with high aspect ratios were successfully synthesized by utilizing ammonium boron trifluoride (NH₃BF₃) as a precursor material for the first time. Our results reveal that when the vapors of NH₃BF₃ react with magnesium chloride (MgCl₂), a perfect crystalline MgF₂ phase can be obtained on a large scale. The MgCl₂ utilized in this study plays a crucial role in nanowire growth and morphology control, resulting in ultrapure and highly crystalline MgF₂ nanowires. The prepared nanowires have an average diameter of 60 nm and lengths up to tens of micrometers. Property studies indicate that the MgF₂ nanowires are completely transparent in nature as confirmed by UV-Vis spectroscopy. We believe that these nanowires provide promising building blocks for the future optical nanodevices.

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1. Introduction

One-dimensional (1D) luminescent nanowires have received intense research attention as building blocks in photonics, nanoscale electronics, display and advanced bioanalysis.^1–6 The study of nanostructured materials revealed that mechanical, electrical, chemical and optical properties of these materials are entirely different from their bulk counterpart due to holes and electron confinement, surface effects and geometrical confinement of phonons.⁷ Therefore, the required properties for potential applications can be obtained by tailoring their size from bulk to nano.^8,9 For example, the improvements to optical properties, such as refractive index, can result in reduced optical reflection on glasses and windows.¹⁰ Among fluorides and especially the alkaline earth fluorides,^11–13 nanoscale magnesium fluoride (MgF₂) is one of the least examined materials.^14,15 Metal fluoride nanomaterials are of great interest because of their potential applications in optoelectronics, optics, biological labelling, catalysis fields, and so on.^16–19

MgF₂ is one of the most interesting materials because of its low refractive index, low chemical reactivity even at elevated temperatures, high thermal stability, high corrosion resistance and significant hardness.^20–23 MgF₂ is a wide band gap (10.8 eV) material having the rutile structure where magnesium (Mg²⁺) ions are surrounded by six fluoride (F⁻) ions and each F⁻ ion is surrounded by three Mg²⁺ ions.²⁴ It has been widely used as a protective coating on glass optics and as an antireflective because of its low refractive index. In addition, due to wide band gap, it is widely used as promising UV transparent material.^25–27 Since it hosts Lewis acidic centers on its surface and possesses relatively chemically inert surface, MgF₂ is extensively used as a catalyst support.^19,24

The traditional approaches utilized to synthesize MgF₂ are thermal decomposition and fluorination of suitable starting material with reactive fluorinating agents at elevated temperatures. So far, many efforts have been made with respect to the preparation of MgF₂ and many methods have been employed for the synthesis of nanosized MgF₂ for its potential applications. Murthy et al.²⁸ utilized sol–gel fluorination process to produce an amorphous MgF₂ for catalytic applications. Later on, Kruger et al.²⁹ utilized non-aqueous sol–gel method as simple and cheap procedure instead of common PVD technique which needs high temperature and corrosive gases such as F₂ or HF to develop a low refractive index MgF₂ film. Other researchers also tried to synthesize nanoscale crystalline MgF₂ powders by precipitation route for optical applications. In this respect, Sevonkaev and Matijevi³⁰ produced MgF₂ nanostructures with different morphologies (spherical, cubic, prismatic and platelike) via the precipitation rate by aqueous solutions of MgCl₂ and NaF at different pH values. The synthesized MgF₂ had a particle size about 50–100 nm. Nandiyanto et al.³¹ also followed the precipitation route to synthesize MgF₂ and changed the size and shape of the particles by varying the molecular ratio of NH₄F to MgCl₂ to obtain amorphous nanosized MgF₂ with cubic morphology at a size of 6 nm.

Although various synthetic routes have been employed, the studies related to MgF₂ 1-D nanostructures have seldom been carried out. Template-directed approaches are effective in producing nanotubes and nanowires with controllable dimension. Minhua Cao et al.³² reported a solution method to synthesize MgF₂ nanorods by utilizing cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol as soft template, however, the surfactant of CTAB was very hard to remove after the synthesis procedure. In addition, the utilization of very expensive and complex materials (hydrocarbons) could increase the cost of MgF₂ nanorods products. It is highly important, therefore, to study the MgF₂ 1D nanostructures, not only with respect to the fundamental research on low dimensional fluoride nanostructures, but also in regard of their potential applications as interconnectors and active components of future optoelectronic nanodevices.³³

Chemical vapor deposition (CVD) is one of the most promising methods for the production of 1D nanostructures because it offers high flexibility in obtaining the nanowires. In this paper, we report on the synthesis of highly crystalline MgF₂ nanowires based on the CVD method for the first time. A simple precursor NH₃BF₃ and anhydrous MgCl₂ have been proposed for the production of MgF₂ nanowires with extreme purity and high yield. Ammonium boron trifluoride, a cheap and a simple inorganic material was successfully utilized in this study to synthesize highly pure MgF₂ nanowires without use of any catalyst, when the vapors of HF reacted with the vapors of MgCl₂, a high yield of pure product is obtained. The highly crystalline MgF₂ nanowires have diameters in the range of 40–120 nm, lengths of tens of micrometers, displaying very high aspect ratios. Optical property studies indicate that the synthesized MgF₂ nanowires are absolutely transparent. The as-prepared MgF₂ nanowires can be considered as promising nanomaterials for applications in optics and optoelectronic nanodevices.

2. Experimental

2.1. Synthesis of MgF₂ nanowires

The MgF₂ nanowires were synthesized by CVD reaction of NH₃BF₃ in the presence of anhydrous MgCl₂. In a typical synthesis (Fig. 1(a)), 1.5 g of MgCl₂ and 5 g of NH₃BF₃ were utilized in producing MgF₂ nanowires. Solid NH₃BF₃ precursor was placed at one end of the quartz tube located near to the intake of Ar gas for easy transportation of NH₃BF₃ generated vapors to the reaction chamber. An aluminum oxide boat loaded with lump of anhydrous MgCl₂, having a cover of Cu substrate, was placed in a small ceramic alumina tube, and closed from one end to trap the vapors. The small tube was then introduced into the center of the longer quartz tube. The distance between NH₃BF₃ and MgCl₂ was ∼15 cm and firstly both of them were far away from the heating zone (T₂) when the system was in heating stage. The Ar flow rate was kept constant at 80 sccm as a protective and transport gas in the whole reaction process. The furnace was heated to 850 °C in 80 minutes. When the system was heated to target temperature, the reactants along with the quartz tube were immediately moved to the heating zone of the furnace. The vapors containing the sublimated NH₃BF₃ were transported by a slow Ar stream into the reaction chamber where MgCl₂ was located. The covering of the crucible is apparently essential under these conditions for reaction, as it prevents the MgCl₂ vapors from escaping out before the reaction with NH₃BF₃ starts. After the reaction for 2 h, the furnace was naturally cooled to room temperature. All the toxic exhausts during the running of the experiment were absorbed by NaOH solution cooling trap system. Finally the wool like product was collected from ceramic alumina boat and Cu substrate.

Fig. 1 (a) A schematic diagram of growth stages, reaction conditions and product formation during the CVD reaction. (b) Photo image of the final white wool like product after the CVD reaction.

2.2. Characterization methods

The morphology and structure of the samples were characterized by X-ray diffractometer (XRD, BRUKER D8 FOCUS with Cu Kα radiation), scanning electron microscope (SEM. HITACHI, SU-8010) and transmission electron microscope (TEM, Philips Tecnai F20, operated at 200 kV) quipped with energy dispersive X-ray spectroscopy (EDS) for compositional analysis. The UV measurements were carried out at room temperature by UV-Vis spectrophotometer (HITACHI, U-3900H). For photo luminescence measurements, excitation and emission spectra were measured at room temperature by using a Hitachi Fluorescence Spectrometer (F-7000).

3. Results and discussions

The white wool like product can be easily collected after the CVD reaction, as shown in Fig. 1(b). The XRD pattern of the product is shown in Fig. 2(a). All peaks positions are appropriately indexed according to tetragonal MgF₂ with the lattice parameters of a = 0.462 nm, b = 0.462 nm and c = 0.304 nm, which are consistent with the calculated values (JCPDS 70-2268). No impurity phases were detected, indicating the highly purity of the product. However, the peak intensities may differ between experimental and calculated structures. The XRD result shows that the experimentally observed (110) peak is clearly enhanced than calculated patterns. This phenomenon could be attributed to the high aspect ratios and preferential growth of the nanostructures.^34,35 A typical SEM image of the product is shown in Fig. 2(b). It consists of uniform 1D nanostructures with high purity and high yield. The diameters of the nanowires are in the range of 40–120 nm, and the entire length reaches up to tens of micrometers, indicating very high aspect ratios.

Fig. 2 (a) XRD patterns of the product. (b) A typical SEM image of the MgF₂ nanowires.

It is noticed that with slightly increasing the deposition temperature, or with the changing of the ratios of reactants, the MgF₂ crystals dimensions increased and morphologies of the grown MgF₂ crystals changed from thin nanowires to very thick fibers as shown in Fig. S1 (ESI†). Our experiments indicate that at higher deposition temperature, the growth rate of MgF₂ nanowires will become larger and the width of crystals will increase.

The microstructures and product morphology were thoroughly studied using high resolution TEM (HRTEM). Fig. 3(a) is the typical low-magnification TEM image of the product, showing 1D wire like structures. The average diameter of the nanowires is 60 nm. Fig. 3(b) shows an individual MgF₂ nanowire and the corresponding selective area electron diffraction analysis (SAED) pattern (inset), which can be indexed as tetragonal MgF₂ single crystal recorded from the [11̄1̄] zone axis with the crystalline parameters the same as the calculated results from XRD measurement. By combining the SAED and TEM results, the growth direction of the MgF₂ nanowire was calculated to be perpendicular to (11̄2) plane, which is in well agreement with the XRD results. From high resolution TEM image depicted in Fig. 3(d), the evidently resolved interplaner distance of ∼0.32 nm, which corresponds well to the d₁₁₀ spacing of the bulk tetragonal MgF₂. All observations show that the nanowires are surely composed of MgF₂. An EDS spectrum (Fig. 3(c)) collected from single nanowire shows that the nanowire is composed of Mg and F, with a molar ratio of ∼1 : 2. Fig. 3(e) shows the corresponding atomic model of the MgF₂ nanowire with Fig. 3(b), showing the arrangement of F and Mg atoms, the growth direction and the side facet of the nanowire.

Fig. 3 (a) Low-magnification TEM image of highly crystalline MgF₂ nanowires. (b) Typical TEM image of an individual MgF₂ nanowire and the corresponding SAED pattern clearly exhibiting the growth direction. (c) EDX spectrum showing the composition of elements in MgF₂ nanowires. (d) High-resolution TEM (HRTEM) image of single nanowire clearly displaying the layers of MgF₂ with an interplaner distance of 0.32 nm. (e) Model of MgF₂ nanowire showing the arrangement of atoms and growth direction.

For the growth of the MgF₂ nanowires, unlike the 1D nanostructures produced with the vapor–liquid–solid (VLS) growth^36–38 which relies exclusively on the formation of solid–liquid interface, the vapor–solid (VS) growth^39–41 mechanism is proposed to be responsible for the production of the present MgF₂ nanowires. It should be noted that there had been temperature gradient in the quartz tube. During the reaction process, the low temperature zone T₁ was adjusted to 250–300 °C and the temperature of zone T₂ was 850 °C, as shown in Fig. 1(a).

Solid NH₃BF₃ source which was placed at zone T₁ decomposes as follows.^42,43

The vapors containing HF and NH₂BF₂ were transported by the Ar gas to the high temperature zone T₂. At higher temperature zone the MgCl₂ vapors were already formed. Then the HF vapors reacted with MgCl₂ vapors and forms MgF₂ by following reaction.

2HF_(v) + MgCl_2(v) → MgF_2(s) + 2HCl_(g) (3)

Furthermore, the vapors are solidified to form MgF₂ nanowires and HCl is evaporated. Both the growth kinetics and thermodynamics determined the chemical vapor deposition process of MgF₂ nanowires in our method. The speed for F sources to release HF and the vapor pressure of HF should be essential to the anisotropic growth of nanowires. We believe that the decomposition of NH₃BF₃ at the reaction temperature leads to a proper atmosphere which is favourable for the growth of MgF₂ nanowires, whereas, for other F sources, such as NH₄F, the speed of HF release is either too violent or too mild. In fact, Nandiyanto et al.³¹ used NH₄F and they obtained MgF₂ powders instead of MgF₂ nanowires. Similarly, by using other Mg sources, MgF₂ powders would be obtained. Skapin et al.⁴⁴ used Mg(NO₃)₂ as a Mg source and they obtained MgF₂ powders. For NH₃BF₃, the HF release pressure might be matched with vapor pressure of MgCl₂ vapors. The nucleation of MgF₂ then began with the arrival of HF vapors. Another important factor that depends on the growth direction of nanowires is the anisotropic surface free energy.⁴⁵ The growth continued along one favourable direction in which surface free energy was minimum, as a result MgF₂ nanowires were obtained. The growth is achieved with only two states (vapor and solid) and involves the stoichiometry controlled condensation and crystallization of high temperature vapors with the process thus held responsible for one dimensional crystal growth. In addition, no liquid bubbles, which are inherent with the VLS mechanism,³⁹ were found at the end of the nanowires from morphologies of the nanowires. Therefore, the MgF₂ nanowires obtained in the present process were grown by VS mechanism. According to VS mechanism, the morphologies of deposited MgF₂ crystals are affected primarily by supersaturating degree of gaseous reactants, which was depended on the deposition temperature and ratios of reactants in the furnace. Finally, at the optimized ratio of reactants, highly crystalline MgF₂ nanowires with an average diameter of 60 nm are accumulated as shown in Fig. 1(b). The growth continued for 2 h at 850 °C which were the optimized temperature and reaction time at which highly pure MgF₂ nanowires were obtained. At lower temperature, in different heating zone other morphologies are obtained. Different morphologies are shown in Fig. S2 (ESI†). One can see that the present CVD method can be one of the most effective techniques for the production of wool like highly crystalline MgF₂ nanowires with high purity and high yield.

Then we studied the photoluminescent properties of the MgF₂ nanowires. Fig. 4(a) shows the typical emission spectrum excited by 215 nm light at room temperature.

Fig. 4 (a) The optical emission spectrum of the MgF₂ nanowires. (b) The optical excitation spectrum of the MgF₂ nanowires.

The spectrum reveals an intense broad UV emission band centered at ∼324 nm. The shape of the spectrum depends on the origin of the sample and on excitation type and energy. This can be explained by the presence of various defects and/or impurities in the sample. Facey and Sibley⁴⁶ reported a luminescence band at ∼410 nm of MgF₂ sample, which was attributed to F center transitions in MgF₂ crystals and absorption band appeared at ∼260 nm which was accepted due to an F center. A survey of the room temperature luminescence was also done by Blunt and Cohen.⁴⁷ They reported that the emission band appeared at ∼420 nm and 560 nm when the MgF₂ crystals are excited by all the broad lines available from high pressure mercury arc below 405 nm line and the absorption band at ∼260 nm was identified due to F centers. In our case, the intense UV emission band at 324 nm is proposed to be attributed to defect related centers. TEM characterization indicates that the MgF₂ nanowires reported in our manuscript are single crystal without any plane defects such as stacking faults and twins. The defects which contribute on the optical properties of MgF₂ nanowires could be some point defects such as vacancy and interstitial. In particular, the negatively charged point defects, either the magnesium–ion vacancy V_mg, or the fluorine–ion interstitial, F_i, give rise to the F centers which likely result in the optical absorption of MgF₂ nanowires at about 267 nm.

An absorption spectrum of the MgF₂ nanowires is shown in Fig. 5.

Fig. 5 The optical absorption spectrum of MgF₂ nanowires.

It clearly indicates that the MgF₂ sample is transparent in the visible range. An absorption band at 267 nm is observed in the spectrum and can be attributed to the F center as reported in the literatures.^46–49 This suggests that MgF₂ nanowires have high crystallinity and in turn it exhibits promising UV transparent properties which are important for their potential applications as a UV range optoelectronic nanodevices and more specifically in optical windows.

4. Conclusion

Bulk amount of highly crystalline MgF₂ nanowires were successfully synthesized by a CVD technique. The utilization of simple NH₃BF₃ compound was successful for the production of MgF₂ nanowires, for the first time. Our findings apparently indicate that when NH₃BF₃ is reacted with MgCl₂ at high temperature in the traditional tube furnace, MgF₂ nanowires can be obtained at large scale. The prepared MgF₂ nanowires have an average diameter of 60 nm and length up to tens of micrometers. More interestingly, optical properties showed that MgF₂ nanowires are fully transparent in nature. The synthesis of MgF₂ nanowires with nanosized diameters and transparent nature indicate a high potential for their utilization in high performance composites and optoelectronic nanodevices. We believe that these nanowires will become the building blocks for optical nanodevices in near future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51332005, 51372066, 51202055, 51402086, 51572068), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT: IRT13060), the Hundred Talents Program of Hebei Province (E2014100011), the Tianjin Research Program of Application Foundation and Advanced Technology (14JCYBJC42200), and the Innovation Fund for Excellent Graduate Student of Hebei Province (No. 220056).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27173h

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