Manganese molybdate nanoflakes on silicon microchannel plates as novel nano energetic material

Nano energetic materials have attracted great attention recently owing to their potential applications for both civilian and military purposes. By introducing silicon microchannel plates (Si-MCPs) three-dimensional (3D)-ordered structures, monocrystalline MnMoO4 with a size of tens of micrometres and polycrystalline MnMoO4 nanoflakes are produced on the surface and sidewall of nickel-coated Si-MCP, respectively. The MnMoO4 crystals ripen controllably forming polycrystalline nanoflakes with lattice fringes of 0.542 nm corresponding to the (1¯11) plane on the sidewall. And these MnMoO4 nanoflakes show apparent thermite performance which is rarely reported and represents MnMoO4 becoming a new category of energetic materials after nanocrystallization. Additionally, the nanocrystallization mechanism is interpreted by ionic diffusion caused by 3D structure. The results indicate that the Si-MCP is a promising substrate for nanocrystallization of energetic materials such as MnMoO4.


Introduction
Energetic materials (explosives, propellants and thermites) have attracted more and more attention recently due to wide applications in both civilian and military. Not only traditional explosives but also the variety and number of materials with good thermal stability and insensitivity have been required intensely. In the past, the traditional crystallite materials played the primary roles, such as amino/aminonitro heterocycles [1], azides [2] and polynitrogen compounds [3,4]. With the rapid development of nanoscience and nanotechnology, morphology is one of the important factors for the sensitivity of energetic materials [5]. In particular, nano energetic materials show dramatic improvement of the energy release, the reliability in initiation, detonation velocity and charge density. In addition, it also causes the decrease in the explosion critical radius [6,7]. Nanocrystalline compounds are known as metastable intermolecular composites (MIC) in the class of thermite materials [8,9], which are pyrotechnic compositions of metal powder fuels and oxidizers. Aluminium is the most common choice for fuels because of its low cost and high boiling point. And oxidizers are generally metallic oxides, such as iron [10], molybdenum [11] and manganese oxides [12]. Combined with aluminium, these thermite materials have high combustion efficiency, fast energy-releasing rate and good safety performance. Sun et al. [13] composed aluminium and molybdenum trioxide nanoparticles which indicate that the reactivity of nanoparticles is significantly higher than that of micrometre-size samples with a reaction range of 200-300 kJ mol −1 . Comet et al. [14] mixed MnO 2 and Al nanoparticles in hexane solution as high-energy thermite. Granier & Pantoya [15] studied the ignition and combustion behaviours of nanocomposite Al/MoO 3 .
Recently, binary metal oxides have attracted much interest due to their novel chemical and physical properties. As we know, molybdenum metal has the ability to form stable oxides with a series of metals (Mg, Mn, Fe, Co, Ni, Zn, etc.). The reports on these molybdates reveal the extraordinary performance of binary metal oxides which represents an interesting group of properties [16,17]. Minakshi et al. [18] synthesized nanoscale ternary molybdate (Mn 0.33 Nio 0.33 Co 0.33 MoO 4 ) using biopolymer as a precursor to improve the energy storage performance. Currently, since both manganate-oxides and molybdateoxides materials are common MIC materials, nanocrystalline manganese molybdate is proposed in an application as an energetic material. Combined with Al as fuel, nanocrystalline MnMoO 4 is the oxidizer of thermite materials. MnMoO 4 has the wolframite structure with the smaller bivalent manganese cations and molybdenum atoms having an overall sixfold coordination [19]. Manganese molybdate obtained by previous methods (such as sol-gel method, a solid-state reaction at high temperature and hydrothermal process) was often crystallized with crystal size larger than few tens of micrometres rather than nanocrystalline materials [20,21]. Lei et al. [22] prepared manganese molybdate rods and hollow olive-like spheres in micrometre-scale. Senthilkumar et al. [21] reported MnMoO 4 with the size of about 20 µm. Watcharatharapong et al. [23] prepared manganese molybdate rods with a length of 10 µm. These MnMoO 4 obtained are large crystallite materials with micrometre dimension. Nevertheless, Mu et al. [24] reported ultrathin manganese molybdate nanosheets of about 200 nm grown on Ni foam. Cao et al. [25] reported MnMoO 4 nanoplates grown on a Ni foam substrate with the size of 500 nm. It was found that MnMoO 4 grown on three-dimensional (3D) substrate is preferred to be nanocrystalline structure. However, the specific mechanism was barely studied. Hence in this study, a patterned 3D substrate, silicon microchannel plate (Si-MCP), is proposed to act as a substrate due to larger aspect ratio and better orderliness than Ni foam. Si-MCP is arranged with large aspect ratio lattices with length of 5 µm and depth of 300 µm. Manganese molybdate polycrystalline nanoflakes are synthesized on this ordered substrate. The growth mechanism is analysed and discussed more easily. Then, MnMoO 4 polycrystalline nanoflakes exhibit novel properties of energetic materials. To research the crystal growth mechanism, a plane substrate is adopted as comparison. Otherwise, different concentrations of solution are prepared to contribute to analysing the growth process of the nanoflakes. Last but not least, the nanocrystallization mechanism based on 3D substrate is expounded in detail.

Synthesis and crystal growth
Analytical grade chemical reagents were used without further purification. Manganese chloride (MnCl 2 ), ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O) and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. The de-ionized (DI) water used to prepare the solutions had a resistivity of 18 MΩ cm. The Si-MCP was fabricated by electrochemical etching and details about the preparation procedures are available in Yuan et al. [26]. A nickel film was deposited on the surface and inner wall of the Si-MCP by flow deposition to produce the MECN (macro and electrically conductive network) in which the Ni film plays the role of improving the adhesion and electrical properties [27].
Then, 1 M (NH 4 ) 6 Mo 7 O 24 · 4H 2 O and 7 M MnCl 2 solutions were prepared with DI water at room temperature. The (NH 4 ) 6 Mo 7 O 24 solution was added slowly to the MnCl 2 solution under stirring to form a homogeneous solution and the pH was adjusted to about 7 by an NH 3 solution [28]. After the precursor solution was transferred to a Teflon-lined stainless steel autoclave liner, the Ni/Si-MCP was added into the solution after immersing in 0.1% Triton X-100 solution for 30 s. The autoclave was then sealed and heated to 140°C for 15 min to produce a deep-brown precipitate containing MnMoO 4 on the surface of the Ni-coated Si-MCP. After cooling to room temperature in air, the product was taken out, rinsed ultrasonically with DI water and ethanol sequentially three times and vacuum dried at 80°C for 6 h. An aluminium film was deposited on the MnMoO 4 /Ni/Si-MCP as fuel by electroplating which is reported in Shi et al. [29].

Sample characterization
X-ray diffraction (XRD) was performed with a Rigaku RINT2000. The surface morphology and microstructure of the MECN and MnMoO 4 were examined using field-emission scanning electron microscopy (FE-SEM; JEOL, JSM-7001F, Japan) equipped with an energy-dispersive X-ray spectrometer. The Raman spectra were recorded from 200 to 1000 cm −1 on an Olympus BX41 Raman Microprobe using a 524.4 nm argon ion laser. Transmission electron microscopy (TEM) was conducted with a JEOL JEM-2100 FEF. The onset temperature and energy release were monitored by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Mettler Toledo, TGA/DSC 1).   [30]. The highest intensity band corresponds to the Mo(1)O(2) symmetric stretching vibration [31].      To further analyse the difference between the large crystallites on the surface and nanoflakes on the microchannels, MnMoO4/Ni/Si-MCP is measured by TEM. Two different materials are separated into two samples. The sample with the crystallites shown in figure 3a is prepared by ultrasonication in ethanol for a sufficiently long time to make sure the crystallites are detached from the surface. And the nanoflakes sample shown in figure 3b is separated centrifugally from the fragments after etching in KOH solution. Figure 3a,c shows that the crystallites are monocrystals with an interplanar spacing of 0.502 nm corresponding to the (200) plane of MnMoO 4 . The selected-area electron diffraction pattern in figure 3a confirms the monocrystals as diffraction spots can be clearly observed [32].   To study the performance of the sample as energetic material, DSC/TGA experiments are conducted at a temperature from 50°C to 1100°C with a heating rate of 10°C min −1 in 50 sccm N 2 . The thermal analysis results are presented in figure 4b based on the mass-corrected values. As the temperature rises, weight loss occurs gradually and at 730°C, the weight curve rises because of the thermite reaction between Al and oxygen/vapour in the sample and environment. The energy generated by the thermite reaction is quite obvious at 960°C. The peak in the heat flow curve reveals the activation energy of the thermite reaction [33].The prominent thermite performance in the DSC/TGA analyses reveals that the nanocrystallization improves the properties of MnMoO 4 and makes it to be a new kind of energetic material successfully and meaningfully.

Nanocrystallization mechanism
When the (NH 4 ) 6  are transferred to both the surface and inner wall of the Si-MCP evenly. In the supersaturated solution, the particles connect the crystals formed afterwards. Following hydrothermal crystallization and ripening, the larger particles grow at the expense of smaller ones in accordance with the Gibbs-Thomson law [34][35][36][37]. Formation of MnMoO 4 on the surface has a similar mechanism.
Our results show that there are two different types of crystals on the MCP. To further assess the crystal growth mechanism, a sample fabricated on a flat silicon substrate is compared. As shown in figure 5a, the silicon substrate undergoes the electroless process similar to Si-MCP (figure 5c). Tiny nickel particles are deposited on the silicon substrate uniformly and the morphology is shown in figure 5b. There are no nano-structured crystals on the plane substrate but instead large MnMoO 4 particles are observed, indicating that the MCP with a large aspect ratio is crucial to the formation of MnMoO 4 nanoflakes. Different solution concentrations are also prepared. The samples immersed in solutions with concentrations 25%, 50% and 75% of the standard one are processed hydrothermally. As the concentration decreases, there are fewer particles on the sidewall as shown in figure 5d-f. When the concentration is 25% of the standard one, the deposited product is a thin film in lieu of particles, indicating that the concentration is also critical to the formation of the nano-structure. The small particles shown in figure 5d do not morph into large crystallites demonstrating that the structure of the Si-MCP affects the crystal growth.
The crystal growth mechanism is illustrated in figure 6a. When the substrate is planar, the ions arrive at the surface from the solution evenly resulting in continuous and stable absorption and reaction. The particles grow to form large crystallites according to Gibbs-Thomson law. If the time is unlimited, the crystals grow until they reach the boundaries meaning that there is no effective absorption point. In comparison, on the channel plate with a large aspect ratio, the ions in the channel decrease as a result of the reaction in crystal ripening, as shown in figure 6b. The outer ions diffuse into the channels continuously due to the concentration gradient. The ions flow parallel to the sidewall on the Si-MCP and the flow direction is along the shearing direction of the crystals on the sidewall. The particles are separated by the shearing stress. The particles absorb and react on the entire surface evenly rather than preferentially according to the growth orientation [38][39][40][41]. Hence, the crystals cannot capture each other to form large crystallites thus forming the polycrystalline nanoflakes. The nanoflakes on the orifice of the microchannel grow by the same mechanism that ion flow obstructs the normal crystal growth. Since the ion flow on the orifice is not absolutely parallel to the side wall inside the channel, the nanoflakes are non-uniform. 0.542 nm corresponding to the (111) plane on the sidewall. The synthesized MnMoO 4 nanoflakes display unprecedented thermite performance which reveals the possibility of new application as energetic material. Last but not least, to research the growth mechanism, a flat silicon substrate was adopted as comparison. Samples with various concentrations of ions were also prepared as comparisons, and can be explained by ionic diffusion caused by 3D structure. The results indicate that the Si-MCP is a promising substrate for nanocrystallization of energetic materials such as MnMoO 4 .

Conclusion
Data accessibility. http://dx.doi.org/10.5061/dryad.g1693 [42]. Authors' contributions. C.Z.: the author who is doing the main experiments. D.W.: the author who is helping to discuss the growth of MnMoO 4 . L.S.: the author who is helping to discuss the thermite performance of MnMoO 4 . Y.Z.: the author who is helping to analyse the experiments. D.X.: the author who is helping to analyse the experiments. S.X.: the author who is mainly working on SEM analysis. R.H. and R.Q.: the authors who are helping with the TEM analysis. W.Z.: the author who is giving help in thermite test process and equipment application. L.W.: the experimental designer. P.K.C.: the experimental co-designer.