Combined Flame and Electrodeposition Synthesis of Energetic

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Combined Flame and Electrodeposition Synthesis of Energetic Coaxial Tungsten-Oxide/Aluminum Nanowire Arrays Zhizhong Dong, Jafar F. Al-Sharab, Bernard H. Kear, and Stephen D. Tse Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl4021446 • Publication Date (Web): 30 Jul 2013 Downloaded from http://pubs.acs.org on August 2, 2013

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Combined Flame and Electrodeposition Synthesis of Energetic Coaxial Tungsten-Oxide/Aluminum Nanowire Arrays Zhizhong Dong1, Jafar F. Al-Sharab2, Bernard H. Kear2, Stephen D. Tse1* 1

Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey 08854

2

Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08854

*corresponding author, email: [email protected]

Abstract A nanostructured thermite composite comprising an array of tungsten-oxide (WO2.9) nanowires (diameters of 20-50 nm and lengths of >10 µm) coated with single-crystal aluminum (thickness of ~16 nm) has been fabricated. The method involves combined flame synthesis of tungstenoxide nanowires and electrodeposition of aluminum. The geometry not only presents an avenue to tailor heat-release characteristics due to anisotropic arrangement of fuel and oxidizer, but also eliminates or minimizes the presence of an interfacial Al2O3 passivation layer. Upon ignition, the energetic nanocomposite exhibits strong exothermicity, thereby being useful for fundamental study of aluminothermic reactions, as well as enhancing combustion characteristics.

Keywords Coaxial nanowire, Aluminum electrodeposition, Thermite, Tungsten-oxide nanowire

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Nanotechnology has stimulated rapid development of thermite nanocomposites1, consisting of nano-scale metallic fuel (e.g. aluminum) and oxidizer (e.g. iron oxide2,3, copper oxide4, molybdenum oxide5,6, tungsten oxide7–9 etc.), impacting applications in propellants, explosives, and pyrotechnics10. Compared to traditional micro-thermite systems, nano-thermite composites can possess significantly improved exothermic properties, such as lower ignition temperature, shorter ignition-delay time, and higher energy-release rate2,5,7,11. Using mechanical mixing, Pantoya and Granier5 produced thermite composites composed of aluminum particles (diameters of 17-202 nm) and sheet-like molybdenum-trioxide nanoparticles (where one of the dimensions is 10 µm, respectively. The tungsten-oxide nanowires grow vertically-well-aligned on the tungsten substrate after 10 minutes of flame-synthesis duration. Figure 2(b) shows a lowmagnification FESEM image of the as-produced tungsten-oxide/Al coaxial nanowires after electrodeposition of aluminum on tungsten-oxide nanowires.

The image reveals that the

electrodeposition process does not obviously change the geometrical structure of the tungstenoxide nanowires—only coating them uniformly. The high-magnification FESEM image inset in Figure 2(b) shows that the diameters of the as-grown composite nanowires increase to 50-80 nm after 15 minutes of electrodeposition. Typical EDS spectrum, as given in Figure 2(e), of the asprepared tungsten-oxide/Al coaxial composite nanowires indicates the presence of elemental aluminum, oxygen, and tungsten, along with small amounts of chlorine and carbon.

The

presence of chlorine is due to residual electrolyte containing AlCl3 from the deposition process. The signal for carbon originates from the carbon tape used in the FESEM/EDS analysis. Figure 2(c) presents an HRTEM image of a single as-grown highly-textured tungsten-oxide nanowire before aluminum film coating, revealing its dislocation-free, single-crystal structure, with 3.78 Å lattice spacing and [110] preferred growth orientation. Selected area electron diffraction (SAED) pattern further confirms that it is tetragonal WO2.9. Detailed discussion and description for the growth of WO2.9 nanowires can be found in our previous work22. Figure 2(d) presents the sideview of a low-magnification TEM image of a single as-fabricated WO2.9/Al coaxial nanowire. The coaxial structure of the nanowire is evinced, with outer diameter Do=82.9 nm, inner diameter Di=50.5 nm, and Al coating layer thickness δ=16.2 nm.


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Figure 2. (a) Low-magnification FESEM image of flame-synthesized WO2.9 nanowires; the inset shows a high-magnification image of the nanowire array. (b) Low-magnification FESEM image of WO2.9/Al coaxial nanowires after electrodeposition; the inset shows a high-magnification FESEM image of the coaxial nanowires. (c) HRTEM image of as-grown WO2.9 nanowire with [110] growth direction; the inset shows the SAED pattern. (d) Side-view low-magnification TEM image of a single as-grown WO2.9/Al coaxial nanowire.


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(e) Typical EDS spectrum of the as-prepared WO2.9/Al coaxial nanowires.

Figure 3. (a) HRTEM image showing the interfacial region between WO2.9 core and Al shell. A typical interface is marked as A, and an example dislocation region is marked as B. Possible [110] and [1 1 0] growth directions for Al are shown. (b) FFT pattern for WO2.9 indicating the [110] growth orientation. (c) FFT pattern for Al indicating the possible growth orientations of [110] and [1 1 0]. (d) Zoomed HRTEM image of region A, where weak reflections from ( 1 10) and (120) of Al2O3 are observed, with d-spacings of 0.24 nm and 0.21 nm, respectively. (e) Zoomed image of region B, with the example dislocation circled.

HRTEM, Figure 3(a), of the interfacial region between the WO2.9 core and the Al shell reveals that aluminum is grown directly on the surface of WO2.9 nanowire forming an atomically abrupt interface. Again, the lattice planes, Figure 3(a), of the core nanowire has an average dspacing of 3.78 Å, which corresponds to the reflections from the {110} planes of tetragonal


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WO2.922,24. The indexed spots in the fast Fourier transform (FFT) pattern of WO2.9, Figure 3(b), correspond to the (110) plane, indicating the growth orientation of WO2.9 along the [110] direction. The coating Al layer has two d-spacings of 4.06 Å and 2.87 Å, which correspond to the reflections from the {100} planes and {110} planes of cubic Al, respectively. The measured d-spacings match very well with the literature data for aluminum (PDF#97-004-3492). The indexed spots in the FFT pattern of Al, Figure 3(c), correspond to the (110) and (1 10) planes. As shown in Figure 3(a), the growth of the aluminum film may occur along the [110] and/or [1 1 0] directions. The detailed growth mechanism is the subject of ongoing investigation. Analysis of the d-spacings from HRTEM at the interface, Figure 3(d), between WO2.9 and Al (marked as a red dash line) reveals very weak reflections corresponding to d-spacings of 2.4 Å and 2.1 Å, which may come from ( 110) and (120) planes of Al2O3 (JCPDS 76-0144). By further examining the interface, there appears to be a semi-crystalline monolayer where Al2O3 is present. A redox reaction likely takes place for the first atomic layer of Al ions depositing on the WO2.9 nanowire, producing an oxygen-deficient WOX underlayer and resulting in an Al2O3 transition monolayer interface, with subsequent deposition of Al ions forming the coating. The resultant Al2O3 acts as a diffusive barrier for transport of oxygen (from the WO2.9 core to the Al shell), thereby limiting its thickness to one or two atomic layer(s). As a result, our process avoids the formation of the conventional Al2O3 passivation layer between the aluminum coating layer and the tungsten-oxide nanowire core, establishing strong intimacy between Al fuel and metal-oxide oxidizer. Menon et. al.3 used a 10-step approach to fabricate Fe2O3/Al nanocomposites, whereby Fe2O3 nanowires (50nm diameter and 2 µm length) were embedded in a thin aluminum film (25 nm thickness) by physical vacuum deposition of Al. In this work, tungsten-oxide nanowires are coated by nano-scale layers of Al with thickness ~16 9 ACS Paragon Plus Environment

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nm, in a straightforward, fast, and scalable process which can be readily adjusted by electrochemical parameters, such as applied potential, temperature, and reaction time25. Surprisingly, given the high growth rate, forced epitaxy seems to occur, resulting in an atomically distinct interface despite non-negligible lattice mismatch. Furthermore, HRTEM scanning along the length of the WO2.9/Al coaxial nanowire divulges that the aluminum grows as a single crystal surrounding the core metal-oxide nanowire. Although dislocations, for example as seen from Figure 3(e), can be found in the Al film, presumably due to accidents in crystal growth, such growth appears novel, as previous electrodeposition studies of Al produced materials with grain structure. In Jiang et. al.’s work23, aluminum grains of 5-10 µm in size were deposited on tungsten electrodes. Endres et. al.25 reported electrodeposition of nanocrystalline Al (volume-averaged grain sizes ~12 nm) on glassy carbon, where lower current densities and overvoltages formed larger grain sizes. Zein El Abedin et. al.26 deposited micron-thick Al films (composed of fine Al crystallites with an average grain size of 20 nm) on Au films.


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Figure 4. (a) Schematic setup for ignition test. (b) Picture of initial stage ignition/combustion of as-produced thermite nanocomposite. (c) Picture of explosive combustion of as-produced thermite nanocomposite. (d) Comparison test of passing current through a tungsten wire without coaxial WO2.9/Al nanowires on its surface, but coated with Al.

Enhanced exothermic performance is also expected from this thermite nanocomposite. Thus, to verify qualitatively the reactivity of the as-synthesized thermite nanocomposite, a tungsten wire (which is used as substrate) with WO2.9/Al coaxial nanowires grown on its surface is ignited by inducing current through the tungsten wire by applying 30 VAC across it, as shown in the schematic diagram of Figure 4(a). Joule heating ignites the nanocomposite, Figure 4(b), with explosive combustion ensuing Figure 4(c). It is worth noting that the WO2.9/Al coaxial nanowire arrays are chemically stable under ambient conditions prior to the forced ignition experiments described above. A comparison is made with a tungsten wire with no WO2.9/Al coaxial nanowires grown on its surface, but coated with Al (to rule out the role of Al in affecting the circuit). For this case, only glowing heating is observed for the same “ignition” conditions, Figure 4(d). Quantitative characterization of the ignition and combustion behaviors are the subject of future work, where their properties are expected to exceed that for composites obtained by traditional mechanical mixing processes. In conclusion, WO2.9/Al coaxial nanowires are fabricated by a sequential process involving flame synthesis and ionic-liquid electrodeposition. The process establishes a distinct interface between tungsten-oxide and Al, with only a transition Al2O3 monolayer, as confirmed by weak reflections from the HRTEM at the interface. This monolayer of Al2O3 is likely formed due to the deposited Al “plucking off” oxygen atoms from the adjacent WO2.9 (making it oxygen deficient), forming a transition monolayer and diffusive barrier, such that additional oxygen from 11 ACS Paragon Plus Environment

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WO2.9 does not grow the newly-formed interfacial Al2O3 layer further. Such Al2O3 presence is due to transitioning of the material from metal-oxide to metal, which is different than the natural passivation usually encountered surrounding Al nanoparticles. Investigation of the detailed mechanism of deposition and of the parameters affecting the interface condition is the subject of on-going work. This method for thin (nano-scale) metal layer coating on metal-oxide nanowire arrays, wherein metal ions are directly transported to the surface of materials under applied electric field, and where no high vacuum system is needed to prevent the conventional formation of a passivation layer between Al and metal oxides, possesses many applications. As a result, detailed fundamental study, enhanced exothermic performance, and potential scalability at reduced costs are afforded by the described processing route and the resulting structure of this nanocomposite.

Author information Corresponding Author *Email: [email protected] Acknowledgements This work was supported by the Army Research Office (Grant W911NF-08-1-0417) and the National Science Foundation (Grant CBET 0755615). References (1) (2)

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Combined Flame and Electrodeposition Synthesis of Energetic

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