Rapid Synthesis of Thin and Long Mo17O47 Nanowire-Arrays in an Oxygen Deficient Flame

Mo17O47 nanowire-arrays are promising active materials and electrically-conductive supports for batteries and other devices. While high surface area resulting from long, thin, densely packed nanowires generally leads to improved performance in a wide variety of applications, the Mo17O47 nanowire-arrays synthesized previously by electrically-heated chemical vapor deposition under vacuum conditions were relatively thick and short. Here, we demonstrate a method to grow significantly thinner and longer, densely packed, high-purity Mo17O47 nanowire-arrays with diameters of 20–60 nm and lengths of 4–6 μm on metal foil substrates using rapid atmospheric flame vapor deposition without any chamber or walls. The atmospheric pressure and 1000 °C evaporation temperature resulted in smaller diameters, longer lengths and order-of-magnitude faster growth rate than previously demonstrated. As explained by kinetic and thermodynamic calculations, the selective synthesis of high-purity Mo17O47 nanowires is achieved due to low oxygen partial pressure in the flame products as a result of the high ratio of fuel to oxidizer supplied to the flame, which enables the correct ratio of MoO2 and MoO3 vapor concentrations for the growth of Mo17O47. This flame synthesis method is therefore a promising route for the growth of composition-controlled one-dimensional metal oxide nanomaterials for many applications.

provide the heat and oxidizing gases required to evaporate and generate molybdenum oxide vapors from a solid molybdenum source that is placed above the flame. The vapors then deposit onto a temperature-controlled substrate and in a stagnation-point flow configuration. Compared to electrically-heated vacuum deposition methods, the atmospheric pressure and the relatively higher evaporation temperatures of the FVD synthesis allow the total concentration of MoO x vapors to be larger, resulting in denser nucleation of nanowires with smaller diameters, and faster axial growth rates, leading to higher aspect ratio and surface area. In addition, a benefit of this approach is that the partial pressure of oxygen in the synthesis environment can be directly controlled over many orders of magnitude through control over the ratio of CH 4 (fuel) and air (oxidizer) supplied to the flame 14,15,17 . This enables the synthesis of high-purity Mo 17 O 47 nanowires as opposed to MoO 2 or MoO 3 , by correct selection of the CH 4 / air flow ratio. This is in contrast to the hot-filament CVD approach, and most other vapor deposition approaches, in which control over the concentration of oxidizers is typically achieved by flowing or leaking oxidizing gases and controlling the total pressure with a vacuum system 1,18,19 . Finally, the flat flame in the FVD method can be scaled up for large-area deposition onto substrates, and the atmospheric operation of the FVD synthesis is less energy-intensive than maintaining a vacuum [13][14][15][16][17] .
The synthesis of molybdenum oxide nanostructures has previously been investigated using flames. The synthesis of MoO 2 nanostructures in the form of rods with hollow channels by vapor deposition on a probe [20][21][22][23] , or elongated rectangular particles by vapor condensation in the gas phase 24,25 has been achieved by evaporating Mo probes in or near the flame front in a counter-flow diffusion flame. MoO 3 vapors were generated on the oxidizer size of the probe and were then converted to MoO 2 and deposited on the fuel side. In addition, we have previously studied the variation of the fuel/air ratio, the Mo source temperature, and the substrate temperature using the FVD method for the synthesis of single, branched, and flower-like α -MoO 3 nanobelts 16 , and studied the co-evaporation of Mo and W sources to produce W-doped α -and β -MoO 3 nanobelts 13 . However, the present study is the first to demonstrate that the fuel/air ratio of the flame can be controlled to obtain high-purity Mo 17 O 47 nanowires under specific oxygen-deficient (fuel-rich) conditions. Moreover, the chemical kinetics of the combustion reactions are analyzed through calculations using an established CH 4 -air combustion mechanism, to gain insight into the influence of the fuel/air ratio on the gas phase composition which leads to the synthesis of Mo 17 O 47 instead of MoO 3 .

Results
Experiments. MoO x nanostructures were synthesized by FVD directly on Ni and Mo foil substrates by oxidizing and evaporating Mo wires (see Methods). All experiments employed a deposition time of 15 minutes, a peak flame temperature of approximately 1100 °C, a Mo wire source temperature of approximately 1000 °C, and a substrate temperature of 570 °C. More detailed temperature measurements are given in the supplementary information, Table S1. In the flame synthesis method, the burner, flame and vapor source are operating at steadystate. Then, the substrates onto which the deposit occurs are introduced into the flame region at the start of the growth time, and are removed from the flame region at the end of the growth time. The insertion and removal process takes only about 1 second. Therefore, the time that the substrates spend in the flame region is 15 minutes, within an error of only a few seconds. Moreover, after it is inserted into the flame, the temperature of the substrate reaches 90% of the steady-state growth temperature within 2 seconds, and 99% of the growth temperature within 10 seconds, as measured by a thermocouple in contact with the substrate. The substrate temperature reported is the steady-state temperature.
The reaction between CH 4 (fuel) and O 2 (oxidizer) occurring in the flame is given by CH 4 + 2O 2 → CO 2 + 2H 2 O for the stoichiometric case. An equivalence ratio (Φ ) is commonly defined to compare the actual fuel-oxidizer ratio of the flame to its stoichiometric value, as given by equation (1).
Here,  Q are the volumetric flow rates into the flame. Therefore Φ = 1 corresponds to a stoichiometric supply of CH 4 and O 2 into the flame, in which case the product gases will primarily consist of CO 2 and H 2 O, while Φ > 1 corresponds to a fuel-rich flame, in which case the products will additionally contain unburned fuel in the form of H 2 and CO, and Φ < 1 corresponds to a fuel-lean (oxygen-rich) flame, in which case the products will instead contain unreacted O 2 . The O 2 in this experiment was supplied by flowing air, which contains approximately 21% of O 2 , and the same air flow rate of =  Q 18 Oxidizer standard liters per minute (SLPM) was used for all experiments. Assuming that all the gases are ideal and have the same molar volumes, the stoichiometric flow rate of CH 4 corresponding to this air flow rate is then 1.89 SLPM. The flow rate of CH 4 was varied between experiments within the range of = .
Scanning electron microscope (SEM) images of the resulting MoO x nanostructures are shown in Fig. 2. At Φ < 1.06, MoO 3 nanobelts were observed (Fig. 2a), which is consistent with the previous report 16 . As the fuel flow rate is increased to give Φ = 1.06, the morphology of the nanostructures transitions from MoO 3 nanobelts to a mixture of nanobelts and nanowires (Fig. 2b). When Φ is further increased to 1.06 < Φ < 1.11, only densely packed nanowires are obtained (Fig. 2c). The loading density of the Mo 17 O 47 was 0.6 + /− 0.1 mg/cm 2 after 15 minutes of growth. This loading is comparable to that commonly used for nanowire electrodes in Li-ion battery research (~0.5 mg/cm 2 , 0.3 mg/cm 2 ) 26,27 . At Φ = 1.11, the density and length of the nanowires decreases (Fig. 2d), and for Φ > 1.11, no deposit is observed (Fig. 2e). Visually, the growth of MoO 3 nanobelts corresponds to a grey/white sample color, while the growth of nanowires corresponds to a blue/purple color (Fig. 1c). In both cases, the substrate surface is matte, while in the case of no deposit at high Φ , the surface is reflective. The temperature of the Mo source wires (from which the oxide vapor is being generated) increased by 8 °C from Φ = 0.95 to 1.16 (supporting information Table S1). This would result in increased generation of vapor from the Mo source, which cannot account for the change in morphology and composition of the deposited nanostructures from plate-like to wire-like, as this would instead be expected from a decrease in vapor. Moreover, we ensured that the substrate temperature of 570 °C was the same for all samples presented in Fig. 2, and was not influenced by any change in the flame temperature. This was done by carefully adjusting the water flow rate in the substrate cooler to compensate for any change in the flame temperature. Rather, the change in morphology of the nanostructures is primarily due to the change of oxygen partial pressure upon variation of Φ , as will be discussed later.
Higher-magnification side-and top-view SEM images of the nanowires grown at 1.06 < Φ < 1.11 are shown in Fig. 3. The nanowires have diameters of 20-60 nm and lengths of 4-6 um. The oriented nature of the nanowires demonstrates that they grow by the heterogeneous nucleation of vapors on the substrate and subsequent anisotropic growth of the nanowire crystals, rather than by homogeneous nucleation in the gas phase  (Fig. 4). The highest intensity of the signal from the Mo 17 O 47 phase suggests that this signal comes from the nanowires, which according to SEM is the phase present in greatest quantity on the surface of the foil substrate. Moreover, because the relative intensity of the (001) reflection of Mo 17 O 47 is much larger than that of the same peak in the powder standard, we can conclude that the Mo 17 17 O 47 nanowires synthesized by hot wire CVD 1 . In contrast, the MoO 2 and MoO 3 phases appear to have an isotropic texture because the relative intensities of the peaks in their signals match those of the powder standard. This suggests that they are present as polycrystalline layers, rather than part of the elongated nanowire crystals. The XRD signal from MoO 2 persists even when the nanowires are scraped off the substrate, indicating that the MoO 2 is present in an underlying layer formed by oxidation of the substrate during synthesis. As reported previously, the MoO 3 signal can come from the oxidation of the surface of the Mo 17 O 47 nanowires in air, after synthesis 1 . XRD analysis was also performed for the sample shown in Fig. 2e, which was obtained under the condition of Φ > 1.11. The result is given in supplementary information Fig. S1, and shows that there is no difference between the XRD pattern of the pristine substrate and that exposed to the flame synthesis, proving that there is no deposit in this case.
Transmission electron microscopy (TEM) was conducted to further investigate the crystal structure and composition of the nanowires grown at 1.06 < Φ < 1.11. The nanowires were removed from the growth substrate and dispersed onto a TEM grid. A nanowire such as that shown in Fig. 5a, which is oriented parallel to one of the two perpendicular tilt axes of the TEM stage, was selected. The nanowire was first tilted around the pitch axis to maximize the projected length of the nanowire in the TEM image, which serves to roughly orient the nanowire axis perpendicularly to the electron beam. The nanowire was then further tilted around the pitch axis until the crystal planes that are perpendicular to the nanowire axis became precisely parallel to the electron beam, as verified by selected area electron diffraction (SAED) (see diffraction pattern in the inset of Fig. 5b). High-resolution TEM (HRTEM) images of the crystal lattice were then obtained at this orientation, as shown in Fig. 5b. Both the HRTEM images and the SAED spot pattern show a lattice fringe spacing of 0.396 nm, which matches very well with the (001) plane spacing of the orthorhombic Mo 17 O 47 (0.400 nm). To confirm that these (001) planes are indeed perpendicular to the nanowire axis, the nanowire was then rotated about its roll axis over a range of 60°, over which these planes remained parallel to the electron beam. Therefore, this confirms that the nanowires are composed of high-purity Mo 17 O 47 with (001) crystal planes perpendicular to the nanowire axis. This also matches the findings of the XRD analysis, which showed higher relative intensity of the (001) reflection compared to the powder standard. The lattice spacings of planes parallel to the nanowire axis were more disordered, as indicated by the streaks to the left and right side of the bright spots in the SAED pattern. The most distinct features of the Mo 17 O 47 nanowires synthesized here by FVD are their small diameter, long length, and high packing density. The synthesis of Mo 17 O 47 nanowire-arrays was previously reported by hot-filament CVD under vacuum conditions 1 . However, in that case, the nanowires had larger diameters of ~90 nm compared to the 20-60 nm diameters here, and larger lengths of approximately 1 μ m (estimated from SEM images in that report) after a growth time of 30 minutes, compared to lengths of 4-6 μ m after a growth time of 15 minutes here. These differences can be explained by the higher 1 atmosphere pressure and 1000 °C evaporation temperature of the FVD synthesis compared to the 1.1 Torr pressure and 775 °C evaporation temperature of the CVD synthesis, which results in higher pressure and supersaturation of MoO x vapors in the FVD synthesis. The decrease of nucleus size with increasing supersaturation is well-known for heterogeneous nucleation of solids from vapor 28,29 , which leads to the smaller nanowire diameters observed here. At the same time, the higher vapor pressure leads to the faster axial growth rates observed here.
Finally, it is noted that at Φ ≈ 1.1 but with a substrate temperature below 500 °C, unique structures with a mixture of morphologies and compositions are grown on the metal foils, as shown in Fig. 6. These structures may be further investigated in the future.

Simulations.
Another key advantage of the FVD method is that the previously defined equivalence ratio (Φ ) of the flame can be controlled to result in the growth of nanostructures composed of a pure phase (either Mo 17 O 47 or MoO 3 ) rather than mixtures of phases. The reason behind this composition control is studied by simulations of the chemical kinetics of combustion, and by thermodynamic calculations. The species concentration profiles as a function of distance above the burner were simulated using Chemkin PREMIX software 30 , employing the GRI-Mech 3.0 chemical kinetics mechanism for CH 4 combustion 31 and the experimentally measured gas temperatures. A representative result for the case of CH 4 flow rate of 2.1 SLPM (Φ = 1.11) is shown in Fig. 7a. The results predict the formation of the flame a few millimeters away from the burner, which is consistent with the experiment, and also provide concentrations for relevant species such as O 2 , H 2 O, H 2 , CO 2 and CO at the Mo evaporation source, which is located at 1.4 cm. The concentrations of these species are reported in Fig. 7b for various different equivalence ratios obtained from different simulations at different CH 4 flow rates, keeping all else constant.
Data on the volatilization of Mo metal in the presence of oxidizers indicates that MoO 3 and MoO 2 molecules are evolved at the temperatures being studied here 32 . The transition from MoO 3 growth at Φ < 1.06 to Mo 17 O 47 growth at Φ > 1.06 can qualitatively be explained by a decrease in the oxidizing nature of the combustion products from the flame as Φ is increased, since at higher Φ the products contain greater fractions of reducing gases such as H 2 and CO as opposed to oxidizing gases such as H 2 O, CO 2 and O 2 . This should lead to a greater fraction of MoO 2 vapors being generated compared to MoO 3 vapors, therefore resulting in the growth of MoO x oxides with 2 < x < 3. However, the oxidizing species are necessary for generating the MoO 2 and MoO 3 vapors in the first place by a process of simultaneous oxidation and evaporation of the surface of the solid Mo source wires. Studies on refractory metals such as Mo and W have shown that the rate of volatilization of the metal to produce gaseous oxides in the presence of O 2 is orders of magnitude faster than that in the presence of the other oxidizers H 2 O and CO 2 , while at the same time, the presence of H 2 and CO suppress volatilization 33,34 . Therefore, the lack of growth at Φ > 1.11 can be explained by the lack of generation of MoO x vapors due to very low concentrations of O 2 and increased concentrations of CO and H 2 in the combustion products. Despite the large concentrations of H 2 O and CO 2 at these equivalence ratios, these oxidizers are unable to oxidize Mo at the studied temperatures.
Once the MoO 2 and MoO 3 vapors are generated, the combustion products can further influence P MoO 2 and P MoO 3 , the partial pressures of MoO 2 and MoO 3 vapors, respectively, by reduction or oxidation in the gas phase. To estimate the ratio of partial pressures of these two species, we assume that the gas phase reaction 2MoO 2 (g) + O 2 (g) → 2MoO 3 (g) is at equilibrium because, as reported previously, the kinetics of oxidation by O 2 are very rapid at these temperatures, while the kinetics of oxidation by H 2 O and CO 2 are much slower 33,34 . Using the definition of the equilibrium constant for this reaction, we obtain equation (2), where P O 2 is the partial pres- sure of oxygen in atmospheres, P 0 is the standard pressure of 1 atmosphere, Δ G is the change in Gibbs free energy for the reaction, R is the universal gas constant (8.314 J/mol.K), and T is the temperature in degrees Kelvin.
The thermodynamic data needed to evaluate Δ G were retrieved from the NIST Chemistry WebBook 35 . The resulting ratio is plotted in Fig. 7c for the oxygen concentrations given in Fig. 7b at the measured gas temperatures downstream of the Mo wires, which increased from 1051 to 1121 °C as Φ was increased from 0.95 to 1.16 (supplementary information Table S1). The final result is that the P P MoO MoO 3 2 ratio decreases with increasing Φ because of a shift in the gas phase equilibrium of the exothermic reaction 2MoO 2 (g) + O 2 (g) → 2MoO 3 (g) towards MoO 2 . This shift happens primarily because the partial pressure of O 2 decreases extremely sharply with equivalence ratio, and to a lesser extent because the temperature of the gas increases gradually with equivalence ratio in the range of interest.
The vapor has the same overall composition as the Mo 17 O 47 nanowires when the the free energy for the reaction MoO 2 (s) + ½ O 2 (g) → MoO 3 (s), which is Δ G = − 98.74 kJ/mol at the substrate temperature of 570 °C and is highly spontaneous for O 2 partial pressures above 5.8 × 10 −13 atm, which is the case for all conditions in this study. Therefore, if MoO 2 vapors are in excess, MoO 2 molecules can adsorb onto the substrate or nanowire surface, some of the molecules can be oxidized to MoO

Discussion
We first discuss the origin of the anisotropic growth of the Mo 17 O 47 crystals in the shape of elongated nanowires. From high-resolution SEM and TEM images, no distinct region, particles or other features were observed at the nanowire tips, suggesting that the growth occurs without the involvement of any chemically-distinct region at the tip of the nanowire as in the "vapor-liquid-solid" or "vapor-solid-solid" mechanisms 36,37 . Rather the growth appears to occur by adsorption of MoO x vapor molecules onto the nanowire surface, followed by subsequent migration and incorporation of the molecules into the nanowire at the tip, which is the "vapor-solid" mechanism. As mentioned earlier, excess MoO 2 can be oxidized to MoO 3 on the nanowire or substrate surface by adsorbed oxygen, resulting in the growth of stoichiometric Mo 17 O 47 nanowires. Although the reason for the nanowire shape of Mo 17 O 47 is not presently known, we hypothesize that it may arise due to similar reasons as the nanowire shape of W 18 O 49 , which has a closely-related crystal structure. W 18 O 49 nanowires are found to grow in the [010] direction perpendicular to their close-packed (010) planes 38,39 . A recent study found, using density functional theory calculations, that the (010) close-packed plane of W 18  Previous studies have investigated the vapor deposition of tungsten oxide nanowires by the generation of tungsten oxide vapors from a hot tungsten filament in the presence of O 2 gas in a vacuum tube furnace, and the subsequent deposition of these vapors onto a substrate 42,43 . There are similarities between these prior studies and the present study, resulting from the chemical similarities between W, WO 2 and WO 3 , and Mo, MoO 2 and MoO 3 , which can generically be called A, AO 2 and AO 3 . However, while these previous studies investigated a range of conditions in which the partial pressure of AO 2 vapor is larger than that of AO 3 vapor (i.e. P AO3 /P AO2 < 1), the present study explores a range of conditions in which the partial pressure of AO 3 is larger than that of AO 2 (i.e. P AO3 / P AO2 > 1), and additionally shows the transition of the growth from AO 3 nanostructures (MoO 3 nanobelts) at high P AO3 /P AO2 to AO 2−x nanostructures (Mo 17 O 47 = MoO 2.76 nanowires) at lower P AO3 /P AO2 . In the previous studies, in which the vapor generated was primarily WO 2 with smaller quantities of WO 3 , if the vapor molecules were incorporated into the nanowires at the same rates with which they adsorbed onto the surface, the observed tungsten oxide nanowire composition should have been similar to WO 2 . However, the actual tungsten oxide nanostructure composition was WO 3 or W 18 O 49 (= WO 2.72 ). Therefore, it was hypothesized that tungsten oxide nanostructures were formed by reactions such as WO 2 (s) + 0.5 O 2 (g) → WO 3 (s) or WO 2 (s) + 0.36 O 2 (g) → W 18 O 49 (s) 42,43 . This process is similar to the possibility we have described in the present study, in which MoO 2 vapors can adsorb and then undergo oxidation to MoO 3 before being incorporated into the Mo 17 O 47 nanowires. Another study similarly examined the vapor deposition of molybdenum trioxide (MoO 3 ) nanowires and tubular structures by the generation of molybdenum oxide vapors from a hot molybdenum filament in the presence of O 2 gas in a vacuum tube furnace, and the subsequent deposition of these vapors onto a substrate 44 , but did not provide analysis of the partial pressures or roles of MoO 2 and MoO 3 , as has been done here. These prior studies on tungsten and molybdenum oxides additionally hypothesized that under the conditions of excess WO 2 or MoO 2 , the deposited WO 2 or MoO 2 would lead to the formation of a "sub-oxide cluster", which directs the one-dimensional (anisotropic) vapor-solid growth of the nanowires 42 not observe any distinct region, particles or other features at the tips of the nanowires and instead hypothesize, as described above, that the anisotropic growth of the nanowires is expected from the anisotropy of the Mo 17 O 47 crystal itself based on the fastest nucleation of new layers on the closest-packed (001) planes.
In conclusion, we have demonstrated a method to selectively grow long, thin, densely-packed, high-purity Mo 17 O 47 nanowire-arrays using rapid atmospheric flame vapor deposition without any chamber or walls. High aspect-ratio (~100:1) Mo 17 O 47 single-crystal nanowire-arrays were grown on Ni and Mo foils at axial growth rates of up to ~0.4 um/min, with diameters of 20-60 nm and lengths of 4-6 um. The atmospheric FVD growth and high evaporation temperatures achieved by the flame resulted in larger total concentrations of MoO x vapors, which produced smaller diameters and faster axial growth rates compared to electrically-heated CVD synthesis under vacuum. As verified by chemical kinetics simulations, the concentrations of oxidizing and reducing gases in the synthesis environment were directly controlled over several orders of magnitude through changes in the CH 4 /air ratio of the flame, which in turn controlled the relative concentrations of MoO 2 and MoO 3 vapors to enable the deposition of high-purity Mo 17 O 47 nanowires. This is a primary benefit of this approach over most other vapor deposition synthesis methods, in which control over the concentration of oxidizers is typically achieved by flowing or leaking oxidizing gases and controlling the total pressure with a vacuum system, which is more energy intensive. This study is the first to grow Mo 17 O 47 nanostructures using a flame, whereas MoO 3 and MoO 2 have been previously demonstrated. The Mo 17 O 47 nanowires synthesized here could find use as active materials in batteries, and as active materials or high-surface-area electrically conductive supports in other electrochemical devices such as sensors, electrocatalysts, and in photoelectrochemical or photovoltaic devices. Finally, the flame synthesis method that has been further developed here is a promising route for the growth of composition-controlled 1-D metal oxide nanomaterials for many applications. Moreover, due to the large deposition area, rapid growth rates, atmospheric pressure and chamber-less operation, this flame synthesis method may also have future promise in large-scale nanomanufacturing applications.

Methods
Nanowire Synthesis and Characterization. The Mo 17 O 47 nanowires were synthesized using a 60 mm-diameter porous-plug co-flow premixed burner (flat-flame McKenna burner, Holthuis and Associates, Sebastopol, CA), which has been described in detail in previous work on flame vapor deposition (Fig. 1) 14 . The CH 4 -air flame forms a flat 2-dimensional sheet above the burner surface. The flows of CH 4 and air are delivered to the burner by calibrated rotameters (Brooks Instrument). The air flow rate was fixed at 18 SLPM for all experiments, while the CH 4 flow rate was varied over the range of 1.7 SLPM to 2.3 SLPM for successive experiments. A constant flow of 22 SLPM air was also fed through a co-annular ring that surrounds the burner to ensure smooth flow by matching the velocity of the combustion product gases at the edge of the burner. The gas temperature in the post-flame region was maintained at approximately 1100 °C. The Mo evaporation source, which consists of five segments of 37 mm-length Mo wire (0.5 mm diameter, annealed, 99.95% purity, Alfa Aesar), is held above the flame by a plain steel mesh (0.254 cm wire spacing, 0.0635 cm wire diameter, McMaster-Carr) and is heated to a constant temperature of approximately 1000 °C. The surface of these Mo wires is continuously oxidized and evaporated during the synthesis to generate the MoO x vapors. The vapors are convected upwards by the flow and deposit onto the substrate, which is either Ni foil (0.127 mm thick, 1 cm × 4.5 cm, 99.9% purity, annealed, Alfa Aesar) or Mo foil (0.05 mm thick, 1 cm × 4.5 cm, 99.95% purity, Alfa Aesar). The substrate is clamped to an internally water-cooled substrate holder (Al plate, 4-pass Cu tube, Lytron Model CP10G14), which is used to position the substrate and control its temperature. The vapor source (Mo wires) and the substrate are centered with respect to the centerline of the burner at heights of 14 mm and 29 mm, respectively, above the top surface of the burner. The temperature of the substrate is controlled by the flow rate of cooling water through the substrate holder (0.265 SLPM), as well as by controlling the distance between the substrate and the holder through the addition of 0.127 mm-thick stainless steel spacers placed between the clamp and the substrate. Temperatures were measured with a K-Type thermocouple (0.158 cm exposed bead, XL sheath, Omega Engineering Inc.). The nanowires were characterized by SEM (JEOL JSM-7000F, 10 kV), XRD (PANalyticalXPert 2, Cu-kα , 45 kV, 40 mA), and TEM (FEI Tecnai G2 F20 X-TWIN FEG).
Combustion Kinetics Simulations. The species concentration profiles as a function of distance above the burner were simulated using Chemkin PREMIX software 30 , employing the GRI-Mech 3.0 chemical kinetics mechanism for CH 4 combustion 31 and the experimentally measured gas flow rates and gas temperatures.