Aluminum Nanoparticle Production by Acetonitrile-Assisted Milling

Article pubs.acs.org/JPCC

Aluminum Nanoparticle Production by Acetonitrile-Assisted Milling: Effects of Liquid- vs Vapor-Phase Milling and of Milling Method on Particle Size and Surface Chemistry Jiang Yu,† Brandon W. McMahon,† Jerry A. Boatz,‡ and Scott L. Anderson*,† †

Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States Propellants Branch, Rocket Propulsion Division, Aerospace Systems Directorate, Air Force Research Laboratory, AFMC AFRL/RQRP, 10 East Saturn Boulevard, Edwards AFB, California 93524, United States

‡

S Supporting Information *

ABSTRACT: Milling aluminum balls together with either vaporor liquid-phase acetonitrile (ACN) leads to production of nanoparticles by mechanical attrition; however, vapor-phase ACN is far more efficient at inducing size reduction, leading to more, smaller, and more uniform particles. The attrition process is also more efficient than traditional milling of particulate starting material and produces nanoparticles with substantially lower contamination levels. This paper is aimed at better understanding the nature of the size reduction process, the chemistry driving it, and the particles it produces. Mass spectrometry was used to probe gases generated during milling, and a combination of X-ray photoelectron spectroscopy, infrared spectroscopy, dynamic light scattering, helium ion microscopy, scanning electron microscopy, and thermogravimetric analysis/mass spectrometry was used to probe the particles and their surface layer. To provide further insight into the chemistry occurring between ACN and aluminum under milling conditions, high-level ab initio theory was used to calculate the structures and energetics for binding and reactions of ACN and its fragments at different sites on an Al80 model surface.

I. INTRODUCTION

Aluminum nanoparticles are of interest as high energy density additives for propellants or explosives, with enhanced combustion rates relative to the micrometer scale aluminum conventionally used.16−29 There are elegant wet chemistry methods based on organoaluminum compounds that grow well-controlled, ligand-capped Al nanoparticles,30−32 but propulsion/explosive applications will require large volume production, thus methods that are efficient, use inexpensive feedstocks, and are easily scalable for large volume production. Reactant-assisted milling appears to be a good candidate, since it also allows the particle surfaces to be functionalized to modify particle reactivity and dispersibility in fuels or other media. Gayko et al.33 analyzed production of aluminum particles in the size range over 20 μm by milling in various solvents and found that the distributions had fewer large (>100 μm) particles for solvents that bind strongly to aluminum (i.e., nitriles, ketones, alcohols) compared to weakly interacting solvents (i.e., alkenes and aromatics), as might be expected from points raised above. Similarly, Kondis described the effects of milling of aluminum powder (∼4 μm) in solutions of milling agents (mostly oxygenated hydrocarbons) and found that

High-energy ball milling is an effective means for producing large quantities of nanoparticles by using mechanical energy to fracture, crush, or abrade feedstock, such as a powder of interest.1−3 The resulting size distribution is determined by the balance between processes like fracturing or crack propagation, which tend to reduce size, and cold-welding processes that lead to aggregation or fusion. For ductile or malleable metals, the balance does not favor nanoparticle production,4−6 unless a strongly adsorbing milling agent is used to enhance size reduction. As first proposed by Rehbinder in the early 20th century,7 binding of an adsorbate reduces the free energy required to create additional surface area and thus enhances crack formation and propagation. Furthermore, dangling bonds on surfaces are capped with adsorbates; this may reduce the tendency toward crack healing and particle aggregation by mechanical cold welding. The effects of solvent (i.e., adsorbates) on mechanochemical synthesis have been studied previously.2,8−10 Bowmaker11 recently reviewed mechanisms for solvent-assisted mechanochemistry, noting the effects of mass transport and reactant mobility as well as adsorbate binding energetics. Decomposition of solvents in reaction with the milled material has also been studied, though the phenomenon is not well understood.12−15 © XXXX American Chemical Society

Received: April 21, 2016 Revised: August 11, 2016

A

DOI: 10.1021/acs.jpcc.6b04054 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(DFT) was used to compute energetics for binding of ACN and various fragments to aluminum and the activation energies required to drive various decomposition reactions.

appropriate agents that might be expected to bind to Al (e.g., 2butanone) led to higher product surface area, corresponding to effective particle diameters near 300 nm.34 We recently reported two examples of nanoparticle production by milling powdered feedstock together with vapor-phase reactants, including boron milled with H235 and aluminum milled with NH3 and monomethylamine.36 Quantum chemistry calculations demonstrated that in these cases, the reactant molecules were likely to dissociate on the boron or aluminum surfaces, leading to strong chemisorption. Rapid size reduction into the nanoscale was observed in each case, which we attributed to a combination of strong chemisorption and the fact that small gaseous reactant molecules can diffuse rapidly to crack tips, facilitating crack propagation. In the case of hydrogen-capped boron is was possible to generate air-stable, unoxidized nanopowder by postmilling reaction of the B−H functional boron with alkenes, producing alkyl-capped particles.37 We recently reported a somewhat different approach, where rather than crushing powdered feedstock particles are produced by milling balls fabricated from the material of interest, such that nanoparticles are abraded or attrited from the ball surfaces.38 In the case of aluminum, we found that milling Al balls together with liquid agents or solutions of agents capable of binding to aluminum surfaces leads to rapid particle production, whereas milling in inert liquids (e.g., hexane) simply polishes the balls, generating no particles. A variety of molecules, and solutions thereof, were examined, and in each case the products were mixtures of nanoparticles and ∼10 μm aluminum flakes. The best reactant was acetonitrile (ACN), which led to rapid production of multigram quantities of a ∼ 1:1 mixture of nano- and micrometer particles. Of the nanoparticles, about 75% of the mass was in the 3−5 nm range, with additional size modes in the 25−50 and 250−500 nm range. Given that the particles were capped with oleic acid (after milling) in order to facilitate size separation and to suspend the nanoparticles for DLS analysis, the 3−5 nm size implies that the aluminum cores were quite small, indeed. This paper focuses on several aspects of aluminum nanoparticle production aided by reaction with ACN. We compare the size reduction obtained in liquid- vs vapor-phase ACN and report experiments and theory to help understand the process and the nature of the surface layer formed on the particles. Aluminum nanoparticle production in vapor-phase ACN is far more efficient than in liquid ACN, and the particle size distribution is different, with ∼95% of the mass in the range between 50 and 150 nm. This size range is large enough to have high oxidizable aluminum content but small enough to suspend well in liquid propellants. In addition, we compare mechanical attrition from aluminum balls vs milling of aluminum powder using tungsten carbide balls. Nanoparticle production is found to be efficient in either case; however, the chemical properties of the resulting particles are quite different. Mass spectrometry was used to probe the headspace in the milling jar to observe chemistry during milling and was also used to examine species desorbing from the surface of the particles when they were heated. The particle size distribution was measured with dynamic light scattering (DLS), scanning electron microscopy (SEM), and helium ion microscopy (HeIM), and the particle surface chemistry was probed by Xray photoelectron spectroscopy (XPS) and several types of Fourier-transform infrared spectroscopy (FTIR). To complement the experiments, high-level density functional theory

II. EXPERIMENTAL AND COMPUTATIONAL METHODOLOGY Particle Production. Most experiments were done using mechanical attrition to generate particles from the surfaces of aluminum balls, with either limited volumes of liquid ACN or purely vapor-phase ACN as the milling agent, contrasting with previous work in liquid ACN.38 As in that work, milling was done using a Retsch PM 400 planetary ball mill with Retsch 250 mL tungsten carbide jars. Replacement lids equipped with two valved ports were fabricated to allow the atmosphere inside the jars to be controlled and monitored.38 The valves can be connected to a gas/vacuum manifold inside the N2-filled glovebox used to load the jars and handle the resulting particles. The manifold is equipped with pressure/vacuum gauges for leak checking and measurement of milling-induced pressure changes and with vials that can be used to sample headspace gases during or after milling. The feedstock used in most experiments consisted of 4−8 mm diameter aluminum balls, and a typical loading was 200 g. When loaded, the headspace volume in the jar was ∼175 mL. Balls were cleaned before use and between milling runs by “wash milling” in liquid ACN under argon atmosphere. As discussed elsewhere,38 this process results in substantial attrition of the ball surfaces and therefore removes contaminants, including the initial oxide layer present on air-exposed aluminum. For some experiments, the balls were then polished by milling under argon, resulting in smooth, shiny surfaces. Cleaned balls were stored in a sealed jar in the glovebox; therefore, the feedstock is largely free of oxide and other contaminants that are unavoidable when milling airexposed feedstock. Milling in ACN Vapor only. To examine the milling mechanism in the absence of any liquid, experiments were done in which the jar was filled with the room-temperature vapor pressure of ACN (∼100 Torr = ∼13 kPa), corresponding to ∼1 mmol of ACN. The Al balls were first polished by milling under 60 psia (∼414 kPa) of argon, as shown in Figure S1a and S1d. In one set of experiments, the jar was evacuated, filled with ACN vapor, and then pressurized with 60 psia (414 kPa) of argon. The jar was then milled for either 30 s, 5 min, or 15 min. In the other set, pressurization by Ar was omitted, i.e., the jar was milled under just the vapor pressure of ACN. To build up enough particles for analysis, the evacuation/fill/mill procedure was repeated for a total of 4 cycles. Milling in Limited Volumes of Liquid ACN (LV-ACN). Because refilling the jar many times to build up a large mass of nanoparticles would be extremely tedious, we did most experiments using a limited volume of liquid ACN in the jar, which was completely consumed in the milling process. HPLCgrade ACN (Fischer Scientific) was degassed using a freeze− pump−thaw procedure. In early LV-ACN experiments, to avoid any possibility of reaction with glovebox N2 under milling conditions, the jar was sealed in the glovebox, attached to the gas/vacuum manifold, and evacuated and filled with argon (99.9999%, ∼60 psia, 414 kPa) three times to ensure complete removal of N2. The jar was then evacuated again, and 5 mL of degassed ACN was drawn into the evacuated jar through the second port. A 5 mL amount of ACN is ∼0.096 mol, i.e., ∼100 times the amount used in vapor milling. B


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In the present experiments, no HCN or other toxic products were observed in the headspace gases. We have seen evidence of HCN production when milling boron in the presence of nitriles. Because it is not clear if HCN production was due to boron or to the use a different milling procedure (boron powder was milled with WC balls in a SPEX 8000 mill) it would be prudent to test for HCN or other toxic products when milling aluminum with nitriles under conditions different from those used here. Analysis Methods. Particle sizes were characterized using HeIM, SEM, and DLS. HeIM was done on a Zeiss Orion Nanofab helium ion microscope using an electron flood gun to maintain charge neutrality. Samples were prepared for microscopy by dispersing the powder by ultrasonication in ACN, diluting until slightly turbid, and then drop casting onto a lacey carbon transmission electron microscopy (TEM) grid. SEM images were taken by a FEI NovaNano 630 highresolution scanning electron microscope. To improve dispersion, the SEM samples were ultrasonicated in a hexane solution of oleic acid and then drop casted on either lacey carbon or continuous carbon/Formvar film TEM grids (Ted Pella Inc.). DLS was performed using both a NICOMP 380 ZLS instrument and a Wyatt NanoStar DLS instrument (for particle sizes up to 5 μm). To improve dispersion for DLS, particles were functionalized with oleic acid by ultrasonication in a 5% solution of oleic acid in n-hexane. Particle suspensions were ultrasonicated just prior to analysis, and the concentration was adjusted until the dispersion was only slightly turbid. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra instrument using monochromatic Al Kα radiation (1486.6 eV kinetic energy) and an analysis area of 300 × 700 μm. Charging of the sample by X-ray irradiation was compensated by flooding the sample with low-energy electrons, tuned to minimize XPS peak widths. The as-milled powdered samples were prepared for XPS by taking dry powder directly from the jar in the glovebox and pressing it onto carbon tape affixed atop a stainless steel stub. Since the as-milled aluminum powder is highly reactive with oxidizers, we attempted to obtain XPS of an unoxidized sample by doing the transfer to the XPS instrument in a Kratos-supplied inert transfer device, modified as follows. The seals were remachined, and an improved mechanism for sealing the transfer compartment was made. In addition, a port was added to allow the compartment to be evacuated and pressurized to minimize air intrusion during transfer and to allow detection of leaks via a built-in pressure gauge. The transfer device was loaded in the N2-filled glovebox, then evacuated and backfilled with an overpressure of argon (99.9999%) using the manifold in the glovebox, transferred to the XPS instrument load lock chamber, and evacuated to allow sample transfer to the instrument manipulator. Mass spectrometry was used to analyze the headspace gases in the milling jar at intervals during milling, both to look for ACN consumption and to observe any gaseous products of milling. Headspace sampling was done by attaching the jar to the gas/vacuum manifold in the glovebox, leaking a small gas sample into a glass vial, and then returning the jar to the mill for further milling. The gas vial was then connected to the inlet system of a quadrupole mass spectrometer housed in a vacuum system with base pressure below 5 × 10−9 Torr and leaking the gases into the system at an ionization region pressure of ∼1 × 10−7 Torr, as monitored using an ion gauge. To calibrate mass spectrometer sensitivity vs mass and to measure cracking patterns for species of interest (ACN, ethane, methane, H2),

In later experiments, 5 mL of ACN was simply poured into the jar in the N2-filled glovebox, which was then purged of N2 by three cycles of evacuating for 2−3 s and pressurization to ∼60 psia (414 kPa) with Ar. In either case, the jar was finally pressurized to ∼60 psia (414 kPa) with argon, sealed, removed from the glovebox, and loaded into the mill. Finally, some LVACN experiments were also done simply using 1 atm of glovebox N2 as the milling atmosphere. In all cases, a sun wheel rotational frequency of 350 rpm was used, i.e., milling was done under a relative centrifugal force of ∼20 g. At the jar temperature (∼100 °C) reached during milling, the vapor pressure of ACN is ∼1.7 atm (∼170 kPa).39 A 5 mL amount of ACN, if all vaporized, would fill the jar to ∼7 times this pressure; thus, milling was initially under conditions of balls wetted with a small amount of liquid ACN, transitioning to milling dry in ACN vapor as the ACN was consumed by reaction/adsorption on the aluminum nanopowder product. The jar headspace was sampled hourly in one experiment for mass spectrometric analysis, demonstrating that complete ACN consumption occurred during the fourth hour of milling. In another, the jar was opened in the glovebox at hourly intervals during milling to allow visual inspection and sampling of the particles present, then resealed, evacuated, and repressurized with argon to continue milling. In both sampling experiments there would have been significant loss of ACN, and roughly 5 g of freely flowing, dry black powder was recovered after 4 h of milling, with additional powder adhering to the Al balls. In experiments where no sampling was performed, more than 10 g of powdered product was recovered, with additional powder left adhering to the surfaces of the Al balls. No significant differences were observed in either the amount or the appearance of the product from milling in Ar vs N2 atmosphere. We also did an experiment to determine how much ACN could be taken up by 1 g of aluminum under milling conditions. One gram of ∼1 mm aluminum flakes (Aldrich, 99.99%) was milled together with 120 g of 3 mm tungsten carbide (WC) media and 2 mL of ACN under Ar. After 3 h milling, the jar was opened in the glovebox, 2 mL of ACN was added, and milling was resumed for 2 more hours. This ACN addition/2 h milling process was repeated until the ACN was no longer consumed. Safety and Handling Considerations. Unpassivated aluminum nanopowder reacts violently with oxidizers such as O2 and H2O and autoignites and burns vigorously upon air exposure, as shown in Figure S2. Particles that are wetted by solvents may cause delayed ignition as they dry. Samples were handled and stored in a N2-filled glovebox; however, note that even ppm contamination of the glovebox atmosphere by oxidizers is sufficient to result in significant oxidization of highly reactive samples after a few hours exposure. For this reason, samples were stored in the glovebox but inside sealed vials to limit oxidizer exposure. Several procedures were used to passivate particles under controlled conditions. A thin layer of particles spread out in the glovebox overnight becomes sufficiently passivated that the particles no longer autoignite in air. Faster controlled oxidation was done in the glovebox load lock chamber, where samples could be exposed to a few Torr of air for a few minutes. Finally, particles could be stabilized by adding the nanopowder to hexane solutions of oleic acid in the glovebox, ultrasonicating to mix thoroughly. Regardless of treatment, it is prudent to handle these materials as if they were pyrophoric, until they have been shown to be safe by testing on small samples. Even after passivation, the particles are highly combustible. C


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could be separated by sedimentation into ∼1 g of black particles, shown by DLS to be primarily in the 1) species give mass 15:16 ratios much greater than 1. Given the absence of any evidence for

fragmentation pattern of ACN.48 Note that the parent ion at mass 41 has the highest intensity and thus provides a quick measure of the ACN concentration in the gas samples. Mass 42 is 13C-substituted ACN, and no higher mass ions were observed. From the experiment in which the jar was opened hourly for visual inspection, we know that in the headspace sampling experiment liquid ACN should have been present during the first 3 h of milling. In the spectrum for gases collected after 1 h, peaks associated with ACN increased, which we attribute to the fact that the jar was warm from milling when the gas was sampled, so that the vapor pressure of the liquid ACN was elevated. More importantly, there were changes indicating the presence of gaseous species other than ACN, i.e., substantial growth in the intensity for mass 2, a smaller increase for mass 16, and significant changes in the intensity distribution in the mass 24−30 range. After the second and third hours of milling, the ACN (mass 41) signal was significantly reduced and there was growth of the peaks at masses 15, 16, 27, 28, 29, and 30. Finally, after 4 h of milling, the mass 41 peak had completely disappeared, indicating that no ACN vapor remained in the headspace, i.e., the ACN had all reacted or adsorbed on the surface of the Al nanoparticles. Other than peaks due to argon, G


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reactions producing smaller amounts of CH4 and C2H6. During the final hour of milling as the liquid ACN is finally all consumed, there is also significant consumption of H2 and a sharp increase in production of methane and ethane. During this same milling period the rate of nanoparticle production increased substantially and micrometer size flakes produced when liquid ACN was still present were destroyed. Consumption of ACN might be expected to have several effects. First, there is loss of the liquid layer that coats surfaces in the mill, which has at least two “mechanical” effects. The liquid layer lubricates and moderates the intensity of collisions, forming a buffer layer between surfaces. It would not be surprising if such lubrication effects were significant; however, the fact that no particles are produced when balls are milled dry in Ar shows that chemical effects are critical. Evaporation of liquid also provides a mechanism for moderation of local high temperatures created during collisions, which have been estimated to reach as high as 2000 K.49 One might expect that collisions of low-density, ductile media like Al balls would generate lower peak temperatures; however, higher local temperatures after the liquid ACN is consumed may still explain the changes in reactions occurring in the final hour of milling, consuming H2 and generating methane and ethane. One question is the extent to which the final gaseous constituents (H2, CH4, and C2H6) themselves have an effect on milling. We previously showed that milling Al balls under pure H2 atmosphere results in neither H2 consumption nor particle production,36 i.e., H2 does not bind to aluminum under these conditions. We did not study Al milling under methane or ethane; however, we observed that milling Al balls in hexane results in no particle production,38 suggesting that alkanes are also inert with respect to Al surfaces under these conditions. In that case, it is not surprising that particles begin to be lost to aggregation and ultimately are reattached to the ball surfaces by cold welding if milling is continued after complete ACN consumption. F. Thermal Desorption Analysis. To probe the chemical nature of the particle surface layer we used a combination of TGA-MS and XPS. As noted above, we also tried IR spectroscopy in both ATR and diffuse reflection modes on several different instruments but were unable to obtain useable spectra. TGA-MS was used to measure the amount and identity of species that desorbed from the particles as they were heated to 900 °C at 10 °C/min. The TGA instrument is located in an N2-filled glovebox, and samples were transferred under N2. Experiments were run in which the furnace and balance regions were purged with either Ar or N2 in order to examine possible reactivity with nitrogen; however, there was some intrusion of glovebox N2 into the Ar flow, estimated mass spectrometrically to have been on the order of a percent (saturation of the Ar peak makes quantitation difficult). Experiments were run with different particle sample sizes and with different Ar purge conditions in order to vary the N2 concentration. Figure 5 compares the mass change vs temperature for samples run with N2 and Ar purge flows. The heating program included several minutes of hold time at 25 °C and again at 50 °C in order to desorb weakly adsorbed molecules. For the Arpurged sample there was essentially no mass change during these hold periods, but for the N2-purged run there was ∼0.2% mass loss as the sample was held in the N2 flow at 25 °C and a further ∼0.2% loss during the 50 °C hold time. The reason for the difference between the N2- and the Ar-purged runs is unclear, but the N2 run was done after the Ar run; thus, it is

production of water (mass 18) or ammonia (mass 17), the only reasonable assignment for mass 16 is to CH4+ generated by EI of methane, and methane can also account for most of the intensity in the mass range from 12 to 15. To put the analysis on a quantitative basis, we measured mass spectra for pure H2, CH4, C2H6, and ACN under conditions identical to those used for the headspace analysis in order to account for any variation with mass of the mass spectrometer’s sensitivity. By fitting the mass spectra after each hours milling to a linear combination of these candidate mass spectra (Figure S9), we extracted the mole fractions of each gas in the headspace, which are shown in Figure 4 (omitting Ar).

Figure 4. Mole percentages of the main constituents of the headspace gas as a function of milling time.

It can be seen that during the first hour of milling there was substantial H2 production, with ∼2−3% of both methane and ethane also produced (note ×5 scale factors). As shown in the raw spectrum (Figure 3), the actual partial pressure of ACN increased in the first hour spectrum; however, in terms of mole fraction, the ACN decreased to ∼50% because of the substantial new contribution from H2. During the second and third hours of milling, H2 production continued, albeit at decreasing rates. The raw intensities for methane and ethane both increased slightly during this period (Figure 3); however, the mole percentages were roughly constant due to the offsetting effects of H2 production and ACN consumption. In the final hour, the ACN was completely consumed and the remaining gas composition changed substantially. Both the absolute intensity (Figure 3) and the mole fraction (Figure 4) for H2 decreased, and those for methane and ethane increased. We have considerable experience with milling under H2 atmospheres35,37 and have shown that our jar retains H2 without significant loss at pressures up to at least 60 psia higher than total pressure in the present experiments. The final headspace gas composition was ∼70% H2, with 14−15% each of methane and ethane. Note that none of the mass spectra show any obvious signals attributable to nitrogen-containing products. The mass spectrometric data suggest that the following chemistry occurs during milling. As long as ACN is present to bind to fresh aluminum surfaces generated by milling, the dominant process appears to be chemisorption of CH3CN. Some or all of the chemisorbed ACN decomposes, mostly by dehydrogenation, generating H2 gas with but with other H


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energetic compounds, usually attributed to rapid gas generation.50,51 Reaction of Al with N2 would not, in principle, generate any gas; however, ΔfH0 for AlN is −318 kJ/mol;52 thus, it is not unreasonable that reaction of molten aluminum nanoparticles with N2 could become energetic enough to cause sudden evaporation of a small volume of Al or possibly to eject small fragments from the balance pan. The absence of this process in the Ar-purged run presumably reflects the slower reaction rates due to the low concentration of N2. An experiment where the furnace was Ar-purged under conditions expected to leave more N2 present also showed a single abrupt mass loss event (Figure S11). During TGA, 18 ion masses were monitored mass spectrometrically: 2, 15−18, 25−27, 29, 30, and 34−41. The raw data for all monitored masses are presented in Figures S12 and S13 for Ar- and N2-purged runs, respectively. Masses within 1 Dalton of N+ and N2+ had high background due to the presence of N2 in the purge gas, and the same issue affected detection of products near masses 36 and 40 in the Ar-purged experiments. By combining mass spectral data from the different experiments, all masses except 28, 36, and 40 could be monitored with reasonable sensitivity. For comparison, a blank experiment was run with an empty sample pan and Ar purge, and this data is shown in Figure S14. Some masses, corresponding to common hydrocarbon EI product ions, have background levels that rise with temperature in both the sample and the blank experiments, indicating that the instrument had significant hydrocarbon contamination. To correct the desorption signals we fit and subtracted smooth background curves using the blank data for guidance. H2 is a special case, because the mass spectrometer background at mass 2 is quite high and temperature dependent. Comparison with the blank experiment suggests that there is substantial H2 evolution during the sample runs; however, we felt that the background was too high to allow reasonable extraction of an integrated intensity. For other masses we believe that the background-subtracted integrated intensities have ∼10−30% uncertainty, depending on the signal and background levels for each mass. The following masses were observed to have significant background-subtracted signals. The numbers in parentheses are the relative integrated intensities: 2 (na.), 15 (69), 16 (100), 17 (2.8), 25 (3.9), 26 (30), 27 (40), 30 (10), 37 (0.3), 38 (0.7), 39 (2.1), and 41 (4.3). For mass 2, high background precludes giving an integrated intensity; however, it is clearly among the highest in the data set. The background-subtracted signals for a subset of the product masses are shown in Figure 6, including those with high intensities and representatives of all observed classes of temperature dependence. Unless noted, the data are from an Ar-purged run. Note that each mass has its own intensity scale, with masses 2, 15, and 16 having the highest intensities of the masses we were able to monitor. In the following analysis, we used standard EI mass spectral fragmentation patterns from the NIST database.48 Note that if a single neutral product fragments to form different mass ions, they all should have identical temperature dependences; however, because more than one neutral may contribute to each ion mass, the temperature dependences for those masses will also have contributions from each neutral precursor. We, therefore, focus on identifying the main desorption products. There are a number of molecules that one might expect to desorb from these particles, including H2, CH4, C2H6, and CH3CN (all seen in the milling headspace), but also other CxNyHz species that might form by decomposition of the

Figure 5. Mass change versus temperature for ∼5 mg samples of aluminum nanopowder produced by milling in ACN for N2 and Ar purging of the TGA instrument. (Inset) Detail of the rapid mass drop observed in the N2-purged run. Note that the Ar-purged run had N2 concentrations on the order of a few percent.

possible that there was more adsorption of solvents or other vapors present in the atmosphere of the TGA glovebox. To allow the mass changes for the two samples to be compared more easily, the results have been normalized to the masses observed just before the start of the main heat ramp. Additional TGA results are given in Figures S10 (different sample sizes) and S11 (different purge conditions). All closely reproduce the effects shown in Figure 5. For the Ar-purged experiment, significant mass loss began at ∼100 °C, with loss rate increasing up to ∼300 °C. Between 300 and ∼430 °C the mass decreased linearly with temperature before leveling out at 5.2% loss at ∼510 °C. At higher temperatures, the mass increased in two stages, as indicated by a factor of ∼4.5 change in slope at around 660 °C (∼0.014%/ degree below, ∼ 0.062%/degree above). This change in slope occurs just at the melting point of bulk aluminum (660 °C), suggesting that the slow mass gain between 510 and 660 °C is limited by diffusion and reaction in solid particles, followed by more rapid reaction after melting. At high temperatures, the mass continued to increase, with some rate slowing that presumably signals the formation of a passivating layer on the sample surface. In the N2-purged experiment, both the temperature dependence and the total mass loss below ∼450 °C was quite similar to the Ar results, indicating that reactions with N2 are unimportant below ∼450 °C. In pure N2, however, the onset of mass gain was at 488 °C, roughly 12 °C below that for the Ar-purged sample, and the mass gain vs temperature was faster and showed additional structure. From this comparison, we conclude that the mass gains observed in both experiments are due to reaction with N2. In pure N2, the initial rate of mass gain (at ∼520 °C) was ∼0.026%/degree, i.e., more than double that seen with the Ar purge, but the rate decreased to ∼0.009%/ degree in the temperature range around 575 °C. At ∼ 620 °C, the rate jumped by a factor of ∼15 to 0.144%/degree, dropping to ∼0.044%/degree in the range around 660 °C. The mass then decreased by ∼5% in a series of three sharp steps (inset to Figure 5) at 712, 716, and 718 °C, before resuming mass gain at a rate of ∼0.02%/degree. The abrupt, stepwise mass losses indicate that the reaction with N2 was vigorous enough to cause sudden loss of discrete amounts of mass, corresponding to 50− 100 μg/event. Sudden mass loss is frequently seen in TGA on I


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methane product. This methane contribution has been subtracted out in Figure 6 and in the integrated intensities given above. EI of higher hydrocarbons and also of ACN results in negligible fragmentation to mass 17; therefore, given the composition of the sample and absence of significant water desorption, the only reasonable assignment is NH3+ arising from ammonia desorption. This small desorption component has the highest observed onset temperature at ∼260 °C, and then falls slowly to baseline at ∼660 °C. The contribution from EI of NH3 to the much larger mass 16 signal is negligible. H2. Mass 2, despite the high background, is clearly one of the major signals, and EI of CH4, NH3, ACN, and hydrocarbons generates little or no mass 2 signal, accounting for at most ∼5% of the large mass 2 signal. We therefore attribute most of this signal to EI of H2 desorbing from the sample. Because the mass 2 background level is high (Figures S12−S14), it is possible that the high-temperature tail in the mass 2 data in Figure 6 is partly or entirely an artifact of inaccurate background subtraction. That said, it would not be surprising if dehydrogenation of a CxHyNn surface layer continued to produce H2 at high temperatures. C2H6. Mass 30 has substantial integrated intensity and a temperature dependence that is significantly different from those for all other masses, indicating that its molecular precursor is also different. This signal appears near 100 °C, peaks at ∼310 °C (well below the peaks for other masses), and drops back to baseline by ∼520 °C. Mass 30 has little or no low intensity in EI of CH3CN or of C3 or higher hydrocarbons,48 and given the prominence of ethane in the headspace gas (Figure 3) we attribute this signal to desorption of C2H6. EI of ethane would also contribute to masses 25, 26, and 27, accounting for ∼25−35% of these signals. In comparing mass 30 to other masses in Figure 6, note that its intensity in EI of ethane is only ∼26% of that for mass 28, which we cannot monitor due to interference from N2. Alkenes. Masses 41, 39, 38, 37, 27, 26, and 25 all have similar temperature dependences, rising slowly at ∼100 °C, peaking between 340 and 380 °C, and then going abruptly to 0 at ∼490 °C. The fact that masses 26 and 27 peak at somewhat lower temperatures than the rest of this group is expected, because EI of ethane contributes to these signals. The high intensities for 27 and 26 imply, however, that there must be another major product contributing, and the only real possibility is C2H4. Note that in EI of C2H4 the mass 27 and 26 peaks are only 62% and 53%, respectively, or the dominant 28 peak. For the higher masses (37−41), EI of propene gives a reasonable match to the observed intensities but would contribute only a few percent of the large 26 and 27 signals. While we did not monitor higher masses, it seems unlikely that C5 or larger hydrocarbons would be produced by heating of these samples, and other C3 and C4 hydrocarbons can be eliminated because they would give either substantial mass 29 signal (propane, butane, butene) or a large 39:41 ratio (propyne, butyne, butadiene), neither of which is observed. EI of ACN could also account for the small mass 41 signal, but only about one-half the signals at 39 and 38, and a negligible fraction of the mass 26 and 27 signals. To summarize, this “alkene” group of masses is reasonably fit by a combination of C2H4 and C3H6 in a roughly 85:15 ratio; however, we cannot rule out a small contribution from ACN, although theory suggests intact ACN would desorb at much lower temperatures (see below). Because of high H2 background and uncertainties regarding the mass dependence of the TGA-MS transmission, we cannot

Figure 6. Temperature scans for major masses observed in TGA-MS. All but mass 41 were measured with Ar purge. Top frame shows the mass loss (Ar purge) for comparison.

surface layer. The temperature dependences for the various masses can be divided into at least five groups, indicating that there are at least five main contributions to mass loss from these samples. CH4. Masses 15 and 16 have essentially identical temperature dependence, first appearing near 100 °C, coincident with the onset of mass loss (green vertical dashed line), peaking at ∼440 °C and then dropping to baseline near 580 °C. These are, other than mass 2, the highest intensity peaks in the mass distribution, and the mass 16:15 ratio is ∼1.4:1. EI of hydrocarbons other than methane generates relatively little CHx+ signal, and the mass 16:15 ratios range from near 0 to ∼0.2. EI of water would also contribute to mass 16; however, this is precluded by the absence of mass 18 signal. We conclude that the dominant contribution to the mass 16 and 15 signals is EI of methane and that methane desorption is a major source of mass loss during TGA. NH3. A small signal is observed at mass 17, only ∼15% of which can be explained by 13CH4+ from EI of the major J


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particle diameter to be ∼50 nm, with a surface area of ∼45 m2/ gram. Thus, the number of ACN molecules consumed in producing the particles amounts to ∼100/nm2a factor of 10 higher than the upper limit monolayer density. Similarly, in vapor-phase milling, the 0.9 mmol of ACN used would cover ∼56 m2 at monolayer density, corresponding to more than a gram of 50 nm particleswell over the actual amount produced. Of course, these estimates are rough, because the actual particles are not spheres, and have a size distribution. There probably is also some ACN trapped in internal surfaces of fractured or cold-welded particles. Nonetheless, ACN bookkeeping suggests that particle production consumes well over a single monolayer’s worth of ACN. The TGA and mass spectrometry results provide additional insight. In the headspace analysis, no ACN remains at the end of milling and no N-containing gaseous products are observed. In TGA, the samples lose ∼5% of their mass, primarily as CH4 and C2Hn, with little loss of N-containing species. The surface layer composition, thus, must include all of the nitrogen in the ACN, most of which remains on the particles during pyrolysis. The surface layer must also have a CxHy component that pyrolyzes to generate the dominant desorption products. For 50 nm diameter Al particles, 5% mass loss would require ∼1 nm effective thickness of pyrolyzable material (with density typical of a hydrocarbon). Given that some of the hydrocarbon constituent of ACN is already lost during milling, the TGA results also clearly support the idea of a surface layer that is much thicker than one monolayer. The only piece of evidence that seems contrary to this conclusion is the observation that the particles are high pyrophoric. Apparently, this surface layer, although relatively thick, is highly reactive in contact with air. X-ray photoelectron spectroscopy (XPS) provides additional insight into the particle surface composition. We made several attempts to avoid oxidation of the highly reactive aluminum nanoparticles by transferring samples to the XPS instrument under an overpressure of argon; however, as the results show, it proved impossible to prevent oxidation. Indeed, little difference was observed between the results for the Ar-transferred sample and a second sample that was exposed to air in the process of transfer. The problem is that sample recovery from the mill jar, mounting onto an XPS sample holder, loading into the transfer device, transport across campus, and insertion into the XPS instrument together result in at least several hours of exposure to “inert” atmospheres. If those atmospheres contain even 1 ppm of O2 or other oxidizer, the samples get ∼107 Langmuir exposure, which apparently suffices to oxidize the surface layer. Figure 7 shows a survey scan for a sample prepared by milling in argon. The gold signal is the result of our having adding a few milligrams of gold powder to the sample as a binding energy reference for charging correction. Note that the sensitivity factors for the Al 2p, N 1s, O 1s, and C 1s peaks are 0.193, 0.477, 0.711, and 0.296,54 i.e., the signals for O and N would be exaggerated relative to that for Al by factors of 3.7 and 2.5, respectively, even if the sample were homogeneous. In fact, we expect that most of the Al is in the particle bulk, while N, O, and C are present mostly in the particle surface layer, with higher XPS detection efficiency. Nonetheless, it is clear that the sample is significantly oxidized and that there is also significant nitrogen content, as expected from the headspace mass spectral measurements, which show that the nitrogen in the ACN remains on the particles. Significant C 1s intensity is also expected from the considerations discussed above.

quantitatively assign the 5.2% mass loss to specific species. Qualitatively, the mole fractions desorbing from these samples appear to be, in descending order, H2 ≥ CH4 > C2H4 ≈ C2H6 ≥ propene ≫ NH3. The low molecular weight of H2 means that it contributes little ( 500 °C is H2; however, the low molecular weight means that contribution of the H2 high-temperature tail to the total mass loss would be negligible. About 40% of the NH3 also desorbs above 500 °C; however, the intensity for this product is so low that it also corresponds to a negligible fraction of the total mass loss. The only major “heavy” product with significant desorption above 500 °C is methane, but its high-temperature component constitutes only ∼5% of the total methane desorption. Taken together, we can estimate that the measured 5.2% peak mass loss accounts for at least 95% of the total mass loss. G. Particle Surface Layer. There are several pieces of information indicating that the surface layer is not simply a single layer of chemisorbed ACN or decomposition products thereof. First, the consumption of ACN in both the LV and the vapor-milling experiments is much larger than required to generate a single monolayer on the surface of the resulting particles. The maximum monolayer packing density for ACN/ aluminum is unknown; however, from considerations of the Al lattice spacing (0.286 nm53) and the calculated geometry of chemisorbed ACN (see below), we can estimate an upper limit on the monolayer density to be roughly 10 molecules/nm2. In that case, the 5 mL of ACN used in the LV-ACN experiments would cover a surface area of at least ∼6 × 103 m2. Over 10 g of particles is recovered in those experiments, with additional powder adhering to the ball surfaces; thus, total production might be generously estimated as ∼12 g. From DLS and SEM we can estimate the surface-area-weighted average effective K


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The Al 2p spectrum also has a feature just below 72 eV, which is as the low end of the 72.0−72.9 eV range expected for metallic aluminum.54,58,59,63−65 Given the fact that the particles are pyrophoric, we attribute this signal to unoxidized Al0 in the core of the particles, with binding energy somewhat lower than expected, most likely due to overcompensation for sample charging. There was no significant difference in the Al3+/Al0 ratio for the samples transferred under Ar and with air exposure, indicating that adventitious oxidizer exposure during transfer and handling was sufficient to fully passivate the Al surface. The only significant difference was a slight attenuation of the entire Al 2p signal for the air-exposed sample, attributed to adsorption of adventitious species on the sample surface. The N 1s spectrum is dominated by a peak at 398.63 eV, with a tail extending to higher binding energy. The peak is at the high end of the range expected for metal nitrides54,59,60 and about 1 eV above the binding energy reported for N 1s in AlN (397.361), presumably reflecting oxidation during transfer to the XPS instrument (e.g., nitrites have N 1s near 404 eV54). The tail to higher binding energies could also reflect oxidation of the nitrogen, but organic nitrogen is also in this range.66−68 The C 1s spectrum shows a peak at ∼285 eV with a substantial tail to higher binding energy, fit by a second component peaking near 287 eV. The fact that the sample was exposed to acetonitrile and other solvent vapors in the glovebox is expected to lead to adventitious carbon signal, which is often in the 285 eV range for sp2-hybridized carbon55,69−74 but also for C bound to N (285−288 eV54) or C bound to O (∼286.554). The important observation is that there is no significant signal at the energy expected for aluminum carbide (282.4 eV62). The O 1s spectrum has a single broad peak centered at 531 eV, in the range expected for aluminum oxides,55 consistent with formation of an oxidized surface layer during transfer to the XPS instrument. Finally, the integrated intensities of the fitted Al3+ and Al0 features are in a ∼4.8:1 ratio, which can be used to provide another estimate of the thickness of the oxidized surface layer. The effective attenuation length (EAL) for aluminum photo-

Figure 7. XPS survey of aluminum particles transferred under argon, mixed with gold powder.

Figure 8 compares high-resolution scans of the Al 2p, N 1s, C 1s, and O 1s regions for the sample transferred under Ar. First, consider the Al 2p spectrum, shown at the top left, which is dominated by a peak at ∼74 eV. The Al 2p binding energy range typically reported for Al3+ in Al2O3 is 73.6−74.7 eV;55−58 however, the Al 2p binding energy reported for air-exposed (i.e., surface-oxidized) aluminum nitride is also reported to be in the 74 eV range,58−61 and the Al 2p binding energy for Al4C3 is 73.6 eV.62 Mass spectrometry shows that nearly all of the nitrogen in the ACN remains on the Al nanoparticles, and TGA-MS shows that the nitrogen is largely retained during heating, consistent with there being strong nitride-like Al−N bonding. There may also be some carbide-like Al−C binding; however, the observation in both headspace analysis and TGAMS is that there is copious H2 and CxHy desorption, suggesting that much of the carbon and hydrogen is in easily pyrolyzable form. The theory discussed below provides additional insight. The XPS results, together with the fact that fresh samples are highly pyrophoric, suggest that the particles are capped with a reactive AlNnCmHx layer that converts to a passivating AlNnCmOpHx layer upon exposure to even low concentrations of oxidizers during the inert transfer process.

Figure 8. High-resolution XP spectra of the Al 2p, C 1s, N 1s, and O 1s regions for an ACN-milled aluminum nanoparticle sample transferred under argon. L


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The Journal of Physical Chemistry C electrons passing through alumina is ∼2.8 nm,75 and the EAL for passage through an AlNnCmOpHx overlayer should be similar. To extract a layer thickness, we also need to know the Al number density in the AlNnCmOpHx layer, and for want of other information, we approximate this by the density of Al in alumina (0.0473 Al/Å3). If the sample were a flat aluminum surface, a ∼5.1 nm thick AlNnCmOpHx overlayer would give the observed Al3+:Al0 intensity ratio, but for nanoparticles, signal from sides/edges and asperities enhances the sensitivity to the surface layer. For spherical particles with a uniform surface layer, it is straightforward to construct an EAL-based model in which the Al3+:Al0 ratio is averaged over the XPS-detectable region of the sphere, and for 60 nm spherical particles (Figure 1), the overlayer thickness would be ∼3.5 nm. To the extent that the particles are nonspherical, this should tend to further reduce the overlayer thickness required to account for the experimental Al3+:Al0 ratio. Taken together, the XPS, ACN consumption, and TGA mass loss results all show that the particle surface layer is relatively thick, with initial AlNnCmHx composition that oxidizes readily in exposure to air or adventitious oxidizers, igniting or passivating the particles, respectively. H. Computational Results. CH3CN + Al80. Acetonitrile (ACN) binds weakly to the surface of the Al80 cluster, generating a molecularly chemisorbed complex, henceforth denoted as ACN:Al80. In order to assess the variability of the binding energies as a function of specific binding site, four distinct configurations of ACN chemisorbed to Al80 were computed. These local minima have Al−N distances ranging from 2.05 to 2.65 Å with binding energies ranging from 6 to 10 kcal mol−1 relative to separated Al80 + ACN and are shown in Figure S15. ACN should desorb from such complexes via firstorder kinetics, with rate constant k = A·exp(−Eads/RT),76 and for any reasonable assumption for A, the desorption lifetime would be below 20 μs at room temperature. As a result, we conclude that the pyrolysis products observed during TGA-MS must originate from a much more stably, dissociatively bound precursor. Fragmentation of CH3CN. As an initial investigation into the potential chemical reactivity of ACN:Al80, the energetics were computed for C−H and C−C bond dissociation and subsequent covalent binding of the resulting fragments to Al80. As shown in Figure 9, two local minima derived from formal C−H bond dissociation in chemisorbed ACN were found, one of which is 3 kcal mol−1 less stable (Figure 9a) and the other 1 kcal mol−1 more stable (Figure 9b) than separated ACN + Al80, i.e., they are less stable than molecularly adsorbed ACN:Al80. A transition state was located connecting the ACN:Al80 isomer in Figure S15c with the CH2CN− Al80−H local minimum in Figure 9a, as shown in Figure S16. The barrier for this reaction is 24 kcal mol−1, which is higher than the energy required for simple desorption of ACN but nonetheless should be energetically accessible under the elevated temperatures present during ball milling. In contrast to the relatively high energies of the C−H dissociation products, the chemisorbed products of fragmentation of the C−C bond in ACN:Al80 (i.e., H3C−Al80-CN, Figure 9c) are lower in energy than separated ACN + Al80 by 14 kcal mol−1, i.e., they are more stable than chemisorbed ACN:Al80. Relative to ACN:Al80, C−C bond dissociation is energetically favored over C−H fragmentation, with the former process exothermic by 4−8 kcal mol−1 and the latter endothermic by 5−13 kcal mol−1. Finally, several isomerization/rearrangement

Figure 9. DFT-optimized structures of H2CCN−Al80−H (a and b) and H3C−Al80−CN (c.) Energies (in kcal mol−1) are relative to separated ACN + Al80 and include ZPE corrections. A portion of the Al80 substrate has been cropped in order to show the chemisorbed species in greater detail.

reactions of chemisorbed ACN are discussed in the SI and shown in Figures S17−S20. These results show the presence of low-barrier (14−16 kcal mol−1) pathways for the formation of CH3−CN−Al80 in which both the nitrile carbon and the nitrogen are bonded to the Al80 surface, although the barriers are still larger than the energy required for simple desorption of ACN (6−8 kcal mol−1.) In contrast, formation of CH2−CH N−Al80 (see Figure S20) must cross a barrier of nearly 60 kcal mol−1 and thus is unlikely to occur to a significant extent. Scission of the C−C bond to form H3C−Al80−NC (see Figure S21) likewise has a large barrier of 44 kcal mol−1, although the overall reaction is exothermic by 22 kcal mol−1. Formation of H2. Several pathways leading to formation of H2 were considered. The simplest route, recombination of neighboring chemisorbed H atoms, was reported in our recent study of NH3 and CH3NH2 reactions on aluminum38 to have a barrier of 25 kcal mol−1 and to be endothermic by 4 kcal mol−1. A “bimolecular” elimination of H2 from adjacent chemisorbed ACN molecules to form H2CCN−Al80−NCCH2 + H2 is endothermic by 13−15 kcal mol−1, as shown in Figure 10a, 10b1, and 10b2 and also Figure S22. Note that the bimolecular H2 elimination could lead to formation of the local minimum shown in Figure 10b1 or to the slightly more stable conformation depicted in Figure 10b2, but the barrier for this process is unknown. Figure S23 illustrates a second bimolecular pathway via elimination of H2 from two chemisorbed methyl fragments, which is endothermic by 28 kcal mol−1 and consequently is less favorable than recombination of chemisorbed H atoms, which has a barrier of 25 kcal mol−1. The final mechanism considered is elimination of H2 from adjacent chemisorbed CH3 and H species (i.e., H3C−Al80−H → H2C− Al80 + H2,) for which two distinct pathways were found, as shown in Figures S24 and S25. These reactions are endothermic by 18 and 20 kcal mol−1, respectively, with corresponding barriers of 60 and 46 kcal mol−1. Because these reactions are unlikely to occur to a significant extent due to the large barriers, IRC calculations were not performed. Therefore, of the four pathways of H2 formation considered here, M


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state shown in Figure S29 connects the indicated reactants and products. In summary, C−C bond cleavage to form chemisorbed CH3 and CN is energetically more favorable than C−H fragmentation in chemisorbed ACN. Isomerization of ACN:Al80 to form CH3−CN−Al80, in which both the nitrile carbon and the nitrogen atoms are bonded to Al80, is an exothermic process with a barrier of only 14−16 kcal mol−1. Multiple mechanisms of formation of H2 were considered, and all were found to be endothermic. Although recombination of chemisorbed H atoms is the least endothermic pathway to H2 (4 kcal mol−1), the corresponding barrier is 25 kcal mol−1. Bimolecular elimination of H2 from neighboring chemisorbed ACN molecules is endothermic by only 13−15 kcal mol−1, although the barrier for this process is unknown. Elimination of H2 from adjacent chemisorbed CH3 and H is endothermic by 18−20 kcal mol−1 and crosses a large barrier of 46−60 kcal mol−1. Three pathways leading to CH4 were considered, and all were found to be endothermic. The recombination of chemisorbed CH3 and H is endothermic by only 3 kcal mol−1 but crosses a barrier of 43 kcal mol−1, whereas bimolecular elimination of CH4 from two chemisorbed ACN molecules is uphill in energy by 6 kcal mol−1 but for which the barrier is unknown. Formation of CH4 via H atom transfer between chemisorbed methyl groups is uphill in energy by 14 kcal mol−1, with a barrier of 48 kcal mol−1. Bimolecular elimination of CH3CH3 from chemisorbed ACN molecules is exothermic by 4 kcal mol−1, whereas the recombination of two chemisorbed methyl groups is endothermic by 7−15 kcal mol−1. Formation of HCN and NCCN, which are not observed to be significant products, either during milling or in TGA-MS, is predicted to be endothermic by 20 and 48 kcal mol−1, respectively. The corresponding barriers are 31−37 kcal mol−1 for HCN formation and 50 kcal mol−1 for generation of cyanogen. In considering the theoretical energetics together with the headspace and TGA mass spectrometry results, it is important to keep several points in mind. It was only feasible to calculate energetics for one or two adsorbate species on the Al80 cluster, whereas under milling conditions, the surface is certainly saturated with adsorbates, and it is not unlikely that the energetics are affected by both adsorbate−adsorbate interactions and adsorbate effects on the structure and electronic properties of the aluminum surface. In addition, milling collisions may generate instantaneous, localized high temperatures that may drive reactions that would not occur thermally.

Figure 10. DFT-optimized structures of H3CCN−Al80−NCCH3 (a) and bimolecular elimination products H2CCN−Al80−NCCH2 (b1 and b2), CN−Al80−NCCH2 (c), and CN−Al80−NC (d.) Energies (in kcal mol−1) are relative to separated 2ACN + Al80 and include ZPE corrections. Portions of the Al80 cluster have been cropped to show the chemisorbed species in greater detail.

recombination of chemisorbed H atoms is the most favorable (i.e., least endothermic) route. Formation of CH4 and CH3CH3. As reported in our recent study,38 the formation of CH4 from the recombination of chemisorbed H and CH3 fragments is calculated to be endothermic by only 3 kcal mol−1, but this reaction traverses a barrier of 43 kcal mol−1. An alternative route is the elimination of CH4 from neighboring chemisorbed ACN molecules, i.e., CH3CN:Al80:NCCH3 → CN−Al80−NCCH2 + CH4. As shown in Figure 10a and 10c this pathway is endothermic by only 6 kcal mol−1, although the barrier for this reaction is unknown. Another pathway to formation of methane is by H atom transfer between adjacent chemisorbed methyl groups, i.e., CH3−Al80−CH3 → CH2−Al80 + CH4. As shown in Figure S26, this reaction is endothermic by 14 kcal mol−1 and must cross a barrier of 48 kcal mol−1. Similarly, the formation of ethane from the recombination of two chemisorbed methyl groups was calculated to be endothermic by 7−15 kcal mol−1.38 The analogous bimolecular elimination of ethane from two chemisorbed ACN molecules, as shown in Figure 10a and 10d, is found to be exothermic by 4 kcal mol−1. However, the barriers for these reactions are unknown. Formation of HCN and NC−CN. The simplest route to formation of HCN is via recombination of neighboring chemisorbed H and CN fragments. Two reaction pathways for this process have been found and are illustrated in Figures S27 and S28. Interestingly, the nitrile group is predicted to bind to Al80 via either the carbon (Figure S26a) or the nitrogen atom (Figure S27a) with essentially identical binding energies. Additionally, both atoms may bind to Al80, with the C−N bond axis parallel to the cluster surface, but this configuration (not shown) is ∼10 kcal mol−1 less stable than the singly coordinated isomers. As shown in Figures S27 and S28, these pathways are endothermic by ∼20 kcal/mol and cross barriers of 37 and 31 kcal mol−1, respectively. Formation of cyanogen (NC−CN) from the recombination of neighboring chemisorbed nitrile groups (shown in Figure S29) is endothermic by 48 kcal mol−1 and has a barrier of 50 kcal mol−1, although IRC calculations were not performed to show that the transition

IV. SUMMARY AND CONCLUSIONS The results above have focused on Al nanoparticles generated by milling in either vapor or limited volumes of ACN, and the conclusions about particle properties and the production mechanism are clearly limited to the conditions studied. The main findings are as follows: Milling aluminum balls in inert gas or liquid results in polishing, with no particle production. In vapor-phase ACN, there is rapid particle production; however, the small amount of ACN available limits particle production. Milling in a small volume of liquid ACN leads to efficient production of aluminum nanoparticles, consuming the ACN. When liquid ACN is present, both micro- and nanoscale particles are produced, and during this phase the main product detected in the headspace is H2. When only vapor-phase ACN is present, the rate of particle production increases, most of the micrometer size particles are destroyed, presumably by N



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conversion to nanoparticles, and the composition of headspace products shows consumption of H2 with concomitant increases in production of methane and ethane. Under no conditions was there evidence for N-containing products in the headspace, and since the final product is a dry powder, the implication is that the nitrogen in the ACN reactant all ends up on the powder. Clearly, ACN is an effective aluminum-milling agent, binding to and decomposing on fresh surfaces, thereby lowering the energy required to create new surface area. The results for ACN vapor diluted in Ar indicate that an important factor in efficient size reduction is the ability of the milling reactant to quickly reach spreading crack tips. This observation may explain why small molecule reactants, like ACN, are considerably more efficient at promoting size reduction than large capping molecules such as oleic acid.38 The imaging results suggest that the nanoparticles have flattened, plate-like structure. DLS shows that most of the mass is in the form of particles in the 50 and 150 nm size range, in reasonable agreement with sizes inferred from HeIM and SEM, when shape is taken into account. ACN consumption and TGA all suggest formation of a relatively thick layer on the particle surfaces with AlNnCmHx layer composition. During TGA the particles lose ∼5% of their initial mass, mostly as H2 and small hydrocarbons, before undergoing rapid mass gain above 500 K due to reaction with N2. Any O and most of the N in the capping layer is retained on the particles as the layer decomposes. Nonetheless, exposure of freshly milled samples to air results in vigorous ignition, and samples exposed to low levels of oxidizers in the glovebox atmosphere slowly become passivated. XPS of passivated samples shows that the particles have metallic aluminum (Al0) cores, with oxidized Al3+ in the surface layer. The binding energy for this Al3+ is in the ranges reported for Al nitrides, oxides, and carbides, and XPS also confirms the presence of substantial N, O, and C concentrations in the surface layer, i.e., the capping layer has AlN n C m O p H x composition, and the Al3+:Al0 ratio suggests a thickness of a few nanometers for the passivated particles, consistent with the thick surface layer indicated by ACN consumption and TGA mass loss. DFT calculations predict that ACN adsorbs molecularly on aluminum but with binding energies of only 6−10 kcal mol−1 and that the energetically most favorable reaction pathway of a single chemisorbed ACN molecule is cleavage of the C−C bond to form chemisorbed CH3 and CN fragments (4−8 kcal mol−1 exothermic, with unknown barrier height). In contrast, C−H fragmentation to produce chemisorbed CH2CN and H species is 7−13 kcal mol−1 endothermic and must traverse a barrier of 24 kcal mol−1. Isomerization of ACN:Al80 to structures in which both the carbon and the nitrogen atoms of the nitrile group are bonded to the Al cluster is approximately thermoneutral, with small barriers of 14−16 kcal mol−1. The DFT predictions suggests that while initial binding of ACN may be molecular, the strongly bound capping layer must result from dissociation processes, probably initiated by C−C bond scission in adsorbed ACN. This conclusion is supported by the fact that we do not observe desorption of intact ACN to an appreciable extent in TGA-MS but rather see mostly H2 and hydrocarbons like CH4 and C2Hn desorbing at temperatures well above the range where one might expect physisorbed molecules to desorb.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04054. Photos relating to nanoparticle production and combustion, SEM micrographs, headspace mass spectral analysis, additional TGA, raw TGA-MS data, and additional images of theoretical structures of stable and transition structures relating to reaction of Al80 with ACN; raw data for the graphs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 801-585-7289. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Air Force Office of Scientific Research under AFOSR BRI Grant FA9550-12-10481 and FA9550-16-1-0141. XPS was done using University of Utah shared facilities of the Micron Microscopy Suite sponsored by the College of Engineering, Health Sciences Center, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah. HeIM was done by Dr. Doug Wei of the Carl Zeiss Microscopy, Ion Microscopy Innovation Center. The attempt to obtain FTIR spectra of the samples at Argonne National Lab was made by Bo Hu and Tianpin Wu at the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. A grant of computer time from the DoD High Performance Computing Modernization Program at the five DoD Supercomputer Resource Centers (Air Force Research Laboratory, Army Research Laboratory, Engineer Research and Development Center, Maui High Performance Computing Center, and Navy) is gratefully acknowledged.

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REFERENCES

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Aluminum Nanoparticle Production by Acetonitrile-Assisted Milling

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