Article pubs.acs.org/crystal
Reducing the Size of Nanocrystals below the Thermodynamic Size Limit J. W. Drazin,† D. A. Kazerooni,‡ E. P. Gorzkowski,‡ C. R. Feng,‡ S. B. Qadri,‡ R. Goswami,‡ B. N. Feigelson,‡ and J. A. Wollmershauser*,‡ †
American Society for Engineering Education at U.S. Naval Research Laboratory, Washington, D.C. 20375, United States U.S. Naval Research Laboratory, Washington, D.C. 20375, United States
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ABSTRACT: In material systems with competing phase stabilities, simple growth techniques for nanocrystal synthesis are generally restricted to producing the thermodynamically lowest energy crystal structure. Alumina is one industrially and scientifically relevant example where the excess enthalpy of high specific surface area structures (i.e., nanocrystals) forces synthesized nanocrystals to adopt crystal structures that deviate from those observed in large crystals and have very different properties. Prior room temperature thermodynamic calculations determine that alumina nanocrystals smaller than 12 nm are either γ or amorphous, and numerous experimental works confirm the difficulty of producing α-alumina nanocrystals smaller than ∼15 nm. In this work, a direct and simple, high-energy processing route was developed in order to reduce as-grown ∼50 nm α-alumina starting crystallites down to sub-10 nm crystallites; below the room temperature thermodynamic crossover size limit. The nanocrystals were subsequently washed to remove WC-Co contamination on the nanocrystal surfaces incurred during the high-energy process. The smallest crystallite size of high-energy processed powder was 8.7 nm. High-resolution TEM establishes that the individual crystallites have different crystallographic orientations with nominal sphericity revealing a truly nanocrystalline powder. The developed processing route provides an industrial and scalable procedure that provides a new avenue for nanocrystalline α-alumina synthesis and, potentially, other metastable nanocrystals.
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very small (on the order of a few nanometers).4 Similarly, zirconia can be forced into the cubic structure without dopants when the as-grown crystal size is 98%, Sigma-Aldrich, St. Louis, MO) was dissolved in deionized water to form a clear, dilute solution. The cationic solution was then added dropwise to a stirred 5 M excess ammonium hydroxide solution where aluminum hydroxide formed as a white precipitate. The final mixture was washed with 190 proof denatured ethanol and centrifuged (5000 rpm for 4 min) three times to remove and replace the excess ammonia solution. The white precipitate was then dried at 70 °C for 48 h. The dried powder was heated to 1200 °C and held at temperature for 2 h to calcine the hydroxide to α-alumina with a crystallite size of 50 nm. It is noted that ∼50 nm was the smallest possible crystallite size that we were able to produce in the lab such that the powder was 100% αalumina. Unfortunately, all attempts at seeding the synthesis with 50 nm α-Al2O3 powder (as reported by Li and Sun35) were unable to lower the calcining temperature or the crystallite size. Therefore, it was decided to take the smallest crystallite size producible (and commercially available) and reprocess the powder by shearing/ grinding the agglomerates and crystals as a direct synthesis approach is limited by thermodynamics. A commercial (99.9+% Stanford Advanced Materials, Irvine, CA) α-alumina powder was purchased to compare with the lab-synthesized powder. The alumina powder was ball milled in a SPEX 8000M using a cobalt cemented tungsten carbide vial in 3 g batches. The milling media consisted of four 6 mm and two 12 mm WC balls (WC-Co, McMaster-Carr, Elmhurst, IL) giving a ball to powder mass ratio (BMR) of ∼10:1. The batches were milled in 30 min intervals and reoriented at each interval to ensure that the powder did not preferentially collect inside the vial during milling. After ball milling, the powder was washed using a 5% (v/v) 15.4 M HNO3 in 30% H2O2 solution to dissolve the residual WC-Co contamination from ball milling similar to a previous work.36 The dissolution was performed at 95 °C, while under magnetic stirring and finished in under an hour. The mixture was then washed with 190 proof denatured ethanol three times. X-ray diffraction (XRD) analysis was performed on a 18 kW rotating anode Rigaku X-ray Diffractometer (Rigaku, Tokyo, Japan) operated at 50 kV and 200 mA using a copper target. The crystallite size and lattice parameter were calculated using JADE 9.6v software to
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RESULTS AND DISCUSSION XRD analysis of the lab-synthesized and commercial powders shows that high-energy ball milling effectively reduces the crystallite size of the powders while retaining the alpha crystal structure. Figure 2 shows the X-ray diffraction patterns of the lab-synthesized and commercial powder before and after milling for 270 min. The starting crystallite sizes for the labsynthesized and commercial α-Al2O3 are 50 and 60 nm, respectively, as determined by diffraction peak broadening analysis. XRD of the alumina samples after ball milling for 270 min shows no new alumina crystal structure peaks, and diffraction peak broadening analysis determines that the crystal size is below 10 nm for both powder sources. The XRD analysis clearly shows the milled powders retain their alpha structure even though the crystallite size was reduced below the thermodynamic crossover limit; 9.6 and 8.7 nm for the labsynthesized and commercial powder, respectively. The powders, which were white before milling, developed a greyish color after milling. The color was the result of the WC-Co milling media wearing and contaminating the surface of the nanopowder. This contamination is also observable in the diffraction pattern. The major peaks seen at 31.5°, 48.2°, 64.2°, and 73.2° 2θ in Figure 2b are associated with the WC structure. In order to remove the WC contamination, a modified washing procedure was employed using nitric acid and hydrogen peroxide. The hydrogen peroxide oxidized the WC such that the product was dissolvable in nitric acid.36 With the low concentrations of cobalt cement used, nitric acid was sufficient to prevent a cobalt passivation layer from forming; however, aqua regia, instead of nitric acid, could also be employed with excess cobalt media. After acid washing, the powders returned C
DOI: 10.1021/acs.cgd.6b01748 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Interestingly, the commercial powder appears to accumulate WC contamination at an earlier milling time, and to higher degree, when compared to the lab-synthesized powder. An explanation for the difference in XRD observed contamination levels at similar crystallite sizes could be due to the agglomeration state of the powders: i.e., the amount of free surfaces in the powder. BET analysis shows that the final labsynthesized and commercial powder, after milling and washing, have surface areas of 62.101 ± 0.217 and 42.983 ± 0.120 m2· g−1, respectively. Therefore, the WC surface contamination is distributed over different surface areas, and given that the rate of reduction in crystallite size is similar between the two powders, it is probable that the WC accumulates at similar concentrations per unit time. In such a case, the higher specific surface area of the lab synthesized powder will allow thinner WC surface layers compared to the commercial powder. Consequently, the XRD line scan perceives a smaller local diffracting volume of WC contamination in the lab synthesized powder and the XRD calculated concentration profile lags behind the commercial powder. It is pointed out that though the final surface areas of the synthesized powders are not in agreement with theoretical specific surface area determinations (6000/density/crystal size), the final surface area represents roughly a 500% increase in surface area (initial surface area of lab-synthesized α-Al2O3 is ∼12 m2·g−1 due to high calcination temperature). The lower surface area than theoretically expected is likely due to the fact that the starting powder was heavily agglomerated and ball milling is not completely effective at breaking all hard agglomerates under the current conditions. As a result, a tail in the size distribution toward larger crystal sizes may persist. Regardless, this is the first time that a simple procedure, with high yield, has demonstrated the feasibility of producing α-Al2O3 with high specific surface areas and crystallite sizes at or below the thermodynamic crossover limit. Also available by diffraction-based analysis is the lattice parameter evolution. Figure 4 shows the evolution of the lattice parameters of the alpha alumina with increasing milling time (decreasing crystallite size in the figure). Both lattice directions are shown to increase with milling time. At the smallest crystallite size, the volume increase of the unit cell is ∼0.5%. Ball milling studies on other oxide ceramics indicate that a small
to their bright white color, and the XRD of the powders after acid washing, shown in Figure 2 (in blue), no longer host WC peaks and show no phase or crystallite size change of the alumina. X-ray fluorescence (XRF) analysis of the powder after washing showed W and Co elemental concentrations at lower levels than impurity elements, which were introduced during powder synthesis (i.e., W and Co contamination levels are lower than Ga and Fe, which are present at ∼≤0.002%). The elimination of the WC contamination peaks from the diffraction pattern, low level of cobalt and tungsten in the XRF spectra, and the return of the powder to a bright white color signify that the washing was sufficient to remove the WCCo contaminate. Figure 3 shows that there is a practical limit to the minimum obtainable crystallite size with the current high-energy ball
Figure 3. Crystallite size and WC-Co contamination of the powders as a function of milling time. Data points indicated by filled squares (■) are the in house synthesized powder, while data points indicated by open squares (□) are the purchased Stanford Advanced Materials commercial powder. Black refers to the crystallite size, while blue refers to the WC-Co contamination.
milling conditions. The time dependency of the crystallite size and the WC contamination levels of the two powders plotted in Figure 3 show that the crystallite size of both powders followed very similar, almost overlapping, exponential decaying functions plateauing near ∼9 nm. The similarity in trends of the two powders suggests that this technique may not be well suited to produce nanopowders much smaller than ∼9 nm but demonstrates the universal nature of the approach. The plateau in the final sub-10 nm crystallize size might be explained by a Hall−Petchian-type phenomenon where the hardness of the alumina powder competes with the milling media as the size of the powder decreases.37 Chrokab et al.37 show that the strength of nanocrystals increases as the crystal size is reduced. For Si this corresponded to >2.5 times increase in strength when the crystal size is reduced down to ∼19 nm. In the present case, the “bulk” large crystal hardness of alumina and WC are ∼2338 and ∼26 GPa,39 respectively, so even a slight increase in the hardness of alumina will place the system in a condition where the newly formed α-Al2O3 nanocrystals become at least as hard as the WC milling media. Even if the plateau at ∼9 nm is an artifact of the employed milling conditions, the WC contamination limits the efficiency of the method for producing much smaller crystal sizes.
Figure 4. XRD measured lattice parameters “a” and “c” as a function of crystallite size, which is inversely related to the milling time. As milling time is increased and crystallite size is reduced, both lattice parameters are shown to slightly increase. At the smallest crystallite size (270 min of milling), the volume expansion of the unit cell is ∼0.5%. D
DOI: 10.1021/acs.cgd.6b01748 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
has a high level of agglomeration, which would be expected because of high electrostatic attraction of the sub-10 nm particle size.42 High-resolution TEM shown in Figure 6 of the final labsynthesized powder after ball milling for 270 min and contamination removal reveals that the small crystals in Figure 5 have unique crystallographic orientations and are distinct individual particles. The observed crystallite size of the particles matches well with the XRD size, being on the order of
