Article pubs.acs.org/cm
Two-Step Process in Homogeneous Nucleation of Alumina in Supersaturated Vapor Shinnosuke Ishizuka, Yuki Kimura,* Tomoya Yamazaki, Tetsuya Hama, Naoki Watanabe, and Akira Kouchi Institute of Low Temperature Science, Hokkaido University, Hokkaido Sapporo 060-0819, Japan S Supporting Information *
ABSTRACT: Multiple pathways in crystal nucleation are now known to be more common than previously predicted; it is, therefore, crucial to understand the early stages of crystallization. Even in relatively simple vapor-phase homogeneous nucleation, the process has significant potential diversity. Here, we experimentally show crystalline Al2O3 nanoparticles forming via precisely two steps in the nucleation process from supersaturated vapor with a moderate cooling rate. In situ FT-IR measurement of nucleation allowed us to observe the formation of Al2O3 nanoparticles. Liquid-like particles first nucleated from the vapor before crystallizing. The crystalline phase was preserved by quenching without further transformation into the most stable α-Al2O3 polymorph. The precipitated phase changed from δ-Al2O3 for pure Al2O3 to γ-Al2O3 or θ-Al2O3 by adding Sb or Cr, respectively. We demonstrate that a twostep process occurs in homogeneous nucleation of refractory materials from supersaturated vapor, which may facilitate polymorphic control in industry and improve understanding of cosmic dust formation in space. Stable α-Al2O3 has a large surface free energy value of 2.64 J m−2 compared to that of transition Al2O322 (e.g., 1.5−1.7 J m−2 for γ-Al2O3). A much lower surface free energy value of 0.97 J m−2 has been measured for amorphous Al2O3 by calorimetry.23 The large difference in surface free energies suggests potential diversity in the sequence of Al2O3 nucleation. Indeed, the synthesis of Al2O3 nanoparticles via homogeneous nucleation from vapor forms metastable24−29 or amorphous23,30−32 phases. The appearance of metastable Al2O3 in nucleation is thermodynamically plausible. With increasing surface area-tovolume ratio, a stability crossover from α-Al2O3 to γ-Al2O3 and from γ-Al2O3 to amorphous Al2O3 occurs for surface areas exceeding 75 and 370 m2 g−1, respectively.22,23 Therefore, γAl2O3 can form at the beginning of crystallization in a particle smaller than 20 nm, which corresponds to the sphere size of the stability crossover between γ-Al2O3 and α-Al2O3 for a sphere. However, even if the particle size is much larger than 100 nm, α-Al2O3 is hardly ever obtained in a condensation experiment.23 The metastable alumina family is also observed in rapid solidification of undercooled levitated melt25,33 and in thermal decomposition of AlOOH.34−36 The mechanism of the formation of the resulting phases is still unclear. We knew that understanding the pathways for nucleation is the key and conducted in situ Fourier transform infrared spectroscopy (FT-
1. INTRODUCTION Nucleation, the creation of a new material phase from another, has long been enigmatic. Recent advances in the understanding of crystalline nucleation have been achieved by considering multiple transformation pathways that are fundamentally consistent with Ostwald’s step rule.1 In this scheme, metastable phases with lower surface energies than that of the stable phase nucleate as intermediates. The metastable phases precipitated during nucleation sequences appear in many forms, such as metastable polymorphs, amorphous solid particles, liquid droplets, clusters with characteristic structures, and crystals with different chemical compositions from the final crystalline structure. All are critical in determining the crystalline size, shape, polymorph, and crystallinity in systems and materials including inorganics,2−8 organics,9−11 proteins,12,13 colloids,14−16 and ices.17,18 However, the early stage crystallization pathways in homogeneous nucleation from supersaturated vapor are unclear, especially for refractory materials. The homogeneous nucleation of refractory materials from supersaturated vapor is a fundamental process in synthesis techniques of some materials, and in the astrochemistry of evolved stars, where cosmic dust particles form via nucleation.19 The homogeneous nucleation of refractory oxide nanoparticles, such as Mg2SiO4 and Al2O3, in hot vapor ejected from the star triggers subsequent heterogeneous reactions in space. Al2O3 exhibits a number of crystalline structures20,21 (e.g., α, κ, χ, γ, η, θ, λ, δ, σ, ν, ε, β, ι). © XXXX American Chemical Society
Received: September 23, 2016 Revised: November 3, 2016 Published: November 3, 2016 A
DOI: 10.1021/acs.chemmater.6b04061 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the experimental apparatus. enabled the acquisition of FT-IR spectra at different heights relative to the evaporation source in the experimental chamber. The measurable region was 70 mm along the vertical axis. This is larger than 30 mm of the inner diameter of the windows. The relative distance between the optical path and the evaporation source was measured by a laser measurement sensor (CD22, OPTEX FA Co.). The chamber was connected to a turbo-molecular pump (50 L/s). After evacuating the chamber to below 1.0 × 10−4 Pa, the gate valve was closed and high-purity O2 (99.9%) and Ar (99.9999%) were injected. The pressure was first raised to 2.0 × 103 Pa with O2 gas, and then increased to a total pressure of 4.0 × 104 Pa with Ar gas. The gas pressure was measured by a crystal ion gauge (ANELVA, M-601GC) at 1 Pa. The evaporation source was a V-shaped Ta boat (80 mm in length, 10 mm in width, and 0.1 mm in thickness, with a purity of 99.95%); this was connected to Cu electrodes to permit rapid electricalresistance heating. The temperature of the evaporation source was monitored using a radiation thermometer (λ = 0.8−1.6 μm; FTZ2, Japan Sensor Co.) during the experiments. The emissivity of the Ta metal was assumed to be fixed at 0.3 to calculate the temperature. Several tens of milligrams of metal powder at the evaporation source were rapidly heated to 2073−2273 K by applying AC voltage to the electrodes. We evaporated Al powder (99.9%) or its blends thereof with other metal powders. The amounts of Al and additive in the blended powders were regulated using an electronic weighting instrument. We prepared Al powders mixed with 25% of Mg (99.9%), Ti (99.99%), V (99.5%), Cr (99.9%), Fe (99+%), Co (99.8%), and In (99.999%) and mixed with 10% of Zn (99.999%) and Sb (99.999%) in atomic ratios, respectively. The resulting gas flowed upward by thermal convection generated by the evaporation source before cooling to induce the homogeneous nucleation of nanoparticles, which became visible as smoke. Within 1− 3 s after heating at 2073−2273 K, the evaporation source was quenched by turning off the applied voltage. A typical temperature− time plot of the evaporation source is shown in Figure S1. An optical path for FT-IR just above the evaporation source permitted measurement of the IR spectra of the nascent condensates. The height of the measurement position above the evaporation source was changed between 5, 20, 40, and 60 ± 2.5 mm. A series of experiments were performed independently under the same atmospheric conditions.
IR) measurements to study the transformation of alumina during homogeneous nucleation from the vapor to obtain general insights into nucleation sequences of refractory materials. In previous condensation experiments of crystalline silicate37 and titania,38 we suggested that the homogeneous nucleation of a liquid-like phase is the first step and that crystallization from the liquid-like phase is the second step. In this study, we attempted to control the resulting polymorph using small amounts of additives in order to examine differences in the nucleation sequences of the final crystalline structures; in other words, we investigated whether sequences of more or less than two steps are possible. The transformation from metastable Al2O3 to α-Al2O3 in annealing experiments has been both enhanced and inhibited by introducing additives to the precursors.20 The incorporation of the additive influences the activation energy for crystallization and the surface energy, creating crystals of different shapes and optical properties.20,39,40 In previous studies, FT-IR analysis demonstrated that the absorption band profiles in the 10−20 μm range sensitively reflected structural changes.34,41,42 Here, we used additives to control the sequence of nucleation from the vapor phase; the effect of additives on intermediate phases was examined directly in the simultaneous evaporation of Al with added metals.
2. EXPERIMENTAL SECTION Nucleation experiments of Al2O3 nanoparticles were performed in a stainless steel chamber combined with a FT-IR spectrometer (JASCO, VIR200). A schematic of the experimental system is shown in Figure 1. Two viewports consisting of thallium bromoiodide (KRS5) provided an optical path for FT-IR analysis. KRS5 has high transmittance in the near- to mid-infrared (IR) range, where we focus in this study. Transmitted IR light was sensed by a detector unit consisting of an aperture, IR filter (Spectrogon, LP-10000), parabolic mirror, and HgCdTe (MCT) photoconductive detector (Hamamatsu, P5274-01). The viewports were fixed to a U-shaped bench on which the FT-IR and detector unit were set on either side of the chamber. Flexibly extendable bellows were used to connect the chamber and the viewports. In addition, the bench was interlocked to withstand a load motor unit (Oriental Motor, LAS4F40MW-1). This configuration B
DOI: 10.1021/acs.chemmater.6b04061 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials The aperture diameter was adjusted to provide higher spatial resolution with suitable IR signal intensity. When the aperture diameter was 8 or 12 ± 1 mm, the beam diameters at the center of the optical axis where the nanoparticles nucleated were calculated to be 4.5 or 6.8 ± 0.6 mm, respectively. The time evolution of the IR spectra was obtained at 24 or 3.5 spectra/s for frequency resolutions of 4 or 0.5 cm−1 for aperture diameters of 8 or 12 mm, respectively. IR measurements with 0.5 cm−1 were performed to exclude overlapping of the sharp absorptions. All peak positions described in this article were at least enclosed in ±0.05 μm. The nanoparticles produced followed the convection generated by the evaporation source to the top of the experimental chamber before subsequently filling the chamber after heating. In the experiments on pure Al2O3 and Al2O3 co-condensed with Cr, FT-IR spectra were measured with a resolution of 4 cm−1 for the nanoparticles in the gas ascending from the evaporation source, and 0.5 cm−1 for the nanoparticles drifting after heating. In experiments using Al2O3 condensed with other additives, all FT-IR spectra were measured with resolutions of 4 cm−1. The gas temperature, which decreased along the vertical axis from the evaporation source, was obtained using B- and K-type thermocouples under a 4.0 × 104 Pa pure Ar atmosphere. The temperature of the evaporation source was controlled to produce the same conditions as those produced during nucleation experiments. Some ascending nanoparticles were attached to a stainless steel collecting sheet fixed more than 25 cm above the evaporation source. The collected particles were picked up on a standard Cu grid sweated with ethanol and dried for 1 min. The particles were observed by transmission electron microscopy (TEM, JEOL JEM-2100F) at an acceleration voltage of 200 kV. The rest of the particles from the collecting sheet were agitated with KBr powder and formed pellets with 10 mm in diameter under a pressure of 8 × 104 N for subsequent FT-IR measurements.
Figure 2. In-situ FT-IR spectra of nucleating Al2O3 nanoparticles measured at (a) 5 mm, (b) 20 mm, (c) 40 mm, and (d) 60 mm above the evaporation source heated to 2073−2273 K with 4 cm−1 resolution. (e) IR spectrum measured with 0.5 cm−1 resolution for hovering nanoparticles 30 s after the evaporation ended. The subtle absorptions are indicated by the black arrows. The peak positions of (a) and (e) are indicated by the blue lines.
3. RESULTS AND DISCUSSION First, we show the result of the pure Al nucleation experiments. We then expand the discussion to include simultaneous evaporation of Al with additives. 3.1. Nucleation of Pure Al2O3. The evolution of the IR spectra of nucleated Al2O3 nanoparticles in the ascending flow when pure Al metal powder was evaporated in the regulated atmosphere is shown in Figure 2a−d. The absorption features in the IR spectra remained for several minutes after evaporation, because most of the nanoparticles were returned to the optical path by convection. This enabled the accumulation of spectra of sufficiently cooled nanoparticles in the free flying state (Figure 2e). A broad absorption peak at 12.4 μm caused by the Al−O stretching vibration is measured 5 mm above the evaporation source. The peak shifts to a shorter wavelength of 12.1 μm in the ascending gas current at 60 mm above the source and settles at 12.0 μm when the particles are cooled to room temperature. Subtle absorptions at 10.4, 10.9, 11.2, 11.5, 12.0, 12.2, 12.8, 13.6, 14.2, 14.7, 15.1, 15.8, 16.3, 16.8, and 17.2 μm become distinguishable at a height of 40 mm and become sharper as the particles cool. The absorptions are consistent with the characteristic features of crystalline δAl2O3.29 The resulting nanoparticles were examined using TEM imaging (Figure 3). The patterns of lattice fringes confirm the formation of δ-Al2O3 with an average particle radius of 47.7 ± 0.5 nm. The collected nanoparticles were also measured by IR spectroscopy using a conventional KBr embedding method for comparison with the IR spectra obtained from the free flying state (Figure 4). In the KBr medium, the electromagnetic interactions between the embedded nanoparticles and the surrounding KBr often alter spectral properties such as peak wavelength, bandwidth, and relative intensity because of
differences in the dielectric constants of the surroundings.43,44 The sharp absorption at 12.2 μm in the measurement of the free flying particles shows a shift of 0.3 μm toward longer wavelengths, and another sharp absorption peak appears at 13.1 μm, which was obscured in the measurements in the free flying state, in the KBr spectrum. In contrast, the other subtle absorption peaks do not show such shifts. The independent behavior in the peak wavelength shifts in the dielectric medium suggests that the origins of the absorption peaks are not from the same vibrational modes. When the absorbers are embedded in KBr, the peak wavelength of the excitation is shifted to a longer wavelength, unless the absorber is embedded in an Al2O3 particle. The absorber has a different dielectric constant from that of Al2O3 and has a sufficiently small volume fraction relative to the particle. One plausible candidate for the origin of the subtle peaks is a defect-like structure in the Al2O3 particle. In fact, the origins of the subtle peaks have not been identified because of the complex crystal structure of δ-Al2O3. However, similar subtle absorptions have been observed for κ-Al2O3, δAl2O3, and θ-Al2O3.45 This implies that the subtle absorptions arise from the intrinsic periodic structure in transitional Al2O3 crystals. We propose that a periodic defect-like structure is introduced into particles which matches the defective nature of the δ-Al2O3.35 The IR profiles of nanoparticles are influenced by the temperature. From the temperature dependence of the optical constants measured for a single α-Al2O3 crystal, the peak wavelength attributed to the Al−O vibration is expected to shift from 12.94 to 12.73 μm from 928 to 300 K for a spherical C
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Figure 3. (a) Bright-field image of a typical nanoparticle formed in the Al evaporation experiment under O2/Ar atmosphere. (b) High-resolution TEM image of the particle and its FFT analysis. Lattice fringes of 0.292 and 0.339 nm corresponding to (21̅4̅) and (212), respectively, with a crossing angle at 70.6° indicating the formation of a nanoparticle of δ-Al2O3 (JCPDS card no. 56-1186).
wavelength. In contrast, the bandwidth begins to increase at 60 mm above the evaporation source and settles at Δλ = 2.7 μm in the measurement of the drifting nanoparticles. The increase of bandwidth corresponds to the location where δ-Al2O3 forms, and the bandwidth is enlarged through band splitting. The evolutionary sequence in homogeneous nucleation is discussed in section 3.4. The Al2O3 produced with Mg, Ti, V, Fe, Co, Zn, and In show no drastic changes in spectral profiles from that of the pure Al2O3 in our experiments. However, differences in the IR spectra and in the resulting polymorphs were significant for the blends with Cr and Sb. In order to discuss the phase nucleation sequences, we focused on the experiments using these two metal additives. 3.2. Nucleation of Al2O3 with Sb. The simultaneous evaporation of Al and Sb powder under an oxidative atmosphere resulted in the formation of white-yellow nanoparticles. Though Sb powder started to evaporate antecedently to Al powder because of higher vapor pressure, rapid heating resulted in simultaneous evaporation of Al with Sb. Formation of Sb oxide in the early stage of heating was also confirmed from IR spectra. High-resolution TEM images and corresponding fast Fourier transform (FFT) images confirm the formation of spherical γ-Al2O3 nanoparticles similar in size to the δ-Al2O3 nanoparticles observed in the pure Al2O3 experiment (Figure 5). We performed energy-dispersive X-ray spectroscopy (EDS) analyses of 38 individual isolated crystalline Al2O3 nanoparticles. The single-particle analyses for crystalline Al2O3 nanoparticles showed CSb/(CSb + CAl) = 2.5% on average, where CSb and CAl are the atomic ratios of Sb and Al, respectively. Scanning transmission electron microscopy (STEM)-EDS mapping showed at least 0.8% of Sb distributing homogeneously. No γ-Al2O3 nanoparticles with concentrations of 100 nm s−1 as discussed in SI 5. The nucleation of noncrystalline particles agrees with Ostwald’s step rule1 and seems common with some proceeding studies performed in a variety of specific systems. In nucleation of protein crystals, a two-step nucleation process via liquid-like clusters is proposed.12,13 Similar processes are shown in nucleation of heteropolyanions8 and in colloidal systems.15 A liquid-like phase is observed in situ in nucleation of metal particles from a solution with spinodal decomposition.7 In this study, the just-nucleated nanoparticles which have grown to a size of the order of tens of nanometers show the noncrystalline IR feature and the rapid crystallization is introduced in them as the particles cool. We conclude that the noncrystalline phase measured in our study is liquid-like particles. The homogeneous nucleation of liquid-like particles is consistent with our previous studies on condensation of crystalline Mg2SiO437 and TiO2.38 The direct formation of liquid-like particles from vapor and the differences in the nucleated crystalline phases within them causes divergences in IR spectral evolution. The broad IR bands of γ-Al2O3 never appear in IR spectra of the intermediates during δ-Al2O3 formation. In annealing experiments, the γAl2O3 to δ-Al2O3 transformation usually required annealing for 1 h at >1100 K.21 Thus, such a solid−solid transition, requiring high temperature and long waiting times to overcome the activation barrier, is improbable in the formation process observed in this study. This strongly suggests that δ-Al2O3 is directly nucleated from a noncrystalline particle in the process of pure Al2O3 condensation. We, therefore, conclude that the formation of crystals in homogeneous nucleation from supersaturated vapor proceeds through two steps: in the first step, Al2O3 takes liquid-like particles form; in the second step, a G
DOI: 10.1021/acs.chemmater.6b04061 Chem. Mater. XXXX, XXX, XXX−XXX
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as Al2O3,23 TiO2,61 and ZrO2.62 Though the stability of noncrystalline nanoparticles is unknown for most materials, single-step nucleation of crystalline phases would be rare in the homogeneous nucleation of refractory materials. A two-step process in homogeneous nucleation from vapor in a LennardJones system has been demonstrated using Monte Carlo simulations in a low-supersaturation condition.63 In contrast, a change in particle formation mechanism from a two-step to a single step has been suggested with decreases in the nucleation temperature for ices.17,18 However, our results, obtained in extremely high supersaturation, are consistent with a two-step process. Thus, nucleation proceeding in lower supersaturation conditions than those in our study, including numerous fields in industry where physical and chemical vapor deposition techniques are used for material synthesis and for cosmic dust formation regions in space, is suggested to occur in two steps. We further emphasize that a two-step mechanism, which has been recognized in the nucleation of salt,64,8 organic,10 protein,12,13 and colloidal systems,14,15 even those in solution, seems common with our demonstrated results. Though this may be apparent in limited systems, our results will be an aid to obtain the general insight into nucleation steps.
kinetically favorable phase nucleates in the noncrystalline particle. A schematic illustration is shown in Figure 9. We consider that Sb and Cr dissolved in the liquid-like Al2O3 precursor in the same concentration as the final product because the source gas is exhausted just after the nucleation. We consider that the dissolved additives played a crucial role in crystallization and changed the resulting polymorph. The incorporation of Sb or Cr changes the relative thermodynamic stabilities of the transitional phases. This insight into homogeneous nucleation via a two-step phase transition was obtained regardless of thermodynamic equilibria. We thus do not focus on the thermodynamic stability of the resulting Al2O3 nanoparticles with Sb or Cr additives in this study. 3.5. Lack of Formation of Stable α-Al2O3. The IR spectral characteristics of γ-Al2O3 never appeared consistently during the formation sequences of the more stable δ-Al2O3 and θ-Al2O3. The direct nucleation of the relatively stable δ-Al2O3 and θ-Al2O3 phases from liquid-like particles in the second step of the process could occur because the significant undercooling induced a large driving force for nucleation. However, the formation of α-Al2O3 was never observed. This may be caused by a significant kinetic barrier for the nucleation of α-Al2O3, derived from the drastic difference in crystalline structure of the phase. Neutron diffraction analysis of Al2O3 melt under contactless conditions at 2500 K showed that 62, 24, and 2% of Al occurred in the forms of AlO4, AlO5, and AlO6, respectively.57 Ansel et al. and Skinner et al. showed that the molten structure of Al2O3 is dominated by AlO4 and AlO5.58,59 IR spectral studies on numerous crystalline aluminates have empirically suggested that Al−O vibrations of AlO4 arise at 11.1−13.3 μm; those by AlO6 arise at 13.3−18.2 μm in a dielectric medium.28,41,60 In this study, the 12 μm band was considerably stronger than the longer-wavelength band at ∼15−18 μm during measurement of the liquid-like particles and the longer-wavelength band was enlarged as the particles crystallized in all experiments. This suggests that the justnucleated liquid-like particles were mainly composed of AlO4 tetrahedra, and that the proportion of AlO6 increased upon crystallization. The difference of IR spectral properties of justnucleated particles measured in the nucleation of θ-Al2O3 with Cr (Figure 8a) may represent the difference of the proportions of AlO4, AlO5, and AlO6, which possibly affects the resulting polymorph. Unfortunately, it remains unclear whether the liquid-like particles observed in this study are composed of the same molecules as the melt obtained by the thermal treatment of solid Al2O3, because the IR spectrum of AlO5 is unknown. γAl2O3, δ-Al2O3, and θ-Al2O3, in which 25−50% of Al in AlO4 tetrahedra form crystalline structures,20,35 should nucleate with lower activation energies than α-Al2O3, which is composed only of AlO6 octahedra. The large activation energy of α-Al2O3 formation requires a high temperature. This leads to lower undercooling of the melt, and the waiting time for nucleation becomes considerably longer. To obtain α-Al2O3 via homogeneous nucleation from vapor, the newly nucleated liquid-like particles must be kept at a high temperature for a long time. 3.6. Generality of Two-Step Processes in Nucleation. To overcome the energetic barrier for homogeneous nucleation, an extremely high supersaturation in vapor is required. In such a supersaturated vapor, a critical nucleus is less than several tens of molecules in size. This is significantly smaller than the size for the stability crossover from noncrystalline to crystalline forms of refractory materials such
4. CONCLUSIONS By using in situ FT-IR measurements, we have shown that the homogeneous nucleation of Al2O3 crystals in highly supersaturated vapor occurs in a two-step process. The first step is the nucleation of liquid-like particles from supersaturated vapor at a temperature considerably lower than the melting point of Al2O3. The critical nucleus size is less than tens of molecules; such nuclei rapidly grow to 10 nm scale spheres in 10−3 s, which rapidly exhausts the source gas. The liquid-like particles are cooled in the free flying state in approximately 0.1 s. If we assume that the liquid-like particles as molten droplets, the considerable degree of undercooling induces crystallization in the droplets. Crystallization in the liquid-like particles occurs in one step, regardless of the resulting polymorph. The phase transition between solid phases, which requires high temperatures and long waiting times, is impossible in our experimental setup. This sequence was consistently observed for refractory oxides in our previous studies. We believe that the nucleation of liquid-like particles is crucial in determining the resulting crystalline properties of particles formed in many fields, including industrial composition, morphology, and polymorph control and understanding cosmic dust formation in space.
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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.chemmater.6b04061. Results and short discussions of pure Sb and Cr evaporation experiments under the same conditions. Details of estimations of the growth velocity of a nucleus and of the time scale for thermal equilibration between a free flying nanoparticle and its surrounding gas (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yuki Kimura: 0000-0002-9218-7663 H
DOI: 10.1021/acs.chemmater.6b04061 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials Funding
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This work was partly supported by a Grant-in-Aid for a JSPS Fellow (15J02433) and a Grant-in-Aid for Scientific Research(S) from KAKENHI (15H05731). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank F. Saito, S. Nakatsubo, S. Mori, and K. Shinbori of the Technical Division in the Institute of Low Temperature Science, Hokkaido University, for their help in the development of the experimental system.
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