NaAlO2 and γ-Al2O3 Nanoparticles by Pulsed Laser Ablation in

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NaAlO2 and γ-Al2O3 Nanoparticles by Pulsed Laser Ablation in Aqueous Solution I. L. Liu,‡ B. C. Lin,‡ S. Y. Chen,§ and P. Shen*,‡ ‡

Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan, R.O.C. Department of Mechanical and Automation Engineering, I-Shou University, Kaohsiung, Taiwan, R.O.C.

§

bS Supporting Information ABSTRACT: Pulsed laser ablation on an Al target in water spiked with 0.05 and 1 M NaOH was employed to fabricate epitaxial β-NaAlO2 and γ-Al2O3 nanoparticles for X-ray diffraction and electron microscopic and spectroscopic characterizations. A higher NaOH concentration in the pulsed laser ablation in liquid (PLAL) process caused a higher content of NaAlO2 3 4/5H2O and β-NaAlO2 besides the predominant γ-Al2O3 nanoparticles. Upon settling on the silica substrate at room temperature, the NaAlO2 3 4/5H2O tended to develop as micrometer sized plates for the sample fabricated under a relatively low (i.e., 0.05 M) NaOH concentration. However, the crystal structures, d-spacings, mismatch strain, and morphology of the coexisting phases are almost the same for the samples fabricated under different NaOH concentrations. The β-NaAlO2 phase (denoted as N), presumably derived from NaAlO2 3 5/4H2O, was found to form intimate intergrowth with the defective γ-Al2O3 following the crystallographic relationship (011)γ//(110)N; [111]γ//[001]N and alternatively (101)γ//(110)N; [141]γ// [001]N. The epitaxial composite phases have a significant internal compressive stress, (OH-, Naþ) cosignature, and a mixed charge state of Alþ, Al2þ, and Al3þ and hence a smaller band gap (ca. 5.3 eV) for potential applications in the UV region.

I. INTRODUCTION The crystallization of aluminum hydroxide from caustic aluminate solution is the rate-determining step within the Bayer cycle, which is used for aluminum oxide production.1 Recent experimental results indicated that bayerite (R-Al(OH)3) crystallization predominates from dilute sodium aluminate solutions, while for concentrated solutions the precipitated phase is predominately gibbsite (γ-Al(OH)3).2 It was not reported if sodium aluminate forms intimate intergrowth with aluminum hydroxides or oxides having a specific crystallographic relationship under static or dynamic water environment. Dynamically, pulsed laser ablation (PLA) on an Al target at extremely high power density under a specified oxygen flow rate in air typically was employed to fabricate dense γ-Al2O3 nanocondensates of abnormal large size up to micrometer scale.3 PLA in liquid (hereafter referred as PLAL) has an even higher heatingcooling rate and hence a more pronounced pressure effect to form smaller sized and denser nanocondensates,4,5 including a highpressure phase such as diamond.5 PLAL has been used to synthesize bayerite, gibbsite, and boehmite (γ-AlO(OH)) particles6 as well as gram-scale R-Al2O3 nanoparticles by controlling the liquid film thickness and interpulse distance with a nanosecond pulse laser.7 PLAL under a rather high peak power density of 1.8  1011 W/cm2 was also used to synthesize (Hþ,Al2þ)-codoped Al2O3 nanocondensates with γ- and θ-type structures having a significant internal compressive stress due to a significant shock loading effect in water.8,9 Such nanocondensates were found to transform into bayerite plates upon prolonged dwelling in water and then dehydrated as platy γ-Al2O3 following a specific crystallographic relationship.9,10 r 2010 American Chemical Society

Here, PLAL on the Al target in water spiked with NaOH was employed to fabricate NaAlO2 and γ-Al2O3 nanoparticles focusing on their crystallographic relationships, internal stress, defect microstructures, signature of aliovalent ions, and resultant optical properties. Such understandings are of concern to the Bayer process and shed light on the phase behavior of aluminates in natural dynamic settings enriched in hydroxyl and alkali ions.

II. EXPERIMENTAL PROCEDURE An Al (Nilaco, 99.9% pure) plate 1 mm in thickness was immersed in deionized (DI) water with 0.05-1 M NaOH addition within a glass beaker and then subjected to an energetic Nd:YAGlaser (Lotis, 1064 nm in wavelength, beam mode TEM00) pulse irradiation at a specified laser parameter, i.e., 850 mJ/pulse with a pulse time duration of 16 ns at 10 Hz under Q-switch mode on a focused area of 0.03 mm2. (A pulse energy as high as 850 mJ was adopted to enhance shock pressure11 and to reduce absorption in water,12 so that a significant internal compressive stress and a smaller particle size can be achieved for the condensates.) Under such laser conditions, the average power density and peak power density are 2.8  104 and 1.8  1011 W/cm2, respectively, the latter being related to the shockwave-induced pressure according Special Issue: Laser Ablation and Nanoparticle Generation in Liquids Received: July 27, 2010 Revised: November 11, 2010 Published: November 23, 2010 4994

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Figure 1. X-ray diffraction (Cu KR) of NaAlO2 3 4/5H2O (denoted as h-N), β-NaAlO2 (denoted as β-N), and γ-Al2O3 (denoted as γ) condensates fabricated by PLAL under 1 M NaOH with a peak power density of 1.8  1011 W/cm2 for 5 min followed by dwelling in water for 3 days and then centrifugation. The broad diffraction below 30 degree 2θ is due to the silica substrate.

to an analytical model by ref 11. (The severe plasma breakdown effects typically occur at a much higher power density of 10 GW/ cm2.13) The upper surface of the Al target was 5 mm below the water level in a beaker 6 cm in diameter full of DI water ca. 15 cm3 in volume during such an ablation process. An optimal synthesis time of 5 min, i.e., a total of 3000 pulses given 10 Hz, was adopted for a satisfactory yield of nanocondensates that are below the gram scale but enough for detailed characterization in this study. The colloidal solution thus formed by PLAL was centrifuged and then deposited on soda-lime glass for X-ray diffraction (XRD, SIEMENS, D1, Cu KR at 45 kV, 35 mA, and 5 s for each 0.02° increment from 20 to 90 of 2θ angle) and scanning electron microscopic (SEM, JSM6700 at 10 kV) observations. The nanoparticles in the upper portion of the colloidal solution (ca. 1 wt % in concentration) were also settled and collected on Cu grids for transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV) observations coupled with selected area electron diffraction (SAED) and point-count energy dispersive X-ray (EDX) analysis at a beam size of 10 nm. The condensates collected on a silica glass substrate to circumvent the influence of NaOH in solution were used for UV-visible absorption (U-3900H, Hitachi, with a resolution of 0.1 nm in the range of 200-900 nm) and Raman spectroscopic study. The Raman spectrum of the same sample was made using semiconductor laser excitation (633 nm) having a spatial resolution of 1 μm (Horiba HR800). The condensates collected on silica glass were also used for X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MX Photoelectron spectrometer with Mg KR X-ray source) calibrated with a standard of C 1s at 284 eV to analyze the possible presence of Al2þ in the condensates. The condensates mixed with KBr were studied by Fourier-transform infrared spectroscopy (FTIR, Bruker 66v/S. 64 scans with 4 cm-1 resolution) for the extent of OH- signature.

III. RESULTS Phases Fabricated by PLAL in Water Spiked with NaOH. XRD trace (Figure 1) showed the coexisting phases of NaAlO2 3 5/4H2O, β-NaAlO2, and γ-Al2O3 for the representative sample fabricated by PLAL for 5 min in water spiked with 1 M NaOH followed by dwelling in the same solution for 3 days. The broad band from 55 to 85 degree 2θ could be attributed to

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Figure 2. SEM SEI showing platy hydrous NaAlO2 3 4/5H2O and the powdery matrix of β-NaAlO2 and γ-Al2O3 intergrowth as deposited by PLAL under 0.05 M NaOH and then collected on a glass substrate.

an amorphous Al2O3 analogous to that produced by PLAL in pure water8,9 or sol-gel synthesis,14,15 although the influence of Naþ and proton on the amorphous structure needs to be considered in the present case. (XRD showed broad diffraction from 45 to 60 and 60 to 90 degree 2θ for Al2O3 gel.14,15) Electron Microscopic Observations of Defect Microstructures and Crystallographic Relationships. SEM under the secondary electron image (SEI) mode (Figure 2) showed platy crystals and a powdery matrix for the representative sample as deposited by PLAL in water spiked with 0.05 M NaOH. The platy one is hydrous NaAlO2 3 5/4H2O, whereas the powdery matrix is a mixture/intergrowth of β-NaAlO2 and γ-Al2O3 as confirmed by XRD and EDX analysis (cf. supplements 1 and 2, Supporting Information).8 A higher spike (1 M) of NaOH caused the γ-Al2O3 enriched particulates besides the powdery partially hydrated sodium aluminate.8 A TEM bright field image (BFI) coupled with the SAED pattern showed the γ-Al2O3 nanoparticles are predominant for the sample fabricated by PLAL in water spiked with 1 M NaOH and then subject to an electron beam for 30 min (Figure 3). Apparently, these nanoparticles were derived from the as-deposited sodium aluminate and/or γ-Al2O3 nanocondensates during PLAL and subsequent electron irradiation. The lattice image coupled with 2-D forward and inverse Fourier transform showed further that the γ-Al2O3 nanocondensates were coalesced to form misfit dislocations and an incoherent twin plane {111} (Figure 4). Nanoparticles with intimate intergrowth of γ-Al2O3 and βNaAlO2 (denoted as γ and N, respectively) following the crystallographic relationship (111)γ//(001)N and (011)γ//(110)N (hereafter referred to as relationship A) were also identified by lattice image coupled with 2-D forward and inverse Fourier transform in the zone axis [211]γ//[110]N (Figure 5). Figure 6 shows the case of γ-Al2O3 nanocondensate with dislocation half plane parallel to (131) and relic β-NaAlO2 following a specific partial epitaxial relationship, i.e., (111)γ//(001)N, presumably developed from the original relationship A during PLAL and/or electron irradiation. The electron diffraction rings (Figure 3 and supplement 4, Supporting Information) and magnified lattice images in Figures 5 and 6 allow the measurement of interplanner spacings as 0.433 nm for (111)γ, 0.228 nm for (131)γ, 0.199 nm for (004)γ, 0.449 nm for (110)N, and 0.234 nm for (002)N, as 4995

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Figure 3. TEM (a) BFI and (b) SAED pattern of γ-Al2O3 nanoparticles which were derived from the as-deposited sodium aluminate and γAl2O3 nanocondensates fabricated by PLAL under 1 M NaOH and then subject to an electron beam for 30 min.

Table 1. Observed and Calculated d-Spacings (nm) for the Dense γ-Al2O3 and β-NaAlO2 (N) Nanocondensates (hkl)γ

observed (nm)

JCPDS 10-0425

refined (nm)

111 004

0.433 0.199

0.4560 0.1970

0.450 0.195

311

0.228

0.239

0.235

(hkl)Ν

observed (nm)

JCPDS 33-1200

110

0.449

0.4283

ND

002

0.234

0.2609

ND

compiled in Table 1 for the estimation of lattice mismatch strain as discussed later. Occasionally, the nanoparticles showed γ-Al2O3 and β-NaAlO2 intergrowth following an alternative relationship (101)γ// (110)N and [141]γ//[001]N (hereafter referred to as relationship B) (Figure 7). There are penetrating intergrowths of the (202)γ and delaminated (110)N planes to show extra planes and cleavages. 2-D forward and inverse Fourier transform, from the local area of the nanoparticle, indicated that the (110)N planes are in parallel to (202)γ and there are numerous faults parallel to

Figure 4. TEM (a) lattice image of the coalesced γ-Al2O3 nanocondensates. (b), (d), and (c), (e) 2-D forward and inverse Fourier transform, respectively, from the square regions I and II in (a) showing the coalesced nanoparticles in the [110] zone axis with misfit dislocations at the interface in area I and incoherent twin plane {111} in area II. It is the same specimen prepared under 1 M NaOH as in Figure 3.

the lattice fringes of (202)γ. However, the (110)N planes are not parallel to any rational crystallographic planes of the γ-Al2O3. Thus, the epitaxial (110)N planes appeared to be less vulnerable than nonepitaxial (110)N planes during lattice imaging. The combined XRD, SEM, and TEM results of the sample fabricated under 0.05 M NaOH are compiled in supplements 1, 2, 4, 5, 6, and 7 (Supporting Information) showing no appreciable difference in the d-spacings, size, and morphology of the γAl2O3 and β-NaAlO2 composite nanoparticles from the sample prepared under 1 M NaOH. It should be noted however that a higher NaOH concentration (i.e., 1 M NaOH) in the PLAL process caused a higher content of β-NaAlO2 for the determination of its epitaxial relationships with the predominant γ-Al2O3 4996

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Figure 5. TEM (a) lattice image of a nanocondensate with intimate intergrowth of γ-Al2O3 and β-NaAlO2 (denoted as γ and N, respectively) in the zone axis [211]γ//[110]N having facets (111)γ//(001)N denoted by the dotted line and d-spacings labeled in the magnified image inset. (b) and (c) 2-D forward and inverse Fourier transform, respectively, from the square region in (a) showing the crystallographic relationship A, i.e., (111)γ//(001)N and (022)γ//(110)N. The diffractions of γ and N in (b) are circled in yellow and green, respectively. Note double diffraction denoted as D in (b). It is the same specimen prepared under 1 M NaOH as in Figure 3.

nanoparticles in Figures 5 and 6. As for the sample fabricated under 0.05 M NaOH, a rather minor amount of β-NaAlO2 hardly survived electron dosage, and the predominant γ-Al2O3 nanoparticles tended to coalesce as multiple twins having {111} twin boundaries and a [110] asymmetric tilt boundary (113)/(111), similar to that formed by PLA on Al under oxygen gas.3 Still, relic β-NaAlO2 was occasionally observed to follow (001)N//(111)γ, i.e., partial relationship A, with the twinned γ-Al2O3 as shown in supplement 6 (Supporting Information). Spectroscopic Analyses. Figure 8a shows the Raman spectrum of the γ-Al2O3 and hydrous/anhydrous NaAlO2 nanocondensates for the representative sample fabricated by PLAL under conditions of 1 M NaOH followed by dwelling in water for 5 days. The Raman bands at 654 and 526 cm-1 are significantly shifted from the wave numbers 620 (O-H bending) and 535 cm-1 (Al-O-Al bridge symmetrical stretching) reported for the sodium aluminate prepared by hydrothermal synthesis.16 The bands at 236, 286, 325, 381, 455, 747, and 825 cm-1 can be

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Figure 6. TEM (a) lattice image of a γ-Al2O3 nanocondensate in the [211]γ zone axis with relic β-NaAlO2 (denoted as N) having d-spacings labeled in the magnified image inset. (b) and (c) 2-D Fourier transform and inverse Fourier transform, respectively, from the square region in (a) showing the partial epitaxial relationship (002)N//(111)γ and dislocations (denoted by T) with half plane parallel to (131)γ edge on. It is the same specimen prepared under 1 M NaOH as in Figure 3.

attributed to hydrous/anhydrous NaAlO2 nanocondensates in view of the mode assignment for the polymorphs of sodium aluminum silicates NaAlSi3O8, i.e., monoclinic or triclinic albite17 and orthorhombic kumdykolite.18 (Albite shows the strongest bands at 505 and 408 cm-1 (T-O-T symmetrical stretching vibration, T = Si or Al) and moderate bands at 250, 270, 280, and 814 cm-1 (lattice vibration modes),17 whereas kumdykolite shows characteristic strong bands at 220, 456, and 492 cm-1 besides the medium to weak peaks at 265, 282, 406, 856, and 979 cm-1.18) The sample fabricated by PLAL under 0.05 M NaOH showed weaker and broader Raman bands of the minor sodium aluminate (supplement 7a, Supporting Information) than that prepared under 1 M NaOH. The FTIR spectrum of the representative sample shows further the band at 979 cm-1 (Figure 8b) which can be attributed to the Al-O vibration mode of γ-Al2O319 or the Al-O-H bending mode of sodium aluminate in view of the assignment of ref 16. The bands at 3450 and 1653 cm-1 are the O-H bond stretching and bending, respectively,20 due to water absorbed on the powder surface and/or the cavity site of AlO4 rings in the 4997

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Figure 8. (a) Raman and (b) FTIR spectra of the γ-Al2O3 and hydrous/anhydrous NaAlO2 nanocondensates fabricated by PLAL under 1 M NaOH with a peak power density of 1.8  1011 W/cm2 for 5 min followed by dwelling in water for 5 days and then centrifugation.

Figure 7. TEM (a) lattice image of the γ-Al2O3 nanocondensate with relic β-NaAlO2 (denoted as N) layers in an alternative crystallographic relationship B. (b)(d) and (c)(e) 2-D forward and inverse Fourier transform, from the square regions I and II, in (a) showing, respectively, the epitaxial relationship B, i.e., (202)γ//(110)N and [141]γ//[001]N, and relatively stable planes (202)γ//(110)N as well as numerous faults (denoted as red lines) parallel to (202)γ. Note that double diffractions are denoted as D in (b) and (d). It is the same specimen prepared under 1 M NaOH as in Figure 3. -1

NaAlO2 3 4/5H2O lattice. The bands 2922 and 2851 cm are due to EtOH used for FTIR sample preparation, and 1403 cm-1 belongs to carbonate absorbed from air.21 The band at 560, 747, and 1100 cm-1 (a shoulder near 979 cm-1) can be ascribed to β-NaAlO2 in view of previous assignments at 559, 711, and 1100 cm-1 for crystalline sodium aluminate.22 (Note the scheme of phase evolution on progressive heating could be expressed as sodium Dawsonite, amorphous, transition alumina (γ/η), and crystalline sodium aluminate.22) The sample fabricated by PLAL under 0.05 M NaOH showed much weaker bands of the minor sodium aluminate (supplement 7b, Supporting Information) than that prepared under 1 M NaOH, although the wave numbers are basically the same. XPS of the as-condensed sample consisting of hydrous/ anhydrous NaAlO2 and γ-Al2O3 (Figures 9a to 9c) showed

binding energy at 1073 eV for Na 1s, 532 eV for O 1s and OH, 118 eV for Al 2s, and 69.5, 72.0, and 74.3 eV for Al 2p in the charge state of Alþ, Al2þ, and Al3þ, respectively, as shown by the Lorentzian fits. The UV-visible absorption spectra of the colloidal hydrous/anhydrous NaAlO2 and γ-Al2O3 solution fabricated under 0.05 and 1 M NaOH show nearly the same absorbance corresponding to a minimum band gap of ca. 5.3 eV, based on their intersections with the baseline at 232 and 236 nm, respectively (cf. supplement 8a and 8b, Supporting Information).

IV. DISCUSSION Formation Mechanism of Hydrous/Anhydrous Sodium Aluminate. Sodium aluminate can be formed by the action of

sodium hydroxide on elemental aluminum23 or by heating above 800 °C on sodium Dawsonite.22 The present PLAL synthesis of hydrous/anhydrous sodium aluminate, however, involved oxolation of Al-O-H to form specific cage sites for the incorporation of Naþ ions in a dynamic hydrothermal solution. (In this regard, the NaAlO2 3 5/4H2O structure consists of single layers parallel to (001) made of corner-sharing AlO4 tetrahedra, joined into 4and 8-rings, and the Na ion and water molecule oxygens occupy the cavity of 8-rings.24 Whereas β-NaAlO2 is based on a threedimensional framework of corner-linked AlO4 tetrahedra joined into 4-rings and 6-rings, the Na lies in a tetrahedral cavity.24) The species produced in solution is expected to contain the [Al(OH)4]-

4998

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Figure 9. (a, b, c) XPS with specified binding energies of elements (cf. text) for the γ-Al2O3 and hydrous/anhydrous NaAlO2 nanocondensates in the same specimen as in Figure 8.

unit in which the aluminum atom is four-coordinated and could be linked by oxo- or hydroxo-bridges, especially for dilute solutions analogous to the static synthesis in concentrated sodium aluminate solutions.16 In general, at pH 8-12.5, the square planar Al(OH)4- predominates, whereas above a pH of 12.5, linear AlO2- would show up,25 which can be explained as the following equation: Al(OH)4- f AlO(OH)2- þ H2O f AlO2- þ H2O. In other words, AlO2- can be formed with the stepwise release of two water molecules under such a moderate base condition. In the case of extremely basic conditions, such as in the present PLAL solution spiked with 1 M NaOH for pH close to 14, extensive polymerization and water loss would favor γ-Al2O3 and β-NaAlO2 at the expense of NaAlO2 3 4/5H2O. Upon settling on the silica substrate at room temperature, the NaAlO2 3 4/5H2O tended to develop as micrometer sized plates for the sample with a high water/NaOH ratio, i.e., 0.05 M NaOH (Figure 2), but not for the case of 1 M NaOH (cf. supplement 3, Supporting Information). The oxolation of Al-O-H in the present dynamic PLAL process with NaOH addition also formed an amorphous phase

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presumably having short-range order and/or medium-range order of 4-, 5-, and/or 6-cooridnated Al ions in the oxygen framework. Apparently, the copresence of Naþ and proton has modified the amorphous Al2O3 structure to show considerably different broad diffraction (55 to 85 degree 2θ corresponding to 0.17-0.11 nm in the real space) from that under the influence of proton as in the case of PLAL8,9 and sol-gel synthesis of Al2O3 in water.14 The size and interspacing of AlO4, AlO5, and AlO6 polyhedra are of concern to the amorphous structure.26 In this regard, liquid alumina at 10 bar (0.001 GPa) and 3000 K was theoretically calculated to have 64% Al atoms in 4-fold coordination and 31% in 5-fold coordination, whereas only 3% of Al atoms are in 6-fold coordination.26 Under such a case, the average Al-O bond lengths in the [AlO6] and [AlO4] units are 0.182 and 0.173 nm, respectively. (Note that the unit volumes of AlO4, AlO5, and AlO6 are 0.304, 0.1346, and 0.3858 nm3, respectively.) On the other hand, the X-ray synchrotron radiation study of the levitated sample showed that Al2O3 undergoes a major structural rearrangement on melting with an Al coordination change from octahedral to tetrahedral, and the coordination remains 4 for both the stable and supercooled liquid state.27 The diffraction study of Al2O3 liquid at high temperatures27 showed two peaks in total structure factor corresponding to 0.176 and 0.308 nm which can be assigned to intermediate-range order about the Al-O and O-O tetrahedrally coordinated bonds, respectively, similar to that observed in many network liquids. By contrast, the present condensates have a shorter Al-O distance (0.11-0.17 nm) because of the following factors: (1) measurement at room temperature rather than high temperature as adopted by ref 27; (2) structure densification via a dynamic PLAL route; (3) additional influence of proton and Naþ. Such dense [AlO4] tetrahedral structure units are expected to act as homogeneous nuclei to form hydrous/anhydrous sodium aluminate. It is an open question if the present β-NaAlO2 condensates were derived from the high-temperature and/or high-pressure polymorphs upon rapid cooling in the dynamic PLAL process. (As summarized by ref 28, the NaAlO2 has four polymorphs, including the low-temperature, low-pressure β form having orthorhombic symmetry (a = 0.53871 nm, b = 0.70320 nm, c = 0.52180 nm); this form is what we mentioned in this article. β-NaAlO 2 undergoes a reversible nonreconstruction phase transition to its higher symmetry γ form (a = 0.55323 nm, c = 0.7058 nm) on heating above 470 °C. The other two are high-temperature cubic polymorph δ-NaAlO2 and high-pressure rhombohedrally distorted sodium chloride-type polymorph R-NaAlO2 with a = 0.2868 nm and c = 1.588 nm.) Internal Stress of γ-Al2O3 and Hydrous/Anhydrous NaAlO2 Condensates. The internal stress of the present condensates is complicated by the coexistence of γ-Al2O3 and hydrous/ anhydrous NaAlO2. Still, the vibrational mode of the common structure unit, i.e., 4-coordinated tetrahedral Al-O-Al vibration in the coexisting phases, may shed light on the extent of internal stress. The IR band of such a vibration mode is at 457 cm-1 for the present condensates, significantly at higher wavenumber than 432 cm-1 for the analogue material, i.e., NaAlSi3O8 (albite), under ambient conditions but close to 460 cm-1 for the albite having been shocked to 45 GPa.29 This implies an internal compressive stress up to ca. 40 GPa for the present condensates assuming the pressure dependence of the IR band shift for the 4-coordinated tetrahedral T-O-T (T = Si or Al) vibration in 4999

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Table 2. Lattice Misfit Strain ε, i.e., (dγ - dN)/dN, for the Exact/Nearly Coincided Plane Normals of β-NaAlO2 and γ-Al2O3 relation A 3  (022)γ//2  (110)N

εAa

εBa

(202)γ//(110)N

þ1.9%

3  (141)γ//(001)N

þ7.09%

εAb

relation B

εBb

2  (022)γ//(110)N (111)γ//(001)N

-4.78% 0%

2  (202)γ//(110)N 5  (141)γ//2  (001)N

þ9.9% þ2.4%

5  (211)γ//3  (110)N

þ2.64%

(111)γ//(001)N 4  (211)γ//3  (110)N relation A

þ1.9%

relation B

þ12.61% -1.14%

Unconstrained misfit based on the ambient cell parameters of γ-Al2O3 (a = 0.7900 nm, JCPDS file 10-0425) and β-NaAlO2 (a = 0.53868 nm, b = 0.70334 nm, c = 0.52182 nm, JCPDS file 33-1200). b Constrained misfit based on the in situ d-spacings (cf. Table 1). The in situ cell parameters are assumed to be the same for the two relationships in the misfit calculation although the d-spacings of (141)γ//(001)N are not available in the zone axis [141]γ//[001]N of Figure 7. a

albite29 is valid in the present case. The composition, pressure, and structure dependences of the other vibration modes are beyond the scope of this study, and the pressure-dependent Raman shift of sodium aluminate is not available, to the author’s knowledge, to shed light on this problem. The internal stress of the predominant γ-Al2O3 can be alternatively estimated from the lattice parameters and known equation of state (EOS). The lattice parameter of dense γ-Al2O3 condensates was determined as a = 0.779 nm based on leastsquares refinement of the d-spacings in lattice images (Table 1). (The observed d-spacings from the lattice images in Figures 5, 6, 7, and 8 are accurate within (0.002 nm. The d-spacings of JCPDS file 10-0425 in Table 1 were based on the lattice parameter a = 0.7900 nm of synthetic γ-Al2O3 at room temperature and pressure.30 The refined d-spacings of the present dense γ-Al2O3 condensates (a = 0.779 nm) are also given in Table 1.) Despite an accuracy as large as (0.002 nm, this cell parameter is significantly smaller than γ-Al2O3 under ambient condition (a = 0.7900 nm). The internal compressive stress of the lattice turned out to be 7.2 GPa for the as-formed γ-Al2O3 nanoparticles if the Birch-Murnaghan EOS of nanosize γ-Al2O3 with bulk modulus Bo = 152 GPa and Bo0 (i.e., pressure derivative of Bo) = 6.8 GPa31 were used for the calculation. The internal compressive stress is 6.8 GPa if the Birch EOS and Bo= 153 GPa of the nanosize γ-Al2O3 particle32 were used for the calculation. It should be noted, however, that a mixture of phases is in general more compressible, meaning a lower bulk modulus. The coexistent β-NaAlO2 also suffered a significant internal stress for smaller basal-layer spacings than the ambient values, although the (110) d-spacing appeared to be larger than the ambient value (Table 1), indicating a complicated internal stress state. However, the EOS of β-NaAlO2 is not available for its internal stress determination. Lattice Correspondence of γ-Al2O3 and β-NaAlO2. The present experimental result indicated that the β-NaAlO2 formed intimate intergrowth with the γ-Al2O3 following epitaxial relationship A or B when synthesized by PLAL in aqueous solution with NaOH addition. The basal layer (001)N, a close packed plane in terms of AlO4 tetrahedra joined into 4- and or 6-rings of β-NaAlO2,24 is in parallel to (141)γ for relationship B,

whereas it is in parallel to the close packed plane (111)γ of γAl2O3 for relationship A. This indicates different reconstructive schemes upon Naþ detachment from the cavities of AlO4consisted rings in β-NaAlO2 or its higher symmetry polymorphs. Regarding the interfacial strain energetics, the unconstrained and constrained lattice misfit strains ε, i.e., (dγ dN)/dN, for the exact/nearly coincided plane normals of βNaAlO2 and γ-Al2O3 for the relationships A and B based on the stereographic projections in Appendices 1 and 2, respectively, are compiled in Table 2. The constrained misfit for (111)γ//(001)N turned out to be null for relationship A. This indicates a directional coherency-controlled transformation analogous to the coherency-controlled growth habit of precipitates in minerals.33 By contrast, the (110)N planes in parallel to the lattice fringes of (202)γ were preferentially preserved, whereas the nonepitaxial (1 10)N layers were vulnerable for relationship B (comparing Figures 7b and 7d) indicating that the directional multiple match of the adjoined planes is of concern in such a case.

V. CONCLUDING REMARKS A higher NaOH concentration (i.e., 1 M versus 0.05 M) in the PLAL process caused a higher content of NaAlO2 3 4/5H2O and β-NaAlO2 besides the predominant γ-Al2O3 nanoparticles. Upon settling on the silica substrate at room temperature, the NaAlO2 3 4/5H2O tended to develop as micrometer size plates for the sample fabricated under a relatively low (i.e., 0.05 M) NaOH concentration. However, the crystal structures, d-spacings, mismatch strain, morphology, and optical properties of the coexisting phases are almost the same for the samples fabricated under different NaOH concentrations. The β-NaAlO2 and γ-Al2O3 nanocondensates with signature of aliovalent ions, defects, and specific crystallographic relationships in terms of interfacial strain energetics, as determined in this study, are of concern to the Bayer process and have the following implications. Sodium aluminate is in fact a very caustic substance (pH ∼ 14) commonly used as an additive in paper manufacturing, water treatment, and pH adjustment in many applications.34 The UV absorption of sodium aluminate in aqueous solution at about 234 nm has been attributed to tetrahedrally coordinated Al(OH)4-.35 The predominant [AlO4] units in the present hydrous sodium aluminate showed also UV absorption, although at a shorter wavelength (210 nm),8 because of the coexistence of anhydrous sodium aluminate and dense γ-Al2O3. Still, having a smaller band gap (ca. 5.3 eV), the present nanocondensates may have potential applications in the UV region for extremely basic and energetic aqueous solutions. Cosmologically, the dense and epitaxial βNaAlO2 and γ-Al2O3 nanocondensates via the present PLAL process may shed light on natural dynamic settings in a presolar system regarding the evolution of minerals in the presence of water and light elements.36 ’ APPENDIX 1 Stereographic projection of the epitaxial relationship A between γ-Al2O3 (upright hkl) and β-NaAlO2 (italic hkl) in the zone axis [211]γ//[110]N. The ambient lattice parameters a = 0.53871 nm, b = 0.70320 nm, and c = 0.52180 nm of β-NaAlO223 were used for the plot. Note that the close packed (111) layer of the γ-Al2O3 turns out to be parallel to the (001) layer of NaAlO2 5000

dx.doi.org/10.1021/jp107030h |J. Phys. Chem. C 2011, 115, 4994–5002

The Journal of Physical Chemistry C consisting of corner-sharing AlO4 tetrahedra joined into 4- and/ or 6-rings.24

ARTICLE

supported by the Center for Nanoscience and Nanotechnology at NSYSU and National Science Council, Taiwan, ROC.

’ REFERENCES

’ APPENDIX 2 Stereographic projection of the epitaxial relationship B between γ-Al2O3 (upright hkl) and β-NaAlO2 (italic hkl) in the zone axis [141]γ//[001]N. The ambient lattice parameters a = 0.53871 nm, b = 0.70320 nm, and c = 0.52180 nm of β-NaAlO223 were used for the plot. Note that the close packed (111) layer of the γ-Al2O3 is not parallel to the (001) layer of NaAlO2 consisting of cornersharing AlO4 tetrahedra joined into 4- and/or 6-rings.24

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional XRD, SEM, TEM, Raman, FTIR, and UV-visible absorption results of the samples fabricated under 0.05 M and/or 1 M NaOH are compiled in supplements 1-8. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

* Fax: þ886-7-5254099. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. C. N. Huang for some PLAL runs with us and Miss S. Y. Shih for the help on XPS analysis. This work was

(1) Bayer, K. J. Patent DE-PS 43977, 1887. (2) Lia, H.; Addai-Mensaha, J.; Thomasb, J. C.; Gerson, A. R. J. Cryst. Growth 2005, 279, 508. (3) Pan, C.; Chen, S. Y.; Shen, P. J. Phys. Chem. B 2006, 110, 24340. (4) Lu, Y. F.; Huang, S. M.; Wang, X. B.; Shen, Z. X. Appl. Phys. A: Mater. Sci. Process. 1998, 66, 543. (5) Yang, G. W. Prog. Mater. Sci. 2007, 52, 648. (6) Lee, Y. P.; Liu, Y. H.; Yeh, C. S. Phys. Chem. Chem. Phys. 1999, 1, 4681. (7) Sajti, C. L.; Sattari, R.; Chichkov, V. N.; Barcikowski, S. J. Phys. Chem. C 2010, 114, 2421. (8) Liu, I. L. PhD thesis, National Sun Yat-sen University, Kaohsiung, Taiwan, 2010. (9) Liu, I. L.; Shen, P.; Chen, S. Y. J. Phys. Chem. C 2010, 114, 7751. (10) Liu, I. L.; Chen, S. Y.; Shen, P. J. Nanosci. Nanotechnol. 2011, in press. (11) Fabbro, R.; Fournier, J.; Ballard, P.; Devaux, D.; Virmont, J. J. Appl. Phys. 1990, 68, 775. (12) According to the pure water transmission spectrum of light (Kruusing, A. Underwater and water-assisted laser processing: Part 1-general features, steam cleaning and shock processing. Opt. Lasers Eng., 2004, 41, 307-327 and literature cited therein), the absorption length Δ is about 20 mm for the laser beam with a wavelength of 1064 nm. According to Beer-Lambert law, Ix = Io exp(-x/Δ) where Io is the entrance light intensity and Ix is the light intensity on the target surface at a distance x below the water level. Ix can be calculated as 660 mJ given x = 5 mm and pulse energy Io = 850 mJ in the present condition of PLAL. (13) Berthe, L.; Fabbro, R.; Peyre, L.; Tollier, L.; Bartnicki, E. J. Appl. Phys. 1997, 82, 2826. (14) Rousseaux, J. M.; Weisbecker, P.; Muhr, H.; Plasari, E. Ind. Eng. Chem. Res. 2002, 41, 6059. (15) Lovas, H.; T
oth, M.; Madarasz, J.; Josepovits, V. K.; T
oth, Z. J. Anal. Appl. Pyrolysis 2007, 79, 471. (16) Watling, H. Appl. Spectrosc. 1998, 52, 250. (17) Heymann, D.; H€orz, F. Phys. Chem. Miner. 1990, 17, 38. (18) Hwang, S. L.; Shen, P.; Chu, H. T.; Yui, T. F.; Liu, J. G.; Sobolev, N. V. J. Mineral 2009, 21, 1325. (19) Maruyama, T.; Arai, S. Appl. Phys. Lett. 1992, 60, 322. (20) Al-Abadleh, H. A.; Grassian, V. H. Langmuir 2003, 19, 341. (21) Eslami Farsani, R.; Raissi, S.; Shokuhfar, A.; Sedghi, A. World Acad. Sci., Eng. Technol. 2009, 50, 430. (22) Contreras, C. A.; Sugita, S.; Ramos, E. J. Mater. Online 2006, 2, 1. (23) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Oxford: Butterworth-Heinemann, 1997. (24) Kaduk, J. A.; Pei, S. J. Solid State Chem. 1995, 115, 126. (25) Carreira, L. A.; Maroni, V. A.; Swaine, J. W. J. Chem. Phys. 1966, 45, 2216. (26) Hung, P. K.; Nhan, N. T.; Vinh, L. T. Modell. Simul. Mater. Sci. Eng. 2009, 17, No. 025003. (27) Ansell, S.; Krishana, S.; Richard Weber, J. K.; Felten, J. J.; Nordine, P. C.; Beno, M. A.; Price, D. L.; Saboung, M. L. Phys. Rev. Lett. 1997, 78, 464. (28) Thompson, J. G.; Melnitchenko, A.; Palethorpe, S. R.; Withers, R. L. J. Solid State Chem. 1997, 131, 24. (29) Ostertag, R. Proceedings of the 14th Lunar and Planetary Science Conference, Part 1 Shock experiment on feldspar crystals. J. Geophys. Res. 1983, 88, B364. (30) Rooksby, H. P. In X-ray Identification and Crystal Structures of Clay Minerals; Bradley, W. F., Ed.; Mineralogical Society: London, 1951; p 264. (31) Zhao, J.; Hearne, G. R.; Maaza, M. J. Appl. Phys. 2001, 90, 3280. 5001

dx.doi.org/10.1021/jp107030h |J. Phys. Chem. C 2011, 115, 4994–5002

The Journal of Physical Chemistry C

ARTICLE

(32) Chen, B.; Penwell, D.; Benedetti, L. R.; Jeanloz, R.; Kruger, M. B. Phys. Rev. B 2002, 66, 1441011. (33) Hwang, S. L.; Shen, P.; Yui, T. F.; Chu, H. T. J. Appl. Crystallogr. 2010, 43, 417. (34) N€aykki, T.; Raimo, A.; Paavo, P.; Antero, K.; P€aivi, N. Talanta 2000, 52, 755. (35) Ma, S. H.; Zheng, S. L.; Xu, H. B.; Zhang, Y. Trans. Nonferrous Met. Soc. China 2007, 17, 853. (36) Hazen, R. M.; Papineau, D.; Bleeker, W.; Downs, R. T.; Ferry, J.; McCoy, T. L.; Sverjensky, D.; Yang, H. Mineral evolution. Am. Mineral. 2008, 93, 1693.

5002

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