H+- and Al2+-Codoped Al2O3 Nanoparticles with Spinel-Type

J. Phys. Chem. C 2010, 114, 7751–7757

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H+- and Al2+-Codoped Al2O3 Nanoparticles with Spinel-Type Related Structures by Pulsed Laser Ablation in Water I. L. Liu,† P. Shen,† and S. Y. Chen*,‡ Department of Materials and Optoelectronic Science, Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan, ROC, and Department of Mechanical and Automation Engineering, I-Shou UniVersity, Kaohsiung, Taiwan, ROC ReceiVed: January 10, 2010; ReVised Manuscript ReceiVed: March 17, 2010

Pulsed laser ablation in water under a high peak power density of 1.8 × 1011 W/cm2 using Q-switch mode and 1064 nm excitation was used to fabricate (H+,Al2+)-codoped Al2O3 nanocondensates having γ- and its derivative θ-type structure as characterized by electron microscopy and spectroscopy. The as-formed γ- and θ-Al2O3 nanocondensates are mainly 10 to 100 nm in size and have a significant internal compressive stress (>10 GPa) according to cell parameters and vibrational spectroscopy, due to a significant shock loading effect in water. The γ-Al2O3 nanocondensates are nearly spherical in shape but become cuboctahedra when grown to ca. 100 nm to exhibit more facets as a result of martensitic γfθ transformation following the crystallographic relationship (3j11j)θ//(02j2)γ; (02j4j)θ//(3j11)γ. The formation of dense and (H+,Al2+)-codoped γ/θ-Al2O3 rather than aluminum hydrates sheds light on the favored phases of the Al2O3-H2O binary at high temperature and pressure conditions in natural dynamic settings. The nanocondensates thus formed have a much lower minimum band gap (5.2 eV) than bulk R-Al2O3 for potential optocatalytic applications. I. Introduction The relative stability of the nanoparticles of γ-Al2O3 with spinel-type structure and R-Al2O3 with corundum-type structure was known to be affected significantly by a static water environment.1,2 The phase behavior of alumina nanoparticles was also seriously affected by a dynamic process such as pulsed laser ablation (PLA). The PLA on the Al target at extremely high power density under a specified oxygen flow rate in air typically caused dense γ-Al2O3 nanocondensates of abnormal large size up to micrometer scale.3 PLA in liquid (hereafter referred as PLAL) has an even higher heating-cooling rate than PLA and hence a more pronounced pressure effect to form smaller sized and denser nanocondensates.4,5 In fact, the PLAL route has been used to fabricate diamond,6-8 ZrO2,9 and Al2O3 nanoparticles.10 The Al2O3 nanoparticles were reported to be 7 to 20 nm in size when fabricated by PLAL using femtosecond Ti:sapphire laser pulses with a repetition rate of 1 kHz and pulse energy of 1.1 mJ.10 However, the phase, shape, and stress state of the alumina formed by PLAL were not clarified. It is not clear whether the Al2O3 nanoparticles thus formed have shape deviating from the periodic bond chain (PBC) model11-13 due to precondensation effect in solution, and whether the lattice is densified or with defects in a specific phase under the combined effects of defect chemistry and shock loading in water. It is also of interest to find out whether aluminum hydrates, such as tohdite, which stabilizes in the corundum pressure-temperature stability field,14,15 or the room pressure phases gibbsite and bayerite with a slight difference in density,16,17 can be formed in a dynamic PLAL process. * To whom correspondence should be addressed. Fax: +886-7-6578853. E-mail: [email protected]. † National Sun Yat-sen University. ‡ I-Shou University.

Here PLAL on the Al target was used to fabricate proton and Al2+-codoped Al2O3 nanoparticles with the spinel-type related structures, mainly γ- and its monoclinic derivative, that is, θ-phase having significant internal compressive stress and defects. The nanocondensates thus formed have a much lower minimum band gap than bulk R-Al2O3 for potential optocatalytic applications. These experimental results and defect chemistry considerations shed light on the protonation-oxolation process and the phase stability of Al2O3-H2O binary in a hightemperature and pressure regime. II. Experimental Section Al (Nilaco, 99.9% pure) plate 1 mm in thickness was immersed in deionized water within a glass beaker and then subjected to energetic Nd:YAG laser (Lotis, 1064 nm in wavelength, beam mode: TEM00) pulse irradiation at a specified pulse energy (i.e., 850 mJ/pulse with a pulse time duration of 16 ns at 10 Hz using Q-switch mode on a focused area of 0.03 mm2). Under such a condition, the average and peak power densities are 2.8 × 104 and 1.8 × 1011 W/cm2, respectively, with the latter being related to the shockwave-induced pressure according to an analytical model by Fabbro.18 The upper surface of the Al target was 5 mm below the water level in a beaker 6 cm in diameter full of deionized water ca. 15 cm3 in volume during such an ablation process. Ten independent PLAL syntheses were performed by 1064 nm to confirm that the experimental results are reproducible. Alternatively, second harmonic 532 nm excitation and a pulse energy of 400 mJ/pulse with an average and peak power density of 1.3 × 104 and 8.3 × 1010 W/cm2, respectively, were used to compare the PLAL results. An optimal synthesis time of 5 min, that is, a total of 3000 pulses at 10 Hz at a real peak power density of 1.2 × 1010W/ cm2 under the combined effects of water absorption19 and beam broadening,20 was adopted for a satisfactory yield of nanocondensates (ca. 1 mg/cm3) yet to circumvent severe plasma

10.1021/jp1002325  2010 American Chemical Society Published on Web 04/09/2010

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breakdown effects, which typically occur near 1 × 1010 to 1 × 1011 W/cm2.21-23 (According to the Beer-Lambert law,19 the 1064 nm laser pulse energy can be reduced to 516 mJ/pulse by water absorption effect. Besides, the focused area can be increased to about 0.27 mm2 due to laser beam coherence loss in the liquid environment.20 Under such conditions, the peak power density was estimated as 1.2 × 1010 W/cm2 for 1064 nm excitation. For the same reasons, the peak power density was reduced to about 8 × 109 W/cm2 when 532 nm excitation was used for the ablation.) The condensates were centrifuged (10 000 rpm for 30 min) for a much higher concentration and then collected on the glass substrate for morphology and structure characterization using scanning electron microscopy (SEM, JSM6700 at 10 kV) and X-ray diffraction (XRD, SIEMENS D1, Cu KR at 45 kV, 35 mA, and 3s for each 0.05° increment from 20 to 90 of 2θ angle). The same sample was also used for X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MX photoelectron spectrometer with Mg KR X-ray source) study calibrated with a standard of C 1s at 284 eV regarding the Al 2p peak about the possible presence of Al+ and Al2+ besides the predominant Al3+ in the condensates. Raman spectrum of the same sample was made using semiconductor laser excitation (633 nm) having a spatial resolution of 1 µm (HORIBA HR800). The centrifuged condensates were also mixed with KBr for Fourier transform infrared spectroscopy (FTIR, Bruker 66v/S, 64 scans with 4 cm-1 resolution) study of the OH- signature. Alternatively, the γ-Al2O3 nanoparticles enriched in the upper portion of the colloidal solution (ca. 1 wt % in concentration) were settled and collected on Cu grids without centrifugation for composition and crystal structure characterization by fieldemission transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV), coupled with selected area electron diffraction (SAED), and point-count energy-dispersive X-ray (EDX) analysis at a beam size of 10 nm. Bright-field images (BFI) taken by TEM were used to study the morphology and agglomeration of the nanocrystals. Lattice images coupled with 2-D Fourier transform and inverse transform were used to characterize the planar defects of the as-formed nanocondensates. The UV-visible absorption of the colloidal solution containing the as-formed nanocondensates was characterized by the Hitachi U-3900H, with a resolution of 0.1 nm in the range of 200 to 900 nm.

Liu et al.

Figure 1. X-ray diffraction (Cu KR) of the γ+θ-type Al2O3 condensates as fabricated by PLAL using a peak power density of 1.8 × 1011 W/cm2 for 5 min at 1064 nm excitation and then centrifuged.

III. Results XRD and SEM. XRD indicated the Al2O3 nanocondensates fabricated by PLAL and then centrifuged from the colloidal solution for a much larger quantity were mainly γ- and θ-type phases (Figure 1). The broad diffraction near 21° 2θ is due to silica substrate. These nanocondensates were mainly 10 to 100 nm in size and agglomerated up to micrometers in size without appreciable impurities as indicated by SEM coupled with EDX analysis (Figure 2). (In fact, our combined XRD, SEM, TEM, UV-vis absorption, and XPS results indicated that the nanocondensates were agglomerated/coalesced and changed partially to bayerite for a slightly different optical property when subjected to prolonged dwelling in water.24) Vibrational, XPS, and UV-Visible Spectra. The γ/θ-Al2O3 nanocondensates showed a broad Raman band below 439 cm-1 (Eg), sharper bands at 489 (T2g) and 603 cm-1 (T2g), and a broad band above ca. 801 cm-1 (A1g) (Figure 3a), which can be assigned as the modes in parentheses analogous to those for the analogous spinel-type oxides.25 The bands at 603 and 801 cm-1 can be alternatively related to the modes of R-Al2O326 for

Figure 2. (a) SEM SEI, (b) point-count (marked as X) EDX of the γ+θ-Al2O3 nanocondensates as fabricated by PLAL. The Si and Pt counts are from glass substrate and Pt coating, respectively. The same specimen as in Figure 1.

an approximate internal stress estimation of the constituent polyhedra as discussed later. Figure 3b shows the corresponding FTIR spectrum of the γ+θ-Al2O3 nanocondensates. The substantial absorption band is centered around 720 to 626 cm-1, in comparison with 785 to 603 cm-1 for the γ+θ-Al2O3 sample obtained by thermal treatment at 1000 °C of bayerite27 and 570 cm-1 for magnetite of the spinel-type isostructure.28 The bands in this region can be attributed to stretching of anions against octahedral and tetrahedral cations.27 The OH signature of the as-formed nanocondensates by PLAL was manifested by an additional strong band at 3421 cm-1 (Figure 3b). The bands at 2923 and 2860 cm-1 are from EtOH used for IR sample preparation. The other absorption bands at 1633, 1389, and 1058 cm-1 are sharp but rather weak, which can possibly be attributed, respectively,

H+- and Al2+-Codoped Al2O3 Nanoparticles

Figure 3. (a) Raman and (b) FTIR spectra of the OH-signified γ+θAl2O3 nanocondensates as the sample in Figure 1.

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7753 to O-H bending, carbonate absorbed from air, and the distorted tetrahedral and octahedral sites of the θ-Al2O3 residing with Al ions of varied valence under the influence of proton dopant. Regarding the valence state of the ions in the present OHsignified γ+θ-Al2O3, XPS spectrum (Figure 4a) shows O 1s (532.4 eV), Al 2s (119.4 eV), and Al 2p (Al2+ and Al3+ at 71.8 and 74.5 eV, respectively) bands according to the Lorentzian fits (Figure 4b,c, respectively) and the assignment of Palacio and Arranz.29 The UV-visible absorption spectrum of the colloidal solution containing the OH-signified γ+θ-Al2O3 nanoparticles shows a distinct absorbance band corresponding to a minimum band gap of 5.2 eV, based on its intersection with the baseline at 240 nm (Figure 4d). TEM. The TEM BFI and corresponding SAED pattern of the as-formed γ-Al2O3 nanocondensates are shown in Figure 5 for the case of PLAL under a peak power density of 1.8 × 1011 W/cm2 using 1064 nm excitation. Upon electron irradiation for minutes, the coexisting amorphous Al2O3 was largely crystallized as γ-Al2O3 (Figure 5c). The particle size and phase identity of the product by the second harmonics (i.e., 532 nm excitation (not shown)) are basically the same as that produced under 1064 nm excitation. Lattice image coupled with 2-D forward and inverse Fourier transform further indicated that the individual γ-Al2O3 nanoparticles were more or less coalesced toward unity, leaving an amorphous relic in the neck area decorated with misfit dislocations (Figure 6). Such nanoparticles showed faults and dislocation half-planes parallel to {220}, although the dislocation halfplane (11j3) was occasionally observed (Figure 7). Lattice image (Figure 8a) coupled with 2-D forward and inverse Fourier transform (Figure 8b,c, respectively) of the γ-Al2O3 nanocondensate also showed its intimate mixture with the θ-type domains following a specific crystallographic relationship;

Figure 4. (a) XPS spectrum of the OH-signified γ+θ-Al2O3 condensates deposited on glass by PLAL as the specimen in Figure 1. The binding energy in (a) is characterized by O 1s (532.4 eV), Al 2s (119.4 eV), and Al 2p (Al2+ and Al3+ at 71.8 and 74.5 eV, respectively) as indicated by Lorentzian fit in (b) and (c) for O and Al, respectively. The C 1s counts are from absorbed hydrocarbon contaminant. (d) UV-visible absorption spectrum of the colloidal solution containing the nanocondensates.

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Figure 7. TEM (a) lattice image of γ-Al2O3 nanoparticle in [141] zone axis, (b) 2-D Fourier transform, (c) inverse Fourier transform from the square region in (a) showing (2j02) fault denoted by white line and dislocation half plane (11j3). The same specimen as in Figure 5.

Figure 5. TEM (a) BFI and (b) corresponding SAED pattern of the γ-Al2O3 nanocondensates as formed by PLAL under a peak power density of 1.8 × 1011 W/cm2 for 5 min using Q-switch mode and 1064 nm excitation. (c) SAED pattern taken after electron irradiation for minutes showing further crystallization of the γ-Al2O3.

Figure 8. TEM (a) lattice image of a partially transformed γ-Al2O3 condensate ca. 100 nm in size as formed by PLAL under a peak power density of 1.8 × 1011 W/cm2 for 5 min, (b,c) 2-D Fourier transform ([233]γ and [121j]θ zone axis) and inverse Fourier transform, respectively, from the square region in (a) showing intimate intergrowth of θ-type domains and γ-type relic following the crystallographic relationship (3j11j)θ//(02j2)γ; (02j4j)θ//(3j11)γ with the possible (1j10)θ/(11j1)γ interface inclined to the zone axis. The same specimen as in Figure 5. Figure 6. TEM (a) lattice image of the γ-Al2O3 nanoparticles coalesced almost as unity yet with amorphous relic in the neck area, (b) 2-D Fourier transform, (c) inverse Fourier transform from the well-crystallized square region in (a) showing (220) faults denoted by white lines, (d,e) 2-D Fourier and inverse Fourier transform from the neck region in (a) showing the coalesced two are in [001] zone axis with misfit dislocations at the interface. The same specimen as in Figure 5.

(3j11j)θ//(02j2)γ; (02j4j)θ//(3j11)γ (hereafter refer to as relation Β), which allows almost parallel alignment of the (1j10)θ and (11j1)γ as discussed later. A possible (1j10)θ/(11j1)γ interface inclined to the zone axes (i.e., [121j]θ and [233]γ) accounts for partially superimposed lattice fringes of the θ- and γ-phase in Figure 8. The partially transformed nanocondensate was full of facets (as

labeled in Figure 8) and faults/dislocations implying a martensitic type γfθ transformation, that is, shear-type allotropic transformation. The γ- and θ-Al2O3 nanoparticles centrifuged from the asformed colloidal solution showed further details of their twinning microstructures. The γ-Al2O3 nanoparticles were typically twinned over {111} planes and faceted by (001) plane (Figure 9a,b). These twins were likely formed by the {111}-specific coalescence event or alternatively by growth twinning. Occasionally, (002) superlattice fringes due to partial transformation to another spinel-type distorted structure, such as orthorhombic δ-phase, were observed in the lattice image of the individual

H+- and Al2+-Codoped Al2O3 Nanoparticles

Figure 9. TEM (a) BFI of γ-Al2O3 nanoparticles up to ca. 100 nm in size which were twinned over {111} planes and faceted by (001) plane as labeled, (b) corresponding SAED pattern in [11j0] zone axis showing twin diffraction denoted as t, (c) lattice image taken from the corner of the particle in (a) (indicated by white arrow) coupled with 2-D forward (inset) and (d) inverse Fourier transform from the square region showing (002)δ superlattice fringes and (004)γ (denoted by black lines) due to partial transformation to orthorhombic δ-phase in [100] zone axis. The same specimen as in Figure 1.

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Figure 11. Stereogram of the crystallographic relationship (3j11j)θ// (02j2)γ; (02j4j)θ//(3j11)γ in the parallel zone axes [121j]θ//[233]γ given ambient lattice parameters a ) 1.174 nm, b ) 0.572 nm, c ) 1.124 nm, and β ) 103.34° for θ-Al2O3 (JCPDS file 11-0517). The hkl in red and italic hkl in black are for γ- and θ-phase, respectively.

when formed by γfδfθ transformation following the crystallographic relationship (100)θ//(001)γ; [010]θ//[110]γ (hereafter refer to as relation Α) in a sintered sample.30 IV. Discussion Theoretical Maximum Shock Pressure via PLAL. The thermodynamic and kinetic factors of laser ablation of solids in liquids can greatly influence the phase transformation based on the understandings of the evolution of the laser-induced plasma.18 In general, the liquid confinement induces a shockwave for high pressure and temperature conditions which may promote dense phases such as diamond6-8 not accessible by the PLA method. The maximum shock pressure generated by the laser plasma in the water-confined regime was given by an analytical model:18

 R +R 3 √Z(g cm

P(GPa) ) 0.01

Figure 10. TEM (a) lattice image of the twinned θ-Al2O3 bicrystals ca. 100 nm in size viewed along [3j22j] zone axis, coupled with (b) 2-D forward and (c) inverse Fourier transform from the square region in (a) showing twin diffraction denoted as t in (b) and semicoherent (011) twin plane (denoted by a solid line) with nearby faults as indicated by the stepwise offset of the lattice fringes outlined in (c). The same specimen as in Figure 1.

γ-Al2O3 particle (Figure 9c,d). The θ-Al2O3 also formed twinned bicrystals, which however have a (011) twin plane as shown edge on in [3j22j] zone axis (Figure 10a). The 2-D forward and inverse Fourier transform images (Figures 10b,c) show that the (011) twin plane is semicoherent with respect to the lattice planes across it and have faults nearby for strain relaxation. The (011) twin plane of θ-Al2O3 was hardly derived from the {111} twin plane of the γ-Al2O3 bicrystals because the plane normals (011)θ and (011j)θ are off from (111)γ by ∼66 and 34°, respectively, based on the stereogram of the relation B (Figure 11). By contrast, the θ-Al2O3 shows (001) multiple twin planes

s )√I0(GW cm-2)

-2 -2

(1)

where R is the fraction of internal energy devoted to thermal energy (typically R ∼ 0.25), I0 is the incident power intensity, and Z is the reduced shock impedance between the target and the confining water defined by the relation

2 1 1 + ) Z Zwater Ztarget

(2)

where Zwater and Ztarget are the shock impedances of the water and the target, respectively. For the aluminum target, Ztarget ) 1.5 × 106 g cm-2 s-1 and Zwater ) 0.165 × 106 g cm-2 s-1.18 The shockwave-induced pressure can then be calculated as 20.4 GPa given a peak power density of 1.8 × 1011 W/cm2 as adopted in this study. In comparison, the size-induced pressure ∆P of the nanoparticles 10-70 nm in size is much lower, in fact 0.07-0.5 GPa based on the Laplace-Young equation ∆P ) 2γ/r, where r is the radius and γ the surface energy 2.55 J/m2 assuming the value of R-Al2O331 can be extended to γ-Al2O3. (The relaxed surface energy is not available for γ-Al2O3 to the author’s knowledge.) It is worthwhile to note that the amorphous

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TABLE 1: Observed and Calculated d Spacings (nm) for the Dense γ-Al2O3 Nanocondensates (hkl)

observed (nm)

JCPDS 10-0425

refined (nm)

111 220 311 400

0.441 0.262 0.236 0.188

0.4560 0.2800 0.2390 0.1970

0.439 0.269 0.229 1.901

Al2O3, either liquid, supercooled liquid, or glass,32 has Al ion in coordination number (CN) of 4 for the nucleation site of γ-Al2O3 having spinel-like structure with cations in CN of 4 and 6.33 The four-coordinated Al3+ ions in liquid aluminum oxide thus may act as nucleation sites for the crystallization of dense γ-Al2O3 nanoparticles by PLA34 and the present PLAL with a much higher shockwave-induced pressure. Observed Internal Stress of the γ-Al2O3 Nanocondensates. The present γ+θ-Al2O3 nanocondensates by PLAL are expected to have a significant internal compressive stress in view of previous photoluminescence R line shift of Cr-doped Al2O3 nanocondensates via PLA.35 The internal stress can be inferred from Raman shift of the individual phases in the present nanocondensates. Unfortunately, the pressure dependence of Raman shift is not known for γ-Al2O336 and θ-Al2O3. Still, the bands at 603 and 801 cm-1 may be related to the modes of R-Al2O326 for approximate internal stress estimation. The changes from 575.9 to 603 cm-1 for the A1g mode and 749.9 to 801 cm-1 for the Eg mode would then indicate an internal compressive stress up to ca. 12 GPa for the constituent polyhedra, assuming the pressure dependence of the Raman shift determined by static compression of submicrometer sized R-Al2O3 powders26 is valid for the present γ+θ-Al2O3 nanocondensates. 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 ) 0.761 nm based on least-squares refinement of the d spacings in lattice images (Table 1). (The observed d spacings from the lattice images in Figures 7, 8, and 9 are accurate within (0.002 nm. The d spacings of JCPDS file 10-042537 in Table 1 were based on the lattice parameter a ) 0.7900 nm of synthetic γ-Al2O3 at room temperature and pressure. The refined d spacings of the present dense γ-Al2O3 condensates (a ) 0.761 nm) are also given in Table 1.) Despite an accuracy as large as (0.002 nm, this cell parameter is significantly smaller than the γ-Al2O3 under ambient condition (a ) 0.7900 nm).37 The internal compressive stress of the lattice turned out to be 25 GPa for the as-formed γ-Al2O3 nanoparticles if the Birch-Murnaghan EOS of nanosize γ-Al2O3 with bulk modulus Bo ) 152 GPa and Bo′ (i.e., pressure derivative of Bo) ) 6.8 according to ref 38 were used for the calculation. The internal compressive stress is 22 GPa if Birch EOS and Bo ) 153 GPa of nanosize γ-Al2O3 particle39 was used for the calculation. It should be noted, however, that a mixture of phases is, in general, more compressible, meaning a lower bulk modulus. Thus, the internal stress level of the present Al2O3 nanocondensates with a mixture of γ- and θ-phases could still be overestimated based on the bulk modulus of γ-Al2O3 alone. This accounts for a higher estimated stress value than that (12 GPa) using Raman shifts of analogue Al2O3 material as mentioned. Finally, the observed internal compressive stress level, either in terms of the constituting polyhedra or the cell volume, is closer to the theoretical maximum shock pressure rather than size-induced pressure as calculated above. Additional effect of extremely rapid heating and cooling, which is up to108 K/s according to the gray-body radiation calculation for the Al2O3

condensates 10 nm in size,3 may also contribute to the retained internal stress of the unit cell. Still, the internal stress level of the present γ-derived Al2O3 nanocondensates is higher than that (several GPa) of the Al2O3 polymorphs without or with Cr3+ dopant by PLA under oxygen background gas.3,35 Oxolation and Defect Chemistry of H+- and Al2+-Codoped Al2O3 via PLAL. Oxolation of high-valence cations such as Al3+ to form O-Al-O bridges in the presence of OH ligands often leads to the formation of polyanions and oxides.40 In general, the Al3+ would tend to occupy octahedral rather than tetrahedral sites as the pH decreases.34 (Below pH 3, octahedral aluminum is observed, whereas tetrahedral aluminum is formed above pH 11 according to Livage.40) In the present PLAL process at pH ) 7, the Al3+ would form both AlO4 and AlO6 units via the oxolation process to form γ-Al2O3 having Al3+ ions in both tetrahedral and octahedral sites assuming the thermal and shockwave effects did not affect much the pH dependence of the oxolation process. It is an open question if the isoelectric point of Al2O3 (pH ) 8.1,41 for the case of ambient pressure and temperature) was considerably changed at high temperature and pressure to affect the oxolation process in the present PLAL study. The present γ- and θ-type Al2O3 nanocondensates have a considerable Al2+ content and OH- signature according to XPS and FTIR results, respectively. In other words, the Al3+ and the doped Al2+ and H+ ion species were co-incorporated in the spinel-type derived Al2O3 when fabricated by PLAL. Under such a complicated case, charge-compensating defect clusters [Hi• + AlAl′] in association with the interstitial proton would occur through the following equation in Kro¨ger-Vink notation:42 H2O + AlO

Al2O3 98 2[Hi• + AlAl′] + 3OOx

Here Hi• signifies single positively charged hydrogen in the interstitial octahedral and/or tetrahedral sites and AlAl′ dominating single negatively charged aluminum in the crystal lattice. In such a defect chemistry scheme, the Al2+ (effective ionic radius not known) would replace a smaller sized Al3+ in the octahedral (effective ionic radius 0.0535 nm) or tetrahedral site (effective ionic radius 0.039 nm).43 It is thus possible that the volume-compensating effect, due to the oversized Al2+ dopant in the Al3+ site, caused volume-compensating oxygen and/or aluminum vacancies and hence further generation of the chargecompensating defects. The defect clustering of Al2+-, Al3+-, and H+-codoped Al2O3 through eq 1 would be effective in a hydrothermal oxolation process for polymerization and formation of defect clusters. Apparently, polymerization is more important than hydrolysis in the present PLAL process to form dense yet H+- and Al2+codoped γ- and θ-type Al2O3 rather than aluminum hydrates such as tohdite,14,15 gibbsite, and bayerite.16,17 This sheds light on the favored phases of Al2O3-H2O binary at high temperature and pressure conditions in natural dynamic settings. Shape and Lattice Correspondence of the γ- and θ-Al2O3 Nanocondensates. Precondensation such as solidification or crystallization from melt/solutions may favor surprising faces not following the PBC predictions.12 For example, the S- and K-faces of Barite crystal were suggested to be favored by dehydration and impurities.13 In addition, diamonds tend to form dendrite with unusual {100} faces rather than {111} under the combined effects of H2O and a large driving force.44 The present γ-Al2O3 nanocondensates are nearly spherical in shape (Figure 6) but became cuboctahedra when grown to

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TABLE 2: Lattice Misfit Strain, ε ) (dθ-dγ)/dγ, for the Exact/Nearly Coincided Plane Normals of Dense γ-Al2O3 and θ-Al2O3 in Figure 8 (Relation B) and Ref 30 (Relation A) εB % (3j11j)θ//(02j2)γ (02j4j)θ//(3j11)γ (1j10)θ/(11j1)γ 0.16° off

+7.77% -13.71% +16.51%

εA % 2(100)θ//3(001)γ (010)θ//(110)γ

-0.02% +2.52%

ca. 100 nm (Figure 9). The precondensation effect as a result of H+ and Al2+ codopant could be effective for the selection of {100} with two PBCs besides {111} with zero PBCs for the present γ-Al2O3 nanocondensate. (Note the {001}, {110}, and {111} of the spinel-type oxide, such as γ-Al2O3 has 2, 1, and 0 PBCs, respectively, in terms of the edge-shared octahedral (cf. Figure 10 of Lin et al.45)) There was further shape change due to martensitic type γfθ transformation at a critical size of ca. 100 nm following a specific crystallographic relationship as mentioned (Figure 8). The H+ and Al2+ codopant appeared to affect the critical size of the polymorphic transformation for γ-Al2O3. In this regard, the γ-Al2O3 condensates produced by PLA on the Al target under oxygen background gas were found to grow up to an unexpected micrometer size, which transformed upon electron irradiation into metastable orthorhombic δ-form full of twin variants.3 The matrix constraint of the ambient sample via a sintering route favors relationship A, whereas the present H+- and Al2+-codoped γ-Al2O3 nanocondensate with a significant internal compressive stress prefers to transform into θ-Al2O3 via an alternative relationship B, which allows almost parallel alignment of the (1j10)θ and (11j1)γ planes, as indicated by the stereographic projection in Figure 11. The misfit strains (dθ-dγ)/dγ for the adjoined lattice planes in relationships A and B, as shown by exact or nearly superimposed plane normals in the stereographic projection, are calculated using the lattice parameters of the present dense γ-Al2O3 nanocondensates against the ambient parameters of θ-Al2O3. The lattice parameters of ambient θ-Al2O3 (a ) 1.174 nm, b ) 0.572 nm, c ) 1.124 nm, and β ) 103.34° after JCPDS file 11-0517) and dense γ-Al2O3 (a ) 0.761 nm) were used for the calculation of εB and εA to compare the mismatch stain. As compiled in Table 2, there is a larger lattice misfit for relationship B of the present nanocondensates than relationship A of the bulk material under a matrix constraint effect.30 There are rather large mixed (+ and -) strains for the coincided planes in relationship B. It is therefore not apparent that relationship B is a cusp. It could also be a local maximum relative to a small rotation to either side of the symmetric operation for lower surface energy. It is not clear either if H+/Al2+-codopant helped change the shuffling/twinning scheme and lattice correspondence of the γfθ transformation. V. Conclusions The nanocondensates fabricated by PLAL (in water) on the Al target under a high peak power density of 1.8 × 1011 W/cm2 for a significant shock loading effect are H+- and Al2+-codoped Al2O3 with densified spinel-type derived structures rather than aluminum hydrates. This sheds light on the favored phases of the Al2O3-H2O binary at high temperature and pressure conditions in natural dynamic settings. The dense and H+/Al2+codoped Al2O3 shows special twins, facets, and lattice correspondence due to the combined composition and internal stress effects on the γfθ-type transformation. Such nanocondensates show a minimum band gap of 5.2 eV, much lower than that (8.8 eV) of bulk R-Al2O346 and therefore may have potential optocatalytic applications in an aqueous environment.

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