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ZnO and ε-Zn(OH)2 Composite Nanoparticles by Pulsed Laser Ablation on Zn in Water B. C. Lin,† P. Shen,† and S. Y. Chen*,‡ † ‡
Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan, R.O.C. Department of Mechanical and Automation Engineering, I-Shou University, Kaohsiung, 84001, Taiwan, R.O.C. ABSTRACT: Wurtzite-type (W)-ZnO and ε-Zn(OH)2 composite nanoparticles following a specific crystallographic relationship (0001)W//(010)ε; [1120]W//[100]ε were fabricated by pulsed laser ablation on a Zn target in water and then characterized by analytical electron microscopy. The composite nanoparticles are platy and equiaxed in shape when fabricated by a peak power density of 1.7 1011 W/cm2 (1064 nm excitation) and 8.3 1010 W/cm2 (532 nm excitation), respectively. The W-ZnO nanoplates showed well-developed (0001) terraces for mutual coalescence and {1010} and {1120} edges for lateral growth. The W-ZnO and εZn(OH)2 composite nanocondensates have an internal compressive stress up to ca. 0.7 GPa and a minimum band gap of ∼3.1 eV for potential optoelectronic applications in water environment.
I. INTRODUCTION Wurtzite (W)-type zinc oxide has a hexagonal close-packed oxygen framework with half of the tetrahedral sites occupied by Zn2þ ion,1 and is a widely used semiconductor material for electrochemical, piezoelectric, and optoelectronic applications.2 In a water environment, various structures of zinc hydroxides are known to occur. The ε-Zn(OH)2 phase having a space group P212121 in which ZnO4 tetrahedra make up a three-dimensional network3,4 is the most stable under ambient conditions. Other types of crystal structures occurring are R-type with a double layered lattice having tetrahedrally as well as octahedrally coordinated Zn2þ and unknown layer structures such as β1-, β2-, and γ-types.5 The high-pressure Zn(OH)2 phases include a hexagonal CdI2-type recovered to ambient condition from 11-12 GPa and 400 ( 40 °C,6 a tetragonal intermediate,7 and a new orthorhombic high-pressure high-temperature phase.8,9 Pulsed laser ablation (PLA) has been used to fabricate W-ZnO in the form of film or nanoparticles. The W-ZnO thin film produced by PLA in O2 gas shows green luminescence due to defect centers associated with the oxygen vacancies.10 The W-ZnO condensates fabricated by PLA on Zn target in a vacuum typically show {1011} artificial epitaxy on glass.11 Such condensates are expected to have a considerable internal stress to transform into rock salt-type structure (R-ZnO) upon electron irradiation in vacuum.12 Thermal oxidation of such dense nanocondensates was also found to cause {1011}- and {1121}specific growth and twinning to form W-ZnO whiskers13 or even tapered W-ZnO whiskers by {hkil}-specific mosaic twinning vapor-liquid-solid (VLS) growth from a partially molten bottom source.14 The internal stress of the W-ZnO nanocondensates coexisting with Zn as fabricated by PLA in vacuum was not determined.15 r 2010 American Chemical Society
PLA in various liquids (PLAL) was alternatively used to fabricate zinc hydroxide/surfactant nanocomposite16 and W-ZnO nanoparticles.17,18 A dense phase of zinc hydroxide, i.e., β-Zn(OH)2 having a CdI2-type hexagonal dense structure with higher coordination number of Zn2þ,6 was found to occur as sheet intimate intergrowth in zinc hydroxide/surfactant nanocomposite.16 On the other hand, nearly spherical W-ZnO nanoparticles were produced by PLAL in water with/without surfactants (cationic, anionic, amphoteric, and nonionic) and the particle size decreases with the increase of surfactant concentration.17 The W-ZnO nanoparticles thus fabricated typically showed UV-visible photoluminescence (PL) exciton emission at 364.7 nm and defect emission at 541.5 nm. The green defect emission intensity caused by oxygen defects of ZnO was found to decrease whereas the exciton UV emission increased as the concentration of amphoteric increased.17 The effect of high temperature on the anisotropic growth of the W-ZnO nanoparticles during PLAL or subsequent aging in surfactant solutions was also reported.18 The equiaxed W-ZnO nanoparticles via the PLAL route turned out to grow as hexagonal nanorods extending along the crystallographic c-axis with an aspect ratio of about 2.5-3 when the solution was about 80 °C, whereas it had the equant shape when PLAL or subsequent aging in solution was conducted at room temperature.18 In this study, PLAL (in water) was used to fabricate densified W-ZnO/ε-Zn(OH)2 nanocomposites under a power density Special Issue: Laser Ablation and Nanoparticle Generation in Liquids Received: July 30, 2010 Revised: September 26, 2010 Published: November 15, 2010 5003
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more than an order of magnitude higher than that adopted previously.17,18 We focused on the combined effects of laser parameter and water media on the size, shape, defect microstructures, internal stress, and optical properties of the W-ZnO/ ε-Zn(OH)2 nanocomposites following a definite crystallographic relationship. The ε-Zn(OH)2 phase was known to occur by eletrochemical oxidation of zinc in NaOH/NH3 solution,4 or sol-gel synthesis leading to W-ZnO with unusual octahedral shape.19 In general, W-ZnO forms after zinc hydroxide under hydrothermal conditions.20 The epitaxial relationship between W-ZnO and ε-Zn(OH)2, as of concern to the production, applications and natural occurrence of W-ZnO in water, however, was not reported until this study.
II. EXPERIMENTAL SECTION Zn plate with negligible impurities (99.9% pure) was subject to energetic Nd:YAG-laser (Lotis, 1064 and 532 nm in wavelength, beam mode: TEM00) pulse irradiation in deionized (DI) water. The upper surface of the 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. Laser beam was focused to a spot size of 0.03 mm2 on the target under laser pulse energy of 800 mJ/pulse using 1064 nm excitation to achieve a peak power density of 1.7 1011 W/cm2 (average power density 2.6 104 W/cm2) given pulse time duration of 16 ns at 10 Hz under Q-switch mode. A pulse energy of 400 mJ/pulse under 532 nm excitation was also adopted to lower the peak power density to 8.3 1010 W/cm2 (average power density 1.3 104 W/cm2) to compare the experimental results. The errors of the power densities due to the combined factors of water absorption and optical path are estimated as 25% and 3% for the present PLAL using wavelength of 1062 and 532 nm, respectively.21 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 3-5 s for each 0.01° increment from 25-65° of 2θ angle). XRD at such a slow scan rate was used to resolve the individual diffractions so that the relative intensity of (1010)W, (0002)W, and (1011)W can be compared with the theoretical ratio, i.e., 57:44:100 (JCPDS file 36-1451), as far as the preferred orientation of W-ZnO is of concern. Scanning electron microscopy (SEM, JEOL6330, 10 kV) was used to study the size distribution and microstructures of the condensates deposited on glass. The composition and crystal structures of the individual condensates collected on the carbon-coated collodion film were characterized by field emission transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV under a high vacuum of 6.6 10-10 Pa and a beam current of 155 μA) coupled with selected area electron diffraction (SAED) and point-count energy dispersive X-ray (EDX) analysis at a beam size of 5 nm. Scanning transmission electron microscopy (STEM) mode was also used to study the general size distribution of the condensates. Alternatively TEM JEOL3010 instrument at 200 kV under a lower vacuum of 3 10-5 Pa and a lower beam current of 76 μA (dark current 73 μA) was used to image successfully the relic ε-Zn(OH)2 which typically transformed within seconds into W-ZnO upon electron irradiation.22 The errors are different for the lattice parameters determined by XRD ((0.0002 nm) and TEM ((0.002 nm) on the basis of SAED pattern and lattice fringe spacings. The UV-visible absorption of the colloidal solution containing the as-formed nanocondensates was characterized by the
Figure 1. XRD (CuKR) of W-ZnO and ε-Zn(OH)2 composite nanocondensates fabricated under a peak power density of (a) 1.7 1011 W/ cm2 (1064 nm excitation) and (b) 8.3 1010 W/cm2 (532 nm excitation) showing (0001) preferred orientation of W-ZnO, as indicated by abnormally high intensity of (0002) relative to (1011) and (1010) (cf. text) for the W-ZnO enriched sample in (a). The ε-Zn(OH)2 phase of orthorhombic structure (cf. text) is enriched in (b).
instrument of U-3900H, Hitachi, with a resolution of 0.1 nm in the range 200-900 nm. The same colloidal solution was dipped onto silica glass and then dried 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 284.2 eV regarding the Zn 2p3/2 peak and O 1s.
III. RESULTS XRD, STEM, and TEM. XRD indicated the nanocondensates as fabricated by PLAL consist of W-ZnO and ε-Zn(OH)2 (Figure 1a). A relatively high peak power density of 1.7 1011 W/cm2 (Figure 1a) caused better development of W-ZnO with (0002) preferred orientation as indicated by its abnormally high intensity than the supposedly most strong diffraction, i.e., (1011) and the second strong diffraction, i.e., (1010) based on JCPDS file 36-1451 as mentioned. In contrast, a relatively low peak power density of 8.3 1010 W/cm2 (Figure 1b) caused more Zn(OH)2 with distinct diffractions 102, 112, 202, 124, and 311, which can be unambiguously identified as due to ε-type (JCPDS 89-0138). Least-squares refinement of the d-spacings measured from the nonoverlapped XRD peaks (Figure 1) of the two phases (Table 1) gave lattice parameters a = 0.3245 and c = 0.5195 ( 0.0002 nm for W-ZnO and a = 0.4852, b = 0.5132, and c = 0.8324 ( 0.0002 nm for ε-Zn(OH)2 which are predominant in the samples prepared by a relatively high and low peak power density, respectively. The cell parameters of both phases are 5004
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Table 1. Observed and Calculated d-Spacings for W-ZnO (hkil)
observed (nm)a
JCPDS 36-1451
refined (nm)
1010
0.2807
0.2814
0.2810
0002
0.2599
0.2603
0.2597
1011
0.2473
0.2476
0.2472
1012
0.1906
0.1911
0.1908
1120
0.1623
0.1625
0.1623
1013
0.1474
0.1477
0.1474
a
0.3249
0.3245
c
0.5206
0.5195
a
Based on XRD of as fabricated nanocondensates (Figure 1a) with an accuracy of (0.0002 nm.
Figure 3. TEM (a) BFI, (b) SAED pattern, and (c) point count EDX spectrum of a nearly spherical W-type ZnO particulate with corrugated surface and superimposed W-ZnO nanocondensates in random orientation, presumably derived from ε-Zn(OH)2 nanocondensates upon electron irradiation. Note circular yet corrugated thickness fringes in (a) are in accord with surface roughening of the particulate. The same specimen as in Figure 2.
Figure 4. TEM (a) BFI, (b) SAED pattern, and (c) point count EDX spectrum of the (Hþ,Znþ)-codoped W-type ZnO nanocondensates in random orientation. The same specimen as in Figure 2.
Figure 2. STEM images taken from two areas (a) and (b) of the sample fabricated by PLAL using 1064 nm excitation under a peak power density of 1.7 1011 W/cm2, showing bimodal size distribution of the W-type ZnO condensates more or less coalesced in (a), and submicrometer particulate with hexagonal shape in (b).
smaller than the ambient values, indicating a significant internal compressive stress as discussed later. STEM observations coupled with EDX analyses from place to place in Figure 2a,b further showed that a relatively high peak power density of 1.7 1011 W/cm2 caused bimodal size distribution of the condensates. The platy nanoparticles more or less coalesced in a close packed manner are 5-80 nm in size,
whereas the particulates are ca. 500 nm in diameter, which are mostly spherical but occasionally hexagonal in shape (Figure 2b). TEM observations indicated that the submicrometer-sized particulates were almost completely changed into W-ZnO, which typically gave a single-crystal SAED pattern (Figure 3), whereas the smaller ones were intimately mixed with ε-Zn(OH)2, which are vulnerable to electron irradiation to become W-ZnO in random orientation, as indicated by ring diffractions (Figure 4). The lattice image of a typical platy ε-Zn(OH)2 nanocondensate in [100] zone axis shows its partial dehydration to form W-ZnO in the [1120] zone axis upon electron irradiation (Figure 5). 2-D forward Fourier transform from local regions of the lattice image indicated that the resultant W-ZnO has (0001) surface predominating over another polar surface 5005
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Figure 5. TEM (a) lattice image of a platy ε-Zn(OH)2 nanocondensate that was partially dehydrated as W-ZnO in the [1120] zone axis upon electron irradiation. (b) and (c) 2-D forward Fourier transform from the square regions I and II, respectively, in (a), indicating that the resultant W-ZnO has a (0001) surface predominating over another polar surface {1101}, which in fact consists of stepwise (0001) facets and (1100) growth ledges. The relic ε-Zn(OH)2 in the [100] zone axis shows a specific epitaxial relationship with W-ZnO, i.e., (0001)W//(010)ε; (1100)W//(001)ε, and hence double diffraction (denoted as D). Note the 1-D commensurate superstructure (denoted as S) shows (0001)W and/or (010)ε fringes in the reconstructed image of area II in (d). The same specimen as in Figure 2.
{1101}, which in fact consists of stepwise (0001) facets and (1100) growth ledges. The relic ε-Zn(OH)2 follows a specific epitaxial relationship (0001)W//(010)ε; (1100)W//(001)ε with W-ZnO. The (0001)W and/or (010)ε 1-D commensurate superstructures also showed up in the two-phase coexisting region. Lattice image coupled with 2-D forward and inverse Fourier transform showed well-developed (0001) surface and corrugated {1010} ledges for the platy W-type ZnO nanocondensates ranging from ca. 10 nm in size to 30 nm in size. Such platy W-type ZnO nanocondensates tended to coalesce as unity over well-developed (0001) surface, as shown in top view in Figure 6. The sample fabricated under a relatively low peak power density of 8.3 1010 W/cm2 has no submicrometer-sized W-ZnO particulates. Instead, W-ZnO formed equiaxed nanocondensates ca. 5-80 nm in sizes which were coalesced in a close packed manner as indicated by STEM (not shown) and TEM image coupled with SAED pattern (Figure 7). The condensates are equiaxed due to competing (0001), {1010}, and {1210} surfaces and growth ledges as viewed in [0001] (not shown) and [1010] zone axis (Figure 8). Optical Spectra. The XPS of the samples fabricated by peak power densities of 1.7 1011 and 8.3 1010 W/cm2 shows significant binding energies at ∼532 eV for O 1s due to hydroxyl species (Figure 9a), and ∼1022 eV for Zn 2p due to Zn2þ in ZnO and ε-Zn(OH)2 at 1021.7 and 1022.5 eV, respectively as shown by the Lorentzian fits (Figure 9b). The binding energies near 523 and 1013 eV are due to the satellite intensity of O2- and Zn2þ, respectively.23 The weak binding near 527 eV for the sample fabricated under a relatively high peak power density of 1.7 1011 W/cm2 is significantly different from the O 1s of Zn(OH)2 at ∼532 eV.24 Such a difference may possibly be due to
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Figure 6. TEM (a) lattice image and 2-D forward (b) and inverse (c) Fourier transform from the square region of platy (Hþ,Znþ)codoped W-type ZnO nanocondensates with corrugated {1010} growth ledges that were coalesced as a unity over well-developed (0001) surface in top view. The same specimen as in Figure 2.
Figure 7. TEM (a) BFI, (b) SAED pattern, and (c) point count EDX spectrum of the (Hþ,Znþ)-codoped W-type ZnO nanocondensates fabricated by PLAL using 532 nm excitation under a peak power density of 8.3 1010 W/cm2.
significant internal stress of W-ZnO with 1-D commensurate superstructure and its intimate intergrowth with ε-Zn(OH)2. The UV-visible spectra of the ε-Zn(OH)2 and W-ZnO composite nanocondensates fabricated under a peak power density of 1.7 1011 and 8.3 1010 W/cm2 showed similar absorbances (Figure 9c). On the basis of the intersection with the baseline at 400.3 and 405.7 nm for the two samples, the 5006
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Figure 8. TEM (a) lattice image and 2-D forward (b) and inverse (c) Fourier transform from the square region of an equiaxed and (Hþ, Znþ)-codoped W-type ZnO nanocondensate ca. 10 nm in size in [1010] zone axis, showing the (0001) terrace edge on and lateral (1210) growth ledges. The same specimen as in Figure 7.
absorbance corresponds to a minimum band gap near 3.1 eV for the ε-Zn(OH)2 and W-ZnO composite nanocondensates in both cases.
IV. DISCUSSION Power Density Dependence of Size and Shape of the Condensates. There is bimodal size distribution for the con-
densates fabricated under a relatively higher power density of 1.7 1011 W/cm2, but not 8.3 1010 W/cm2, indicating a critical peak power density around 1.7 1011 W/cm2 for the generation and rapid solidification/oxidation of the molten plume during the PLAL process. The predominant W-ZnO condensates fabricated under a peak power density of 1.7 1011 W/cm2 typically have a welldeveloped (0001) surface and hence such a preferred orientation. In contrast, the predominant ε-Zn(OH)2 nanocondensates fabricated under a relatively low peak power density of 8.3 1010 W/ cm2 are generally finer in size and less platy when dehydrated as W-ZnO with competing (0001), {1010}, and {1210} surfaces and ledges, as mentioned. Apparently, a higher peak power density favored oxolation of Zn-O-H to form larger-sized W-ZnO condensates with less extent of hydroxylation. Still, the O-H bond caused significant surface modification of ε-Zn(OH)2 and its epitaxial W-ZnO derivative to stabilize (0001) rather than {1011}, which typically occurred for the W-ZnO condensates fabricated by PLA on the Zn target in a vacuum.11 The anisotropic growth to form hexagonal nanoplates rather than nanorods may have something to do with the preference of the dense (0001)W plane and (001)ε plane via a dynamic PLAL process in comparison to the conventional hydrothermal process
Figure 9. (a) and (b) XPS and (c) UV-visible spectra of ε-Zn(OH)2 and W-ZnO composite nanocondensates by 1064 nm (peak power density 1.7 1011 W/cm2, black trace) and 532 nm excitation (8.3 1010 W/cm2, red trace). Note the binding energies of O1s for the hydroxyl species in (a) Zn 2p for ZnO and ε-Zn(OH)2 at 1021.7 and 1022.5 eV, respectively, as shown by the Lorentzian fits inset in (b), and the minimum band gap (cf. text) in (c).
to form nanorods with/without the assistance of surfactants.17,18 The nature of a liquid media, such as dielectric constant and dipole moment, was known to affect the morphology and optical properties of zinc oxide nanostructures prepared by a dynamic PLA process.25 It is an open question how the dipole moment of water affected the space charge and hence the shape of the W-ZnO and ε-Zn(OH)2 composite nanocondensates in this work. Internal Compressive Stress of the Condensates. The internal stress of the W-ZnO phase, which is predominant in 5007
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Table 2. Observed and Calculated d-Spacings for ε-Zn(OH)2 (hkl)
observed (nm)a
JCPDS 89-0138
refined (nm)
102
0.3157
0.3206
0.3159
112
0.2683
0.2721
0.2690
020
0.2565
0.2572
0.2566
310
0.1547
0.1558
0.1543
124
0.1547
0.1550
0.1534
a
0.4905
0.4852
b
0.5143
0.5132
c
0.8473
0.8324
a
Based on XRD of as fabricated nanocondensates (Figure 1b) with an accuracy of (0.0002 nm.
the sample as fabricated under a relatively high peak power density, can be estimated from the lattice parameter and known equation of state. The lattice parameter of the W-ZnO phase was determined by XRD as a = 0.3245 nm and c = 0.5195 ( 0.0002 nm (Table 1). This cell parameter is significantly smaller than that (a = 0.3249 nm, c = 0.5206 nm, JCPDS file 36-1451) of the W-ZnO under ambient condition, indicating a smaller unit cell of the lattice. The internal compressive stress of the lattice turned out to be 0.7 GPa if the Birch-Murnaghan equation of state (EOS) of nanosize W-ZnO with bulk modulus Bo = 142.6 GPa and Bo0 = 3.6 GPa26 was used for the calculation. The internal stress of the ε-Zn(OH)2 phase, which is predominant in the sample as fabricated under a relatively low peak power density, can also be estimated from the lattice parameter and known EOS. The lattice parameter of the ε-Zn(OH)2 phase was determined by XRD as a = 0.4852, b = 0.5132, and c = 0.8324 ( 0.0002 nm (Table 2). This cell parameter is significantly smaller than that (a = 0.4905 nm, b = 0.5143 nm, and c = 0.8473 nm, JCPDS file 890138) under ambient condition. The internal compressive stress of ε-Zn(OH)2, however, was not determined because its bulk modulus and pressure derivative were not known. It is noteworthy that electron irradiation in vacuum significantly caused relaxation of the W-ZnO phase to have lattice parameters close to ambient values according to ring diffractions in the SAED patterns with an accuracy of (0.002 nm. Oxolation of Zn-O-H to Form ε-Zn(OH)2 and W-ZnO. Two important processes have been recognized for the synthesis of zinc oxide in aqueous solutions. Olation is about polymerization of hydroxo species in aqueous solution by reaction of a hydroxo and aquo species:27 Zn-OH þ Zn-OH2 f Zn-OH-Zn þ H2 O
ð1Þ
Subsequent oxolation leads to Zn-O linkage by the dehydration of hydroxo species: Zn2-ðOHÞ2 f Zn-O-Zn þ H2 O
ð2Þ
Chemical reactions involving such condensation reactions must occur in the present PLAL process to form the polynuclear species, which subsequently develop into zinc oxide particles. Zinc hydroxide is in fact amphoteric and complexation by OH- can lead to soluble species such as Zn(OH)3- and Zn(OH)42-,5,27 the latter with tetrahedral coordination of OH- was suggested to act as a 3-D precursor molecule for the formation of octahedral zinc hydroxide.19 (The trend to tetrahedral coordination is due to the high polarizing effect of the relatively small Zn2þ ion with its completely filled 3d-shell; i.e.,
the tetrahedral Zn-O bonds have a considerable degree of covalency.5) Hydrogen bonding of the Zn(OH)42- species generated via the present PLAL process then caused crystallization of ε-Zn(OH)2. It should be noted that the present ε-Zn(OH)2 formed nanoplates rather than octahedra as produced in different liquid media via a sol gel route.19 The shape difference is inherent form the distortion of tetrahedral Zn(OH)42- units under the combined effects of liquid media and internal compressive stress as pertinent to the present dynamic PLAL process. The W-ZnO nanoplates intimately mixed with ε-Zn(OH)2 via the present PLAL route also has significant OH- linkage according to XPS and FTIR results. Such W-ZnO nanoplates are expected to have negatively charged hydroxyl complexes attached to the Zn-terminated (0001) surface rendering the nearby lattice nonstoichiometric Zn1þxO with extra Zn2þ ions residing the interstitial tetrahedral and/or octahedral sites as the charge compensating defects. Formation of ε-Zn(OH)2 Rather Than β-Zn(OH)2. Epitaxial ε-Zn(OH)2/W-ZnO nanocomposite was formed in the present process of PLA on Zn in water. In contrast, PLA on the Zn plate in an aqueous solution of sodium dodecyl sulfate (SDS) was used to fabricate a layered zinc hydroxide/dodecyl sulfate (ZnDS) nanocomposite and the zinc hydroxide was stabilized as the highpressure β-type under the influence of surfactant intergrowth.16 The reasons of forming W-ZnO intergrowth with ε-Zn(OH)2 instead of β-Zn(OH)2 in this study are 2-fold. First, the pressure level in the present PLAL process, as indicated by the estimated internal compressive stress of W-ZnO (0.7 GPa), is an order-ofmagnitude smaller than that required to stabilize β-Zn(OH)2.6 Second, the oxolation process of Zn-O-H in water to form a three-dimensional network ZnO4 polyhedra for ε-Zn(OH)2 as mentioned is different from that in aqueous solution of SDS with hydrophilic headgroups. In the ZnDS composite formation processes,16 the charged inorganic zinc hydroxide species were produced step-by-step by the strong reaction between the ablated Zn species and the water molecules, which concurrently experience assembling with surfactant molecules controlled by the charge-matching mechanism. The preferred coordination of hydrophilic headgroups with zinc coordination sites not only stabilizes β-Zn(OH)2 but also prevents further reaction from forming ZnO nanoparticles.16 The chemical linkage of the surfactant headgroup with Zn sites may exert pressure to favor ZnO6, i.e., the constituent polyhedra of β-Zn(OH)2,6 analogous to the chemically induced stress effect to stabilize high-pressure minerals.1 Energetics of the Epitaxial Relationship between ε-Zn(OH)2 and W-ZnO. According to ref 4 the ε-Zn(OH)2 structure is closely related to that of β-cristobalite having a quarter of tetrahedral interstices in a distorted cubic close packed arrangement of O regularly occupied by the metal atoms. The filled O tetrahedra are twisted against one another in such a way, that strong O(H 3 3 3 O(H hydrogen bonds are favored by the zinc ion. The epitaxial relationship (0001)W//(010)ε; (1100)W// (001)ε observed in this study further suggests that the (010) lattice planes of the ε-Zn(OH)2 nanocondensates were preferably dehydroxylated/delaminated to form the most close packed (0001) basal layer of the W-ZnO. Apparently, the (0001)W// (010)ε interface is preferred to minimize the interfacial and mismatch strain energies for such a phase transformation. In this regard, the unconstrained or constrained lattice mismatch, i.e., ε = (dW - dε)/dε, is indeed small for the adjoined planes across 5008
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Table 3. Lattice Misfit Strain δ, i.e., (dW - dε)/dε, for the Exact/Nearly Coincided Plane Normals of W-ZnO and εZn(OH)2 relation
εA %a
εB %b
(0002)W//(020)ε
þ1.24%
þ1.21%
3 (1100)W//2 (002)ε
-0.35%
þ1.11%
3 (1120)W//2 (200)ε
-0.63%
þ0.35%
(1101)W//(013)ε
þ0.01%
þ1.23%
3 (1103)W//(011)ε
þ0.79%
þ1.26%
a
Unconstrained misfit based on the ambient cell parameters of W-ZnO and ε-Zn(OH)2 as mentioned in the text. b Constrained misfit based on the cell parameters of the as-fabricated W-ZnO and ε-Zn(OH)2 nanocondensates in Tables 1 and 2, respectively.
platy W-ZnO nanocondensates and additional particulates submicrometer in size. The W-ZnO nanoplates are in fact intimately mixed with ε-Zn(OH)2 following a specific crystallographic relationship for beneficial low strain energy. Such nanoplates have internal compressive stress and well-developed (0001) polar terraces for (0001)-specific coalescence and are decorated with corrugated {1010} nonpolar edges, as a result of varied extent of dehydroxylation from epitaxial ε-Zn(OH)2. The W-ZnO and ε-Zn(OH)2 composite nanocondensates have a higher minimum band gap of ca. 3.1 eV, significantly lower than the direct band gap (3.4 eV) of bulk W-ZnO,31 for potential {hkil}-specific catalytic and optoelectronic applications in near violet range and shed light on the phase assemblage of the ZnO-H2O binary in natural dynamic settings.
’ APPENDIX Stereographic projection of the epitaxial relationship (0001)W//(010)ε; (1100)W//(001)ε between W-ZnO (hkl) and ε-Zn(OH)2 (hkil) in the zone axis [1120]W//[100]ε. The ambient lattice parameters of W-ZnO and ε-Zn(OH)2 as mentioned in the text were used for the plot.
Figure 10. Schematic drawing of unrelaxed oxygen atom positions of the (010) plane of ε-Zn(OH)2 (ε, black circles) and the (0001) plane of W-ZnO (W, gray circles) which are superimposed to define a 2-D CSL as outlined in top view.
the (0001)W//(010)ε interface (cf. Appendix) given the lattice parameters compiled in Table 3. In fact, the (0001)W//(010)ε interface can be defined by a rather small 2-D coincidence site lattice (CSL) along the parallel directions [1120]W//[100]ε and [1100]W//[001]ε as shown by the schematic drawing of the superimposed oxygen atom positions of the interface in top view (Figure 10). The (0001)W and/or (010)ε 1-D commensurate superstructures in the two-phase coexisting area (Figure 5d) are presumably due to distortion ordering of ZnO4 tetrahedra in the basal layer during dehydroxylation of ε-Zn(OH)2. It is by no means clear if such ordering has anything to do with the spinodal type analogous to the case of nonstoichiometric Cu1þxZn involving wrong site occupation of the atom.28 A rather low misfit strain of the epitaxial ε-Zn(OH)2 and W-ZnO accounts for their favorable formation in the ZnO-H2O binary system under high pressure and temperature in a dynamic process. In contrast, in the Al2O3-H2O binary, the Al2O3 nanocondensates of spinodal-type related structures, i.e., γ- and θ-type with a significant internal compressive stress, were fabricated via PLA in water.29 Such dense alumina nanocondensates did not form hydrate unless subjected to prolonged dwelling in water to form bayerite, which back-transformed as platy γ-Al2O3 following a specific crystallographic relationship upon electron irradiation.30 It seems that polymerization of hydroxo species is more effective in the ZnO-H2O than in Al2O3-H2O binary via a dynamic PLAL process or in natural dynamic settings.
V. CONCLUSIONS The present PLAL study showed that a relatively high power density input on Zn in water caused extensive development of
’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT We thank Miss S. Y. Shih for the help on XPS analysis and anonymous reviewers for constructive comments. This work was supported by Center for Nanoscience and Nanotechnology at NSYSU and National Science Council, Taiwan, ROC. ’ REFERENCES (1) Putnis, A. Introduction to Mineral Sciences; Cambridge University Press: Cambridge, U.K., 1992; pp 1-457. (2) Jagadish, C.; Pearton, S. J. Zinc oxide bulk, thin films and nanostructures; Elsevier: Amsterdam, The Netherlands, 2006. (3) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Robert E. Krieger Publishing: Malabar, FL, U.S., 1982; Vol 1, p 325. (4) Stahl, R.; Jungb, C.; Lutzb, H. D.; Kockelmannc, W.; Jacobsa, H. Z. Anorg. Allg. Chem. 1998, 624, 1130. 5009
dx.doi.org/10.1021/jp107140r |J. Phys. Chem. C 2011, 115, 5003–5010
The Journal of Physical Chemistry C (5) Lieth, R. M. A. Preparation and crystal growth of materials with layered structures; Kluwer Academic Publishers, D. Reidel Publishing Company: Amsterdam, Holland, 1977. (6) Baneyeva, M. I.; Popova, S. V. Geochem. Int. 1969, 6, 807. (7) Kusaba, K.; Yagi, T.; Yamaura, J.; Miyajima, N.; Kikegawa, T. Chem. Phys. Lett. 2007, 437, 61. (8) Kusaba, K.; Kikegawa, T. Solid State Commun. 2008, 148, 382. (9) Kusaba, K.; Yagi, T.; Yamaura, J.; Gotou, H.; Kikegawa, T. J. Phys.: Conf. Ser. 2010, 215, No. 012001. (10) Kang, J. S.; Kang, H. S.; Pang, S. S.; Shim, E. S.; Lee, S. Y. Thin Solid Film 2003, 443, 5. (11) Huang, B. H.; Shen, P.; Chen, S. Y. J. Eur. Ceram. Soc. 2008, 28, 2545. (12) Huang, B. H.; Shen, P.; Chen, S. Y. J. Eur. Ceram. Soc 2009, 29, 743. (13) Huang, B. H.; Shen, P.; Chen, S. Y. J. Phys. Chem. C 2008, 112, 1064. (14) Huang, B. H.; Shen, P.; Chen, S. Y. Nanoscale Res. Lett. 2009, 4, 503. (15) Huang, B. H. Ph.D. Thesis, National Sun Yat-sen University, Taiwan, 2008. (16) Liang, C. H.; Shimizu, Y.; Masuda, M.; Sasaki, T.; Koshizaki, N. Chem. Mater. 2004, 16, 963. (17) Usui, H.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J. Phys. Chem. B 2005, 109, 120. (18) Ishikawa, Y.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J. Colloid Interface Sci. 2006, 300, 612. (19) Qu, X. R.; Jia, D. C. J. Cryst. Growth 2009, 311, 1223. (20) Shaporev, A. S.; Ivanov, V. K.; Baranchikov, A. E.; Polezhaeva, O. S.; Tret’yakov, Y. D. Russ. J. Inorg. Chem. 2007, 52, 1811. (21) Note 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. Following Beer-Lambert law Ix = Io exp(-x/Δ), where Io is the entrance light intensity (pulse energy) and Ix is the light intensity on the target surface at a distance x below the water level, Ix can be calculated as 620 mJ (22.5% loss) given x = 5 mm and pulse energy Io = 800 mJ. As for 532 nm wavelength, the water absorption loss is negligible given Δ > 10 m. Besides, the possible pulse energy loss is about 3% through two reflectors and one lens having 99% efficiency each. The total errors of the power densities input on target are then 25% (77.5% 0.97 = 25%) and 3% for the present PLAL using wavelengths 1062 and 532 nm, respectively. (22) The ε-Zn(OH)2 nanoparticles were dehydrated instantaneously even under very careful electron dosage as adopted in this study. The SAED pattern taken from relatively large sized particulate superimposed with randomly oriented nanoparticles (e.g., Figure 3) or μμ diffraction (i.e., diffraction limited by the electron beam itself with a diameter down to 15 nm) from the individual W-ZnO/ε-Zn(OH)2 composite nanoparticle show that ε-Zn(OH)2 was very rapidly dehydrated as W-ZnO. Nevertheless, after careful scrutiny of more than a hundred nanoparticles, we were able to find a partially dehydrated εZn(OH)2 plate as indicated by the lattice image coupled with 2-D forward and inverse Fourier transforms in Figure 5. The epitaxial relationship thus obtained for the ε-Zn(OH)2 relic and W-ZnO (Appendix) is justified by the rather small lattice mismatch for a beneficial low strain energy across the interface (Table 3 ). (23) Vickerman, J. C. Surface analysis: the principal techniques; John Wiley & Sons, Ltd.: Chichester, England, 1997. (24) Ballerini, G.; Ogle, K.; Labrousse, M. G. B. Appl. Surf. Sci. 2007, 253, 6860. (25) Singh, S. C.; Gopal, R. Effects of nature of liquid media on the morphology and optical properties of zinc oxide nanostructures synthesized by pulsed laser ablation. EOS Conference on Laser Ablation and Nanoparticle Generation in Liquids 2010 (Angel 2010), Engelberg, Switzerland, 29 June to 1 July 2010.
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(26) Desgreniers, S. Phys. Rev. B 1998, 58, 14102. (27) Govender, U.; Boyle, D. S.; Kenwa, P. B.; O’Brien, P. J. Mater. Chem. 2004, 14, 2575. (28) Porter, D. A.; Easterling, K. E.; Sherif, M. Y. Phase transformations in metals and alloys, 3rd ed.; CRC Press: Boca Raton, FL, U.S., 2009. (29) Liu, I. L.; Shen, P.; Chen, S. Y. J. Phys. Chem. C 2010, 114, 7751. (30) Liu, I. L.; Chen, S. Y.; Shen, P. J. Nanosci. Nanotech., in press. (31) Hvam, J. M. Solid State Commun. 1978, 26, 987.
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