Crystal Perfection in Zinc Oxide with Occluded Carboxyl

CRYSTAL GROWTH & DESIGN

Crystal Perfection in Zinc Oxide with Occluded Carboxyl-Functionalized Latex Particles Rafael

Mun˜oz-Espı´,§

Amreesh

Chandra,‡

2007 VOL. 7, NO. 9 1584-1589

and Gerhard Wegner*

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ReceiVed NoVember 29, 2006; ReVised Manuscript ReceiVed August 3, 2007

ABSTRACT: Carboxyl-functionalized latex particles can be incorporated into zinc oxide crystals growing from an aqueous medium, giving zinc oxide-latex hybrid materials. The intracrystalline polymer particles do not disturb the long-range order of the zinc oxide host. This has been investigated by X-ray diffraction (XRD) and vibrational spectroscopies. The results from Rietveld refinement of XRD data show a slight but distinct compression of the zinc oxide lattice in the presence of latex, which is relaxed by thermal reequilibration of the lattice strains when the polymer is burned off. We suggest that the latex has an indirect effect related to the introduction and stabilization of defects at the interface with the polymer in the course of the crystallization from aqueous media, rather than a direct effect due to mechanical interaction between latex and zinc oxide. The changes in the lattice parameters are independent of the latex content in the material. The crystallization of zinc oxide in the presence of surface-modified latex particles might serve as a model for analogous biological phenomena concerning mineralization of inorganics in the presence of biomacromolecules. Introduction Nature is rich in materials composed of a crystalline inorganic phase and a polymeric component. For instance, bones, teeth, corals, and shells of sea creatures are examples of biological composites in which nucleation and growth of inorganic structures are controlled by the presence of biomacromolecules.1-3 Transferring the knowledge learned from nature, synthetic polymers with different structures have been applied in material science as controlling agents for inorganic crystallization phenomena. This results in a field of study that can be labeled as “bioinspired polymer-controlled mineralization.”4-6 How macromolecules interact with the growing crystals and how the structural features of the resulting products are influenced by their presence are still intriguing issues that attract much research work. Among the different polymers used, double-hydrophilic block and graft copolymers have been shown to be particularly effective in morphology control.4,7,8 The effects observed with block copolymers suggested the investigation of a model system with functional groups anchored to the surface of spherical particles. Such a system, provided by surface-functionalized latex nanoparticles, gives an interesting and novel approach in the study of crystal growth control, due to the possibility of tuning the particle size and the local concentration of functional groups attached to the surface. Latex particles have been applied as templates or structure-controlling agents in the crystallization of zinc oxide and calcium carbonate.9-12 In the present paper, we address the question of whether or not the presence and incorporation of polymers during the growth process of inorganic crystals leads to structural defects or strains in the lattice, affecting the crystal perfection of the host. This is an aspect of importance in understanding of biomineralization phenomena and industrial applications. A certain amount of work has been devoted to the analysis of * Corresponding author: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. Tel.: +49-6131-379-130. Fax: +49-6131-379-330. E-mail: [email protected]. § Current address: Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400. ‡ Current address: School of Biomedical and Molecular Sciences, Department of Chemistry, University of Surrey, Guildford-GU2 7XH, United Kingdom.

biogenic systems, comparing them with the analogous nonbiological materials.13-18 The most studied biominerals are by far calcium carbonate and hydroxyapatite. However, biogenic systems commonly present several polymorphs and have a high structural complexity, which complicates the evaluation of the results. As a simplified approach to the problem, the mineral chosen in this work was zinc oxide, used as a model system, because it crystallizes in the form of a single hexagonal phase, zincite. Similarly, latex particles modified at their surface with carboxylic groups serve as a macromolecular model. Here, the influence of poly(styrene-acrylic acid) latex particles on the crystal perfection and the long-range order of zinc oxide crystallized from aqueous media is investigated by X-ray diffraction (XRD) and vibrational (Raman and infrared) spectroscopies. Experimental Section A detailed description of the synthesis of pure zinc oxide control samples and zinc oxide-latex hybrid materials has been described elsewhere.9 Briefly, a solution of zinc nitrate hexahydrate was placed in a jacketed reactor connected to a thermostated water circulator (95.0 ( 0.1 °C). The desired amount of latex dispersion (from 0 to 9 g‚L-1, calculated from the solid content of the emulsion) was added into the reactor. The reaction started after the addition of hexamethylenetetramine (HTMA), dissolved in a small quantity of water. The concentration in the reaction medium of both reactants, Zn(NO3)2‚6H2O and HMTA, was 0.03 mol‚L-1, and the total volume was 200 mL. The poly(styrene-acrylic acid) [(P(S-AA))] latex used was prepared by miniemulsion polymerization as previously reported.9 The average diameter of the particles was 71 ( 1 nm, as determined by dynamic light scattering, and the surface charge density was estimated to be 1.3 nm-2 (in charged groups per unit area) by titration with a 0.001 N solution of the oppositely charged polyelectrolyte poly(diallyl dimethylammonium choloride). The latex particles are comprised of a core of polystyrene surrounded by a hydrophilic corona, composed of carboxylic groups originating from the acrylic used as co-monomer and hydrophilic segments from the surfactant used in the preparation of the emulsion. Scanning electron microscopy (SEM) images were taken with a fieldemission microscope LEO EM1530 Gemini, working at an accelerating voltage of 1-1.5 eV. Thermogravimetrical analysis (TGA) was carried out with a thermobalance Mettler Toledo ThermoSTAR TGA/SDTA 851 under an oxygen atmosphere (heating rate of 10 °C min-1, RT f 600 °C). TGA-

10.1021/cg060858l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007

Crystal Perfection in Zinc Oxide

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Figure 1. (a) SEM micrograph of a pure zinc oxide reference crystallized in the absence of any additive. (b) Schematic representation of the evolution of the crystal shape with increasing latex concentration. (c-e) SEM micrographs of zinc oxide-latex hybrid materials mineralized in the presence of 0.5, 3, and 9 g‚L-1 of P(S-AA) latex. (f) SEM micrograph showing the cross-section of a hybrid material after burning off the polymer at 600 °C. MS curves were measured with an analogous balance coupled with a Balser MS/Netsch STA449 C mass detector. X-ray powder diffractograms were registered by a Seifert XRD 3000 TT diffractometer with Cu KR radiation (λ ) 1.54 Å, voltage: 40 kV, current: 30 mA) in the range 2θ ) 15-90°. Temperature-dependent diffractograms were measured in the range 15-75° in an analogous device equipped with an MRI TC radiation heating chamber (maximal error in the temperature measurement up to (5%). The heating rate was 10 °C‚min-1, and a stabilization time of 5 min was provided before the beginning of the measurements. The Rietveld refinement of XRD data was carried out using the software FullProf.19,20 Fourier transform IR (FTIR) spectra were obtained by a Nicolet Magna 850 spectrometer. The pellets used for the measurements were prepared as identical as possible to avoid differences caused by matrix effects, mixing homogeneously the samples (1-2 mg, weighed by a precision balance) with KBr (ca. 250 mg, Aldrich). Raman spectra were registered from the original powder samples in a Bruker RFS100 spectrometer equipped with a Nd:YAG laser source (1064 nm, 510 mW), working at a power of 450 mW and accumulating 100 scans.

Results and Discussion Zinc oxide crystallizes in the hexagonal system as zincite, which belongs to the point group 6mm (using the HermannMaugin nomenclature, or C6V in the Scho¨nflies notation) and to the space group P63mc (or C46v). The zincite lattice has the structure of wurtzite, and the cell parameters are a ) 3.2498 Å and c ) 5.2066 Å.21 The ratio c/a is 1.602, slightly smaller than the value for an ideal wurtzite lattice (c/a ) 2x2/3 ) 1.633); that is, the zinc oxide lattice is contracted in the direction of the c-axis with respect to the ideal wurtzite structure.22 The oxygen atoms are hexagonal close-packed and occupy the Wycoff 2b sites at (1/3,2/3, 0); the zinc atoms are placed in the 2b sites at (1/3, 2/3, 1/2 ( ∆z), occupying half of the tetragonal interstices, and are also hexagonal close-packed. When zinc oxide is precipitated in the presence of carboxylmodified latex, the latex particles adsorb at the growing faces and become incorporated into the crystal, giving zinc oxidelatex hybrid materials. The scanning electron microscopy (SEM) images and the schematic diagram shown in Figure 1 illustrate how the aspect ratio of the resulting crystals decreases systematically when zinc oxide is crystallized from an aqueous medium in the presence of an increasing concentration of P(S-AA) latex particles. The decrease of the length, parallel to an increase of

the width, at increasing latex concentration has been explained in terms of a preferential adsorption of the latex to the {001} faces of zinc oxide, which retards the growth in the direction of the c-axis, [001].9 The micrograph of Figure 1f, corresponding to a hybrid sample after burning off the polymer at 600 °C, demonstrates the incorporation of the polymer, as judged from the cross-section of a crystal that contains holes of size coinciding with the diameter of the latex particles. Thermal Analysis. The latex content in the zinc oxide-latex hybrid materials can be estimated by TGA. The samples are heated in an oxidative atmosphere so that the polymer gets pyrolyzed, and, as a consequence, they suffer a weight loss, which is registered by a precision balance. Two steps of weight loss, at around 300 and 450 °C, can be observed in our samples, corresponding to the decomposition of the different components of the copolymeric latex. The latex combustion can be monitored by a TGA equipment coupled to a mass spectrometry (MS) detection system (abbreviated TGA-MS). Figure 2a shows the weight loss for a zinc oxide-polymer hybrid sample containing acrylic-acid-derived latex particles, together with the MS signals corresponding to H2O (m/z ) 18) and CO2 (m/z ) 44). The maxima of the signals coincide with the pyrolysis steps of the latex. Figure 2b presents the weight loss of samples synthesized in the presence of an increasing concentration of P(S-AA) latex. At around 5-6 g‚L-1 of latex in the reaction medium, the latex content of the product seems to achieve a plateau; there is a maximum in the latex incorporation, and a larger overall latex concentration during the crystallization does not lead to higher polymer contents in the final material. Nevertheless, it has to be noticed that at high concentration of latex (>6 g‚L-1), a quantification of the latex content becomes difficult because the excess of latex (i.e., nonincorporated latex and particles merely adsorbed at the crystal surface) cannot be always properly removed, which results in imprecise estimations. The latex content in the hybrid materials reaches values of up to almost 10 wt %, which corresponds to volume percentages of as much as up to 50%, considering a density of 5.606 g‚cm-3 for the pure zinc oxide and an approximated value of 1.00 g‚cm-3 for the latex. X-ray Diffraction and Rietveld Refinement. For all samples, prepared both in the absence and in the presence of latex

1586 Crystal Growth & Design, Vol. 7, No. 9, 2007

Mun˜oz-Espı´ Table 1. Coherence Lengths Calculated by the Scherrer Equation from the (100) and (002) XRD Reflections for Zinc Oxide Samples Prepared in the Presence of Different Concentrations of P(S-AA) Latex. latex concentration [g‚L-1]

L100 [nm]

L002 [nm]

0 0.5 1 3 4 5 6 9

54 48 47 45 44 45 47 53

82 60 72 58 54 55 63 66

Table 2. Coherence Lengths at Different Temperatures Calculated by the Scherrer Equation from the (100) and (002) XRD Reflections for a Zinc Oxide Sample Prepared in the Presence of 3 g‚L-1 of P(S-AA) Latex pure zinc oxide

Figure 2. (a) Weight loss (solid line) from TGA, with coupled MS detection of water (dashed line, m/z ) 18) and CO2 (dotted line, m/z ) 44) for a zinc oxide-latex hybrid sample. The intensities of the MS signals have been rescaled to fit both in the same graph. (b) Latex content estimated from TGA as a function of the latex concentration in the precipitation medium.

particles, hexagonal zincite was the only phase identified by XRD. All observed XRD peaks could be assigned to the reflections of zincite.21 The coherence length (or the size of the coherently scattering domains), Lhkl, was estimated from the width of the XRD reflections by the Scherrer equation, Lhkl ) Kλ/(β1/2 cos θ), where K is a form factor approximated here to unity, λ is the radiation wavelength, and β1/2 is the full width at half-maximum of the peak profile on the 2θ scale in radians.23 The broadening of the diffraction lines occurs for three main reasons: (i) an experimental contribution intrinsic to the instrumental equipment; (ii) a diffraction-order-independent broadening, related to the size of the crystals domains; and (iii) a diffraction-orderdependent broadening, caused by lattice strains. The instrumental broadening can be estimated from a silicon standard, assuming as negligible the size and strains components. When applying the Scherrer formula, it is assumed that the broadening results only from the size of the coherently scattering domains, neglecting the experimental broadening and the contribution of strains. Although this might be considered as an oversimplification, the experimental broadening will be constant for the different measurements the relative trend will not change. When evaluating the data, one has to take into account that the real value of the crystallite sizes can be larger than that calculated by the Scherrer formula.24 Table 1 lists the coherence length calculated for the first two reflections, (100) and (002), for pure zinc oxide and for different hybrids, prepared in the presence of an increasing overall latex concentration. It is observed that the values of L are larger in the direction of the c-axis (L002)

hybrid zinc oxide-latex

temperature [°C]

L100 [nm]

L002 [nm]

L100 [nm]

L002 [nm]

25 200 400 600

46 45 47 48

58 60 55 61

42 42 43 43

50 45 39 43

than in the direction of the a-axis (L001). The values for the hybrid materials are slightly smaller than those of the pure zinc oxide, and the difference is more significant in the case of the domain size calculated from the (002) reflection. In general, the broadening of diffraction peaks induced by organic molecules has also been reported when comparing biogenic with geological calcite.17 However, the effects of latex on the lattice of zinc oxide seem to be different from that of biomacromolecules on biogenic aragonite, as we will show next. In Table 2, the discordance in the trend of the sample at the highest polymer concentration, showing a L larger than for the rest of the hybrids, cannot be easily explained, but it seems to be related to effects taking place at concentrations above a threshold. To investigate the influence of the presence of the latex in the lattice dimensions, the XRD patterns were subjected to Rietveld profile refinement.25 A pseudo-Voigt function was applied to fit the peak profiles, and a fifth-order polynomial was used for the background fits. All parameters (i.e., scale factor, zero correction, background, half-width parameters along with mixing parameters, positional coordinates, and thermal parameters) were varied in the course of the computational refinement. Good fits between the observed and the calculated profiles were obtained after a few cycles of refinement for all the samples. As representative examples, Figure 3 presents the XRD patterns and the Rietveld difference plots for a pure zinc oxide and for two zinc oxide-latex hybrid materials, crystallized in the presence of 3 and 9 g‚L-1 (overall latex concentration at the onset of the crystallization), which corresponds to a latex content in the final hybrids of approximately 7 and 9.5 wt %. The results of the refinement are illustrated in Figure 4, which displays the calculated cell parameters, a and c, and the cell volume, as the latex concentration is varied. The absolute values of a and c are close to those reported.21 Important observations are that the lattice parameters are slightly smaller when the crystal growth occurred in the presence of latex and that they seem to be independent of the latex content. If the presence of the latex would generate a mechanical interaction with the crystal leading to a lattice strain, a higher latex content would enlarge the effect. However, this does not appear to be the case, which could indicate that the compression of the lattice is an indirect consequence rather than a direct effect of the latex particles. As the influence of the synthesis conditions on the

Crystal Perfection in Zinc Oxide

Crystal Growth & Design, Vol. 7, No. 9, 2007 1587

Figure 3. X-ray diffraction patterns and Rietveld difference plots for a samples obtained in the absence and in the presence of 3 and 9 g‚L-1 of P(S-AA) latex. Circles represent the experimental data, and solid lines the calculated profiles. The short vertical bars indicate the position of the diffraction peaks.

packing defects (e.g., vacancies) of zinc oxide is commonly reported, we suggest that the presence of the latex in the precipitation medium originates differences at the crystalpolymer interface that induce changes in the formation of point defects. The slight but distinct compression of the lattice would then be a consequence of the strain exerted by the defects. The results reported here seem to differ from those found by Pokroy et al. for biogenic calcium carbonate.16-18 These authors found that the a and c parameters of biogenic aragonite are larger than that of the geological mineral, whereas b is smaller (as it happens for a and c in our case when latex is present). For the case of biogenic calcite, they reported a unit cell expansion, as compared to geological calcite. However, there is a point that has attracted our attention, “raw” lattice parameters of biological calcite published by Pokroy et al. (cf. Table 2 of ref 17) show compression of a and c parameters, when compared to geological or their control calcite sample. The authors justify this fact by the effect of magnesium impurities in the CaCO3 lattice and point out that Mg incorporation leads to a compression of the lattice. Therefore, according to the authors, a “correction” should be done for comparison, and they suggest calibration curves relating a and c with the Mg content. These explanations coincide with our speculation that certain point defects can exert a lattice compression. Thus, the results found for zinc oxide and for biogenic calcium carbonate are not as contradictory as they may initially appear. Temperature-Dependent X-ray diffraction. The previous observations prompted the investigation of the behavior of the hybrids with the temperature in an attempt to find out the effect of the polymer particles on the zinc oxide lattice. XRD patterns were registered at different temperatures up to 600 °C for a zinc oxide reference, precipitated in the absence of any additive, and for a zinc oxide-latex hybrid, precipitated in the presence of 3 g‚L-1 of P(S-AA) latex. The coherence lengths obtained from the two first peaks by the Scherrer equation are contained

Figure 4. Lattice parameters and cell volume obtained from the Rietveld profile refinement for a pure zinc oxide sample (asterisk) and for hybrids obtained in the presence of an overall P(S-AA) latex concentration (circles).

in Table 2.26 Within the limits of error, the size of the coherently scattering domains remains almost constant at different temperatures, with the exception of the values calculated for the hybrid material from the (002) reflection. This may indicate a reorganization of the lattice, especially along the c-axis, during the decomposition of the latex particles by pyrolysis, which according to Figure 2 starts at around 300 °C and is completed at ca. 450 °C. Rietveld profile refinement was performed analogously as described above, and the lattice parameters and cell volume calculated for the different temperatures are presented in Figure 5. It is observed that the differences between the lattice constants of pure and latex-containing materials are minimized at higher temperature, when the latex is removed, which indicates that the lattice strains initially present in the sample tend to disappear. This similarity in the lattice constants of both materials is kept after cooling down to room temperature. During the heating process, there is a combination of two effects to be considered: the intrinsic thermal expansion of the material and the annealing process as the temperature increases. The slope of the a, c, and volume data for the hybrid is larger than for the pure zinc oxide, suggesting an apparent increase in the thermal expansion. However, as the parameter changes are independent of the latex content (Figure 4), it is more likely that the differences in the slopes are due to a thermal reequilibration of the lattice defects; that is, the stresses originated by the defects are annealed out and the behavior of the system approaches to the one of the pure zinc oxide. Lattice Dynamics: Vibrational Spectroscopies. The effect of latex particles on the crystal structure was further studied by

1588 Crystal Growth & Design, Vol. 7, No. 9, 2007

Mun˜oz-Espı´

Figure 6. FTIR spectra for samples precipitated in the absence and in the presence of P(S-AA) latex.

Figure 5. Lattice parameters and cell volume obtained from the Rietveld profile refinement at different temperatures for a pure zinc oxide (circles) and a zinc oxide-latex hybrid obtained in the presence of 3 g‚L-1 of P(S-AA) latex (squares).

vibrational spectroscopies: FTIR and Raman spectroscopy. With 4 atoms per unit cell, zincite has 12 vibrational modes: one longitudinal-acoustic (LA), two transverse-acoustic (TA), three longitudinal-optical (LO), and six transverse-optical (TO).27 Group theory predicts nine optical modes: an A1 mode, a doubly degenerate E1 branch, two doubly degenerated E2 branches, and two B branches. The A1 and E1 modes are both Raman- and infrared-active, the two E2 modes are only Raman-active, and the B modes are inactive (silent modes).28 FTIR spectra for pure zinc oxide and hybrids are presented in Figure 6. In the absence of latex peaks at 401 and 524 cm-1 were observed. In the presence of P(S-AA) latex, the second peak becomes a shoulder and cannot be clearly identified. FTIR spectra of the hybrid samples prepared with different concentrations of P(S-AA) latex were similar. However, the peak at around 400 cm-1 shows a slight but systematic shift in the maximum absorption peak (minimum transmission) equivalent to up to an energy shift of 5 meV. Our results are similar to those of Andre´s-Verge´s et al.,29-31 who applied the so-called theory of the aVerage dielectric constant32,33 to analyze IR spectra of zinc oxide prepared by precipitation in a way similar to that presented here. In addition to the crystal structure and the chemical composition, three factors may affect the position and the form of the peaks in vibrational spectra: (i) the dielectric constant of the matrix in which the material is diluted, (ii) the particle aggregation, and (iii) the crystal shape. Morphological characteristics of small particles can influence the spectra due to dipole oscillation induced by the vibrational modes. Similar to the mentioned authors, we do not consider the matrix effects for a relative comparison of the samples, because all were

Figure 7. Raman spectra for pure zinc oxide, pure latex, and two examples of zinc oxide-latex hybrids.

prepared by using KBr as a diluting agent. The particle aggregation is also considered to be of less importance, according to the SEM observations. Therefore, differences in the FTIR spectra are assumed to be essentially caused by the differences in the morphology. No vibrational coupling between the latex and the zinc oxide particles was detected by Raman spectroscopy. Figure 7 shows the Raman spectra of pure zinc oxide, hybrid samples, and a latex reference. In pure zinc oxide, two intense peaks are observed at 101 and 436 cm-1, assigned to the two E2 branches and associated with the vibration of the zinc (low-frequency mode) and oxygen (high-frequency mode) sublattices. Peaks at 385 and 409 cm-1 are ascribed to the A1(TO) and E1 (TO) modes, respectively.27,28 Further peaks are identified at 205 and 333 cm-1. The ascription of these peaks is uncertain, but some

Crystal Perfection in Zinc Oxide

authors have attributed peaks at a similar position to the secondorder Raman spectrum arising from zone-boundary phonons.34,35 The Raman modes did not significantly shift as a function of the latex concentration, and no additional Raman modes appear in any of the hybrid materials, which confirms that the crystal perfection is preserved in spite of the latex incorporation. An additional interesting remark, regarding the lower energy spectral region, at around 3000 cm-1, should be made. In this range, a very strong luminescence was observed in the sample grown without additive, which is systematically quenched when the concentration of latex increases. This feature, which could be related to the presence of point defects of in the crystals, has been the object of detailed study in further work.36 Conclusion In analogy to the incorporation of biogenic macromolecules into inorganic structures in biomineralization phenomena, polymers can be occluded in synthetically grown crystals. The mineralization of zinc oxide in the presence of carboxyl-functionalized latex particles leads to the formation of zinc oxidelatex hybrids composed of inorganic and largely undisturbed crystalline material in which the organic polymer particles are embedded. The incorporation of the latex in the growing crystal, up to 10 wt % (equivalent to ca. 50 vol %), does not cause deterioration in the microcrystalline structure, as judged from the Rietveld refinement of the X-ray diffraction data. The lattice parameters, a and c, for the obtained zincite are slightly smaller for the zinc oxide-latex hybrids, as compared to zinc oxide crystallized in the absence of any additive. The lattice compression is independent of the latex content, which seems to suggest an indirect effect rather than a direct consequence of the latex particles by mechanical interaction. This indirect effect could be related to the change in the defect formation and the introduction of stress during the precipitation of the materials in the presence of latex particles. At higher temperatures, parallel to the calcination of the latex, there is a thermal reequilibration, and the initial lattice strains are relaxed. From infrared spectroscopy studies, the morphology of the precipitated hybrids has been found to affect the position and the form of the adsorption peaks corresponding to the transverse optical vibration modes, in correlation with the variation of the aspect ratio in the samples. No significant change in the position of the peaks was observed by Raman spectroscopy, which is consistent with the structural conservation deduced by X-ray diffraction. References (1) Mann, S. Biomineralization; Oxford University Press: New York, 2001. (2) Dove, P. M.; De Yoreo, J. J.; Weiner, S. Biomineralization; Mineralogical Society of America, Geochemical Society: Washington, DC, 2003; Vol. 54. (3) Baeuerlein, E. Biomineralization; Wiley-VCH: Weinheim, 2003. (4) Yu, S.-H.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2124-2147. (5) Xu, A.-W.; Ma, Y. R.; Co¨lfen, H. J. Mater. Chem. 2007, 17, 415449. (6) Gorna, K.; Mun˜oz-Espı´, R.; Gro¨hn, F.; Wegner, G. Macromol. Biosci. 2007, 7, 163-173.

Crystal Growth & Design, Vol. 7, No. 9, 2007 1589 (7) Wegner, G.; Baum, P.; Mu¨ller, M.; Norwig, J.; Landfester, K. Macromol. Symp. 2001, 175, 349-355. (8) Co¨lfen, H. Macromol. Rapid Commun. 2001, 22, 219-252. (9) Mun˜oz-Espı´, R.; Qi, Y.; Lieberwirth, I.; Go´mez, C. M.; Wegner, G. Chem. Eur. J. 2006, 12, 118-129. (10) Lu, C. H.; Qi, L. M.; Cong, H. L.; Wang, X. Y.; Yang, J. H.; Yang, L. L.; Zhang, D. Y.; Ma, J. M.; Cao, W. X. Chem. Mater. 2005, 17, 5218-5224. (11) Hetherington, N. B. J.; Kulak, A. N.; Sheard, K.; Meldrum, F. C. Langmuir 2006, 22, 1955-1958. (12) Wegner, G.; Demir, M. M.; Faatz, M.; Gorna, K.; Mun˜oz-Espı´, R.; Guillemet, B.; Gro¨hn, F. Macromol. Res. 2007, 15, 95-99. (13) Aizenberg, J.; Hanson, J.; Ilan, M.; Leiserowitz, L.; Koetzle, T. F.; Addadi, L.; Weiner, S. FASEB J. 1995, 9, 262-268. (14) Aizenberg, J.; Hanson, J.; Koetzle, T. F.; Leiserowitz, L.; Weiner, S.; Addadi, L. Chem.sEur. J. 1995, 1, 414-422. (15) Aizenberg, J.; Hanson, J.; Koetzle, T. F.; Weiner, S.; Addadi, L. J. Am. Chem. Soc. 1997, 119, 881-886. (16) Pokroy, B.; Quintana, J. P.; Caspi, E. N.; Berner, A.; Zolotoyabko, E. Nat. Mater. 2004, 3, 900-902. (17) Pokroy, B.; Fitch, A. N.; Lee, P. L.; Quintana, J. P.; Caspi, E. N.; Zolotoyabko, E. J. Struct. Biol. 2006, 153, 145-150. (18) Pokroy, B.; Fitch, A. N.; Marin, F.; Kapon, M.; Adir, N.; Zolotoyabko, E. J. Struct. Biol. 2006, 155, 96-103. (19) Rodrı´guez-Carvajal, J. Physica B 1993, 1993, 55-69. (20) The complete program and documentation can be downloaded from http://valmap.dfis.ull.es/fullprof. (21) Powder diffraction file, card 36-1451, JCPDS-ICDD (Joint Commitee on Powder Diffraction Standards-International Center for Diffraction Data), Swarthmore, 1996. (22) Kolb, D. M.; Schulz, H.-J. Optical properties of zinc oxide. In Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland Publishing Company: Amsterdam, 1981; Vol. 7; pp 226-268. (23) Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; John Wiley: New York, 1974. (24) From measurements of a silicion standard, it has been estimated that for the diffractometer used in our experiments from real values could be up to 30% larger than those calculated by the Scherrer formula without taking into account instrumental broadenings. (25) Young, R. A. The RietVeld Method; International Union of Crystallography; Oxford University Press: Oxford, 1996; p 298. (26) The X-ray diffractometer used in the data shown in Table 2 and Figure 5, equipped with a chamber for temperature control, was different from the one used for the conventional measurements at room temperature shown in Table 1 and Figure 4. This explains the slight differences between the values at room temperature. Comparison of the absolute values can only be done for measurements carried out in the same device (i.d., within the same table or figure). (27) O ¨ zgu¨r, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S.-J.; Morkoc¸ , H. J. Appl. Phys. 2005, 98 (4), 041301. (28) Damen, T. C.; Porto, S. P. S.; Tell, B. Phys. ReV. 1966, 142 (2), 570-574. (29) Andre´s-Verge´s, M.; Serna, C. J. J. Mater. Sci. Lett. 1988, 7 (9), 970972. (30) Andre´s-Verge´s, M.; Mifsud, A.; Serna, C. J. J. Chem. Soc. Faraday Trans. 1990, 86, 959-963. (31) Andre´s-Verge´s, M.; Martı´nez-Gallego, M. J. Mater. Sci. 1992, 27 (14), 3756-3762. (32) Ocan˜a, M.; Forne´s, V.; Garcı´a-Ramos, J. V.; Serna, C. J. Phys. Chem. Mineral. 1987, 14, 527-532. (33) Serna, C. J.; Ocan˜a, M.; Iglesias, J. E. J. Phys. C: Solid State Phys. 1987, 20, 473-484. (34) Calleja, J. M.; Cardona, M. Phys. ReV. B 1977, 16, 3753-3761. (35) Rajalakshmi, M.; Arora, A. K.; Bendre, B. S.; Mahamuni, S. J. Appl. Phys. 2000, 87, 2445-2448. (36) Mun˜oz-Espı´, R.; Jeschke, G.; Lieberwirth, I.; Go´mez, C. M.; Wegner, G. J. Phys. Chem. B 2007, 111, 697-707.

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