J. Phys. Chem. B 2006, 110, 1599-1604
1599
Heterostructured Nanohybrid of Zinc Oxide-Montmorillonite Clay Su Gil Hur,† Tae Woo Kim,† Seong-Ju Hwang,*,† Sung-Ho Hwang,‡ Jae Hun Yang,† and Jin-Ho Choy*,† Center for Intelligent Nano-Bio Materials (CINBM), DiVision of Nano Sciences and Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea, and Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 700-742, Korea ReceiVed: August 5, 2005; In Final Form: NoVember 30, 2005
We have synthesized heterostructured zinc oxide-aluminosilicate nanohybrids through a hydrothermal reaction between the colloidal suspension of exfoliated montmorillonite nanosheets and the sol solution of zinc acetate. According to X-ray diffraction, N2 adsorption-desorption isotherm, and field emission-scanning electron microscopic analyses, it was found that the intercalation of zinc oxide nanoparticles expands the basal spacing of the host montmorillonite clay, and the crystallites of the nanohybrids are assembled to form a house-ofcards structure. From UV-vis spectroscopic investigation, it becomes certain that calcined nanohybrid contains two kinds of the zinc oxide species in the interlayer space of host lattice and in mesopores formed by the house-of-cards type stacking of the crystallites. Zn K-edge X-ray absorption near-edge structure/extended X-ray absorption fine structure analyses clearly demonstrate that guest species in the nanohybrids exist as nanocrystalline zinc oxides with wurzite-type structure.
Introduction Over the past decades, aluminosilicate clay minerals have attracted intense research activity because of their ability to form intercalation complexes with diverse guest species.1-3 There have been many reports on various intercalation compounds of the clay minerals containing organic molecules, metal complexes, metal oxides, and so on.4 From the viewpoint of application, pillared compounds obtained by the post-calcination of the intercalation compounds are of special importance in that they have various interesting physicochemical functions such as absorption capacity, catalytic activity, etc. In particular, the pillaring of a transition metal oxide like cobalt oxide can provide an opportunity to develop excellent heterogeneous catalysts with an expanded surface area.5 To date, several transition metal oxide-pillared clay systems, such as CoO/SiO2-,5 Cr2O3-,6 Fe2O3,7 ZrO2-,8 TiO2-aluminosilicate,9 have been reported. Currently, among various transition metal oxides, zinc oxide receives special attention due to its wide applicability as luminescent and lasing materials, photocatalysts, high-frequency piezoelectric resonators, and so on.10-12 Such useful properties of the zinc oxide originate mainly from its semiconducting nature with a wide band gap separation. In this context, several attempts have been made to synthesize nanostructured zinc oxide like 0D quantum dot or 1D nanorod/nanowire for the optimization of its band structure and its functionality through quantum confinement effect.12-14 In addition, there have been several reports on the incorporation of the zinc oxide into the pores of zeolite and MCM41 system.15 However, the pillared system of the zinc oxide with the aluminosilicate clay, which is completely distinguishable from the previously reported materials in terms of immobilization of the zinc oxide in a 2D inorganic lattice, has never been systematically investigated. A recent work on * To whom all correspondences should be addressed. Phone: +82-23277-4370. Fax: +82-2-3277-3419. E-mail:
[email protected]. † Ewha Womans University. ‡ Daegu Gyeongbuk Institute of Science & Technology.
the growth of the zinc oxide in the presence of the clay minerals allows us to predict the possible intercalation of the ZnO into 2D aluminosilicate lattice.16 In this study, we were successful in synthesizing heterostructured ZnOx-montmorillonite nanohybrids without any surface contamination by bulk ZnO through the hybridization of zinc oxide nanoparticles with colloidal nanosheets of montmorillonite clays. We have carried out the systematic characterizations of crystal structure, chemical composition, and physicochemical properties of the resulting nanohybrids using X-ray diffraction (XRD), elemental analysis, thermogravimetry-differential thermal analysis (TGA-DTA), ultraviolet-visible (UV-vis) spectroscopy, and N2 adsorption-desorption isotherm measurement. Also, X-ray absorption spectroscopic (XAS) analysis has been performed at Zn K-edge to probe the electronic and local crystal structures of this heterostructured material. Experimental Section Pristine montmorillonite clay, KSF (Aldrich Chemical Co.), was treated with excess 1 M NaCl aqueous solution several times and then dried at 50 °C. The powder obtained was dispersed in distilled water for 1 week to prepare the colloidal suspension of delaminated clay particles.9 The transparent solution of zinc acetate was obtained by adding zinc (II) acetate into a mixed solution of 1-propanol and distilled water, and then stirring the resultant solution vigorously at room temperature and finally refluxing it at 120 °C for 2 h. ZnOxmontmorillonite nanohybrid was synthesized by reacting the colloidal suspension of the montmorillonite clay with the sol solution of zinc acetate at 80 °C for 5 h in a polytetrafluoroethylene-lined autoclave. Prior to characterization, the product was washed thoroughly with distilled water and then dried. The crystal structure, crystallite morphology, and optical property of the ZnOx-montmorillonite nanohybrids were studied by XRD, field emision-scanning electron microscopy (FE-SEM, JEOL JSM-6700F microscope), and diffuse reflectance UV-
10.1021/jp0543633 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/07/2006
1600 J. Phys. Chem. B, Vol. 110, No. 4, 2006 vis spectroscopy (Perkin-Elmer lambda 35 spectrometer), respectively. Their chemical compositions, thermal behaviors, and surface area/porosity were determined by induced coupled plasma (ICP) spectroscopic analysis, X-ray fluorescence spectroscopy (XRF), CHN (carbon, hydrogen, nitrogen) elemental analysis, TGA-DTA, and N2 adsorption-desorption isotherm measurements, respectively. XAS experiments were carried out at the Zn K-edge with the extended X-ray absorption fine structure (EXAFS) facility installed at the beam line 7C at the Pohang Light Sources in Korea. XAS data were collected at room temperature in a transmission mode using gas-ionization detectors. All the present spectra were calibrated carefully by measuring the reference spectrum of Zn metal simultaneously. Data analysis for the experimental spectra was performed with a standard procedure, as reported previously.17 In the course of EXAFS fitting analysis on the reference ZnO, the coordination number (CN) was fixed to the crystallographic values, whereas the amplitude reduction factor (S02) was allowed to vary. As for the nanohybrids, the CN’s of Zn-Zn shells were set as variables to examine the effect of nanocrystalline nature on the coordination number of distant coordination shells. All the present fitting analyses gave acceptable S02 value of 0.9-1.1. On the other hand, all the bond distances (R), energy shifts (∆E), and Debye-Waller factors (σ2) were set as variables, under the constraint that the latter two parameters were kept the same for two adjacent Zn-O shells at ∼1.96-2.05 Å and two neighboring Zn-Zn shells at ∼3.15-3.23 Å, respectively. Such constraints can be rationalized from the fact that the adjacent shells consisting of the same types of atoms at very close distances would possess nearly the same degree of energy shift and structural disorder. Results and Discussion Powder XRD Analysis. The powder XRD patterns of pristine Na-montmorillonite and ZnOx-montmorillonite nanohybrids are presented in Figure 1. As-prepared nanohybrid exhibits a series of equally spaced (001) reflections in the low angle region, which provides strong evidence for the formation of intercalative nanohybrid with an 1:1 ordered heterostructure. The basal spacing of the as-prepared nanohybrid was determined to be 15.3 Å, corresponding to the gallery height of 5.7 Å. Upon postcalcination at elevated temperature, the (001) reflections move toward the high angle side, suggesting a lattice contraction due to the decomposition of interlayer guest species. A heat treatment at 300 °C gives rise to a shrinkage of c-axis lattice parameter to 13.8 Å, corresponding to the interlayer distance of 4.2 Å. Even after a heat treatment at 500 °C, a broad but distinct (001) peak is still discernible, suggesting the maintenance of a pillared structure. In comparison with other pillared clay compounds,5-9 the present nanohybrids show rather poorer crystallinity, which is surely due to the low structural order of the pristine KSF clay samples rather than to the incomplete formation of pillared structure. In case that a montmorillonite clay (Kunepia Co.) with high structural order was used, we could observe well developed (001) Bragg reflections for the ZnOxmontmorillonite nanohybrid prepared by the same synthetic route (see Supporting Information). Instead of the hydrothermal synthesis, we have also tried to synthesize the ZnOx-montmorillonite nanohybrid through a reaction between zinc acetate solution and exfoliated clay nanoparticles under reflux condition. Such a trial gave only an X-ray amorphous material, highlighting the usefulness of the hydrothermal route to heterostructured compounds. As can be clearly seen from Figure 1b, none of the present nanohybrids show any distinct peaks corresponding to bulk ZnO particles in the high angle region of 30-40°, which
Hur et al.
Figure 1. Powder XRD patterns of (a) the as-prepared ZnOxmontmorillonite nanohybrid and its derivatives calcined at (b) 300 and (c) 500 °C, in comparison with those of (d) the pristine Namontmorillonite and (e) ZnO. The upper and lower panels represent low-angle and high-angle diffraction data, respectively. The asterisk represents a nonhydrated montmorillonite phase existing in the starting pristine material.
Figure 2. TGA (solid lines) and DTA (dashed lines) curves of the as-prepared ZnOx-montmorillonite nanohybrid under Ar atmosphere.
is in contrast with the previous report on the growth of ZnO particles on clay surface.16 This finding clarifies that, under the present synthetic condition, the formation of bulk ZnO particles on the nanohybrid surface becomes effectively suppressed. Elemental Analysis and TGA-DTA. ICP analysis confirms that considerable amount of zinc species are hybridized with montmorillonite clay at the Zn/Si ratio of 0.93. In combination with XRF results, the chemical composition of the resulting nanohybrid was determined to be (ZnOx)3.7-Ca0.2K0.05(Si4Al1.4Fe0.18Mg0.43)O10(OH)21.4H2O, with a cation exchange capacity of 109.6 mequiv/100 g. We have probed the thermal behavior of the present nanohybrid by performing TGA-DTA measurement under Ar atmosphere, see Figure 2. The present nanohybrid shows considerable weight losses over a wide temperature range.
Zinc Oxide-Montmorillonite Clay A main change in the temperature range of 100-200 °C is attributable to the dehydration and dehydroxylation of the nanohybrid. The decomposition of residual organic species is responsible for the weight loss in the higher temperature region with a broad exothermic peak. No marked weight loss occurs beyond 600 °C, indicating the complete thermal decomposition of interlayer zinc species below this temperature. CHN elemental analysis demonstrates that the as-prepared sample contains only a small amount of carbon, indicating that most of the zinc acetates are decomposed into zinc oxide during hydrothermal synthesis.18 This finding was further confirmed by the fact that the hydrothermal treatment of zinc acetate solution at 80 °C leads to the formation of ZnO nanocrystals.19 FE-SEM and N2 Adsorption-Desorption Isotherm Analysis. The crystallite morphologies of the as-prepared ZnOxmontmorillonite nanohybrid and its calcined derivatives were monitored with FE-SEM analysis, as shown in Figure 3. The layered crystallites of montmorillonite with the lateral dimension of 400-500 nm are restacked to form a house-of-cards structure. There is no detectable growth of ZnO particles on the sample surface before and after the calcination, which is consistent with the present XRD results. The N2 adsorption-desorption isotherms of the as-prepared ZnOx-montmorillonite nanohybrid and its calcined derivatives are presented in Figure 4. All of the present nanohybrids show a distinct hysteresis during an adsorption-desorption cycle. The isotherm data for the calcined nanohybrids can be classified as the BDDT-type I and IV shape, along with H4-type hysteresis loop in the IUPAC classification.20 Such a type of isotherm underlines the presence of the open slit-shaped capillaries with very wide bodies and narrow short necks. While the as-prepared nanohybrid and its calcined derivative at 300 °C have a similar surface area of 60 m2g-1 with a negligible micropore area, a heat-treatment at 500 °C results in a distinct increase of the surface area to 82 m2g-1 with a micropore area of ∼13 m2g-1. In fact, as shown in Figure 4, the nanohybrid calcined at 500 °C displays the most prominent hysteresis due to the presence of mesopores in the house-of-cards structure of restacked crystallites. In comparison with the mesopores, the micropores originating from the pillared structure and make a smaller contribution to total surface area even for the calcined sample at 500 °C. In fact, the gallery height of the calcined nanohybrid (4.2 Å) is only slightly greater than the kinetic diameter of the nitrogen molecule (3.64 Å). Furthermore, the presence of the zinc oxide pillar would make the diameter of the micropores narrower, which prevents the efficient introduction and adsorption of nitrogen molecules into the gallery space of the nanohybrids. Diffuse UV-vis Spectroscopy. Figure 5 illustrates UV-vis spectra for the ZnOx-montmorillonite nanohybrids, in comparison with that for ZnO. All of the nanohybrids show a small step with an absorption onset at ∼230 nm, which is attributable to sub-nanocrystalline species in the interlayer space of the host lattice. According to the empirical curves of absorption onset wavelength vs particle size,21,22 the ZnO nanoparticles with a diameter around 10 Å show an absorption onset at around 265280 nm. In this regard, the observed shorter absorption threshold wavelength of the ZnOx-montmorillonites is in good agreement with the gallery height of ∼4-6 Å, estimated from XRD analysis. A peak at 247 nm corresponds to the montmorillonite clay.23 After a calcination at 300 °C, a new peak appears at around 370 nm due to the formation of zinc oxide particles in the mesopore sites. The absorption edge is slightly displaced after a heat-treatment at 500 °C, suggesting a change of Zn
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1601
Figure 3. FE-SEM images of (a) the as-prepared ZnOx-montmorillonite nanohybrid and its derivatives calcined at (b) 300 and (c) 500 °C.
electronic configuration due to the partial formation of covalent bonds between the guest ZnO and the host montmorillonite. It is worthwhile to note here that the lower-energy absorption edge of the calcined derivatives appears at a somewhat shorter wavelength compared to that of the reference ZnO, which is surely due to the nanocrystalline nature of the ZnO in the pores of the calcined nanohybrids. Such a result is further supported by the absence of ZnO peaks in the XRD pattern and no observation of ZnO particles on the surface of the nanohybrid in the FE-SEM image (Figures 1 and 3). Based on the experimental findings presented here, we were able to suggest
1602 J. Phys. Chem. B, Vol. 110, No. 4, 2006
Figure 4. N2 adsorption-desorption isotherms for (a) the as-prepared ZnOx-montmorillonite nanohybrid and its derivatives calcined at (b) 300 and (c) 500 °C.
Figure 5. UV-vis absorption spectra for the as-prepared ZnOxmontmorillonite nanohybrid (solid lines) and its derivatives calcined at 300 (dashed lines) and 500 °C (dot-dashed lines), and the reference ZnO (triangles).
Figure 6. Schematic illustration of the ZnOx-montmorillonite nanohybrid.
a structural model for the calcined nanohybrid, as illustrated in Figure 6. In this material, there are two kinds of the zinc oxide species existing in the interlayer space of pillared compound, and in the mesopores formed by the house-of-cards stacking of exfoliated clay crystallites as well.
Hur et al.
Figure 7. Zn K-edge XANES spectra for (a) the as-prepared ZnOxmontmorillonite nanohybrid and its derivatives calcined at (b) 300 and (c) 500 °C, in comparison with that for (d) the reference ZnO.
Zn K-Edge XANES and EXAFS Analyses. Figure 7 illustrates the Zn K-edge X-ray absorption near-edge structure (XANES) spectra for the as-prepared ZnOx-montmorillonite and its calcined derivatives together with that for the bulk ZnO. The edge positions of the present nanohybrids are similar to that of the reference ZnO, indicating the divalent state of zinc in these compounds. All the nanohybrids exhibit rather poorly resolved XANES features A and B in the energy region of 9664-9668 eV, which are quite different from the spectral features of the bulk ZnO, which shows two distinct peaks at 9662.5 and 9669 eV. Taking into account the fact that such broad and poorly resolved white line peaks are characteristic of nanocrystalline ZnO species,24 we suggest that most of the zinc species in the nanohybrids exist as zinc oxide nanocrystals in the interlayer space of montmorillonite and in the mesopore sites. We have determined quantitatively the local structure of intercalated zinc oxide species by performing EXAFS analysis. The Fourier transforms (FT) of k3-weighted Zn K-edge EXAFS spectra for the as-prepared ZnOx-montmorillonite nanohybrid, its derivative calcined at 500 °C, and the reference ZnO are plotted in Figure 8a, and the corresponding Fourier-filtered oscillations are plotted in Figure 8b. The reference ZnO exhibits two intense FT peaks in the R range of 1.0-3.5 Å. While the first peak at ∼1.6 Å (phase-shift uncorrected) originates from the Zn-O coordination shells, the more distant feature at ∼2.8 Å (phase-shift uncorrected) is related to Zn-Zn bonding pairs in corner-shared ZnO4 tetrahedra. The experimental spectrum of the reference ZnO was quite reproducible with wurzite structure and the obtained structural parameters are summarized in Table 1. The calculated structural parameters match well with the crystallographic values, verifying the reliability of the present fitting analyses. Like the reference ZnO, both of the nanohybrids display two FT peaks corresponding to the Zn-O and Zn-Zn shells at around 1.6 and 2.8 Å (phase-shift uncorrected). Such spectral features are quite distinguishable from the FT spectrum of the reference zinc acetate showing only one peak corresponding to Zn-O bonding pair at around 1.6 Å (not shown). In this regard, the appearance of Zn-Zn peaks in the FT data of nanohybrids strongly suggests that the intercalated zinc species exist in the form of zinc oxide rather than the other phase such as zinc acetate. Also, it is worthwhile to note here that the distant FT peaks beyond 2 Å are somewhat weaker for the nanohybrids than for the bulk ZnO, strongly suggesting the limited crystal growth of ZnO species in the 2D lattice of the
Zinc Oxide-Montmorillonite Clay
J. Phys. Chem. B, Vol. 110, No. 4, 2006 1603 octahedral one. Hence, the zinc species in the as-prepared sample are believed to crystallize with wurzite-like structure. In addition, there are two types of distant zinc neighbors at 3.15 and 3.19 Å for the as-prepared nanohybrid and at 3.17 and 3.21 Å for the calcined derivative. As in the Zn-O shells, the Zn-Zn bond distances of the calcined derivative are more similar to those of the reference ZnO, compared to the as-prepared sample. This finding confirms the fact that the calcination process helps to complete the formation of ZnO pillar. As expected from the FT data, the CNs of Zn-Zn shells are determined to be smaller for the nanohybrids than for the reference ZnO. Conclusion
Figure 8. (a) Fourier-transformed and (b) Fourier-filtered Zn K-edge EXAFS spectra for (i) the as-prepared ZnOx-montmorillonite nanohybrid and (ii) its derivative calcined at 500 °C, in comparison with those for (iii) the reference ZnO.
TABLE 1: Results of Non-Linear Least Squares Curve Fittings for the Zn K-Edge EXAFS Spectra of the As-Prepared ZnOx-Montmorillonite and Its Calcined Derivative at 500 °C and the Reference ZnO sample
bond
CN
R (Å)
σ2 (10-3 × Å2)
as-prepared ZnOx-montmorillonitea
Zn-O Zn-O Zn-Zn Zn-Zn
3.0 1.0 3.0 3.0
2.03 2.05 3.15 3.19
11.53 11.53 13.95 13.95
ZnOx-montmorillonite -500 °Cb
Zn-O Zn-O Zn-Zn Zn-Zn
3.0 1.0 2.8 2.8
1.96 1.97 3.17 3.21
6.13 6.13 10.14 10.14
reference ZnOc
Zn-O Zn-O Zn-Zn Zn-Zn
3 1 6 6
1.97 1.99 3.19 3.23
6.18 6.18 10.55 10.55
a The curve fitting analysis was performed for the range of R 1.1973.221 Å and k 3.35-11.00 Å-1. b The curve fitting analysis was performed for the range of R 1.135-3.405 Å and k 3.45-9.75 Å-1. c The curve fitting analysis was performed for the range of R 1.0743.283 Å and k 3.30-10.85 Å-1.
nanohybrid. After the calcinations at 500 °C, the spectrum of the nanohybrid becomes more similar to that of the bulk ZnO. As shown in Figure 8b, the experimental spectra of both the nanohybrids could be well reproduced on the basis of wurzite ZnO structure. The fitted structural parameters are listed in Table 1. The zinc ions are stabilized in the ZnO4 tetrahedra with Zn-O bond distances of 2.03-2.05 Å for the as-prepared sample and 1.96-1.97 Å for the calcined derivative, which are compatible to the ZnII-O bond lengths in bulk ZnO (1.97-1.99 Å). Although the estimated Zn-O bond distances for the asprepared nanohybrid are slightly longer than those of the reference ZnO, the calculation of CN strongly suggests the stabilization of Zn in a tetrahedral site rather than in an
In this work, we were quite successful in synthesizing mesoporous zinc oxide-montmorillonite nanohybrid with an 1:1 ordered heterostructure without any significant surface contamination by bulk ZnO particles. The formation of such a heterostructured nanohybrid was evidenced on the basis of the XRD, ICP-XRF, UV-vis, and XANES-EXAFS results. After the post-calcination, the zinc oxide species exist both in the interlayer space of pillared compound and in the mesopores formed by the house-of-cards stacking of exfoliated clay crystallites. Nitrogen adsorption-desorption isotherm measurements clarify that the calcination process creates a considerable amount of mesopores due to the house-of-cards-type stacking of exfoliated clay nanosheets. Acknowledgment. This work was supported by grant no. R08-2003-000-10409-0 from the Basic Research Program of the Korea Science & Engineering Foundation and partly by the SRC/ERC program of MOST/KOSEF (grant R11-2005-00803002-0). The experiments at Pohang Light Source were supported in part by MOST and POSTECH. Supporting Information Available: Powder XRD patterns for the pristine montmorillonite (Kunepia Co.), its ZnOxintercalated nanohybrid, the pristine KSF montmorillonite, and its ZnOx-intercalated nanohybrid. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jones, S. L. Catal. Today 1987, 12, 209. (2) Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2004. (3) Comprehensive Supramolecular Chemistry; Atwood, J. L., Macnicol, J. E. D., Vogtle, F., Eds.; Pergamon: Oxford, 1996; Vol. 7. (4) Intercalation in Layered Materials; Dresselhaus, M. S., Ed.; Plenum Press: New York, 1986. (5) Choy, J. H.; Jung, H.; Han, Y. S.; Yoon, J. B.; Shul, Y. G.; Kim, H. J. Chem. Mater. 2002, 14, 3823-3828. (6) (a) Bornholdt, K.; Corker, J. M.; Evans, J.; Rummey, J. M. Inorg. Chem. 1991, 30, 2. (b) Drljaca, A.; Anderson, J. R.; Spiccia, L.; Turney, T. W. Inorg. Chem. 1992, 31, 4894. (7) Burch, R.; Warburton, C. I. Appl. Catal. 1987, 33, 395. (8) Bartley, G. J. J.; Burch, E. Appl. Catal. 1985, 19, 175. (9) Sterte, J. P. Clays Clay Miner. 1986, 34, 658. (10) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (11) (a) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G. C.; Choi, W. AdV. Mater. 2004, 16, 1661. (b) Rensmo, H.; Keis, K.; Lindstrom, H.; Solbrand, A.; Hagfeldt, A.; Lindquist, S. E.; Wang, L. N.; Muhammed, M. J. Phys. Chem. B 1997, 101, 2598. (12) (a) Choy, J. H.; Jang, E. S.; Won, J. H.; Chung, J. H.; Jang, D. J.; Kim, Y. W. AdV. Mater. 2003, 22, 1911. (b) Choy, J. H.; Jang, E. S.; Jang, D. J. Appl. Phys. Lett. 2004, 84, 287. (13) (a) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (b) Zou, B. S.; Valkov, V. V.; Wang, Z. L. Chem. Mater. 1999, 11, 3037. (c) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756.
1604 J. Phys. Chem. B, Vol. 110, No. 4, 2006 (14) (a) Guo, L.; Ji, Y. L.; Xu, H.; Simon, P.; Wu, Z. J. Am. Chem. Soc. 2002, 124, 14864. (b) Yin, M.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126, 6206. (15) Chen, J.; Feng, Z.; Ying, P.; Li, C. J. Phys. Chem. B 2004, 108, 12669-12676. (16) Ne´meth, J.; Rodrı´guez-Gattorno, G.; Dı´az, D.; Va´zquez-Olmos, A. R.; De´ka´ny, I. Langmuir 2004, 20, 2855-2860. (17) (a) Choy, J. H.; Hwang, S. J.; Park, N. G. J. Am. Chem. Soc. 1997, 119, 1624. (b) Choy, J. H.; Kim, Y. I.; Hwang, S. J. J. Phys. Chem. B 1998, 102, 9191. (c) Choy, J. H.; Kim, Y. I.; Hwang, S. J.; Huong, P. V. J. Phys. Chem. B 2000, 104, 7273. (18) Contents (in wt %) of carbon and hydrogen for the as-prepared ZnOx-montmorillonite (C, 1.28%; H, 1.51%) and its calcined derivatives at 300 °C (C, 0.83%; H, 0.93%) and 500 °C (C, 0.47%; H, 0.55%).
Hur et al. (19) Nishizawa, H.; Yuasa, K. J. Mater. Sci. Lett. 1998, 17, 985-987. (20) (a) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619. (b) Carrado, K. A.; Csenesits, R.; Thiyagarajan, P.; Seifert, S.; Macha, S. M.; Harwood: J. S. J. Mater. Chem. 2002, 12, 3228-3237. (21) Hasse, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 482487. (22) Wood, A.; Giersig, M.; Hilgendorff, M.; Vilas-Campos, A.; LizMarzan, L. M.; Mulvaney, P. Austr. J. Chem. 2003, 56, 1051. (23) Han, Z.; Zhu, H.; Bulcock, S. R.; Ringer, S. P. J. Phys. Chem. B 2005, 109, 2673. (24) Park, N. G.; Kang, M. G.; Kim, K. M.; Ryu, K. S.; Chang, S. H.; Kim, D. K.; van de Langemaat, J.; Benkstein, K. D.; Frank, A. J. Langmuir 2004, 20, 4246.
