Electrochemical Synthesis of Highly Oriented Layered Zinc Hydroxide

Highly oriented crystalline Zn(OH)2/p-aminobenzoic acid nanolaminates are ... properties which are not seen in either zinc hydroxide or p-aminobenzoic...
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J. Phys. Chem. C 2008, 112, 3800-3804

Electrochemical Synthesis of Highly Oriented Layered Zinc Hydroxide with Intercalated p-Aminobenzoic Acid Li-Li Xing, Bing Yuan, Shu-Xin Hu, Yu-Dong Zhang, Ying Lu, Zhen-Hong Mai, and Ming Li* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: October 13, 2007; In Final Form: December 20, 2007

Highly oriented crystalline Zn(OH)2/p-aminobenzoic acid nanolaminates are fabricated on solid supports by cathodic electrodeposition. The growth condition is very mild, involving no organic solvent. The nanolaminates have superhydrophobic surface and are transparent in the range of the visible light. The nanolaminates electrodeposited on silicon substrates behave like multiquantum well structures, exhibiting unique optical properties which are not seen in either zinc hydroxide or p-aminobenzoic acid crystals.

Introduction Layered double hydroxides (LDHs) have recently gained much attention owing to their ability to host organic guests in their interlayer spaces to produce LDH-organic hybrid materials.1-22 The resulting layered nanocomposites, called nanolaminates hereafter,23 have applications in many fields, functioning as UV adsorbents,5 chemical sensors,7 drug or DNA transporters,8,9 etc. Many techniques, such as intercalation, ion exchange, and coprecipitation, have been exploited to synthesize suspensions of the LDH-organic hybrids.10-14 For practical applications, they usually need to be cast onto solid supports followed by drying at a certain temperature.15-17 The products were usually composed of misoriented granules in these methods. Highly oriented structures are desirable for many applications.18-20 The layer-by-layer technique has been adopted to fabricate oriented LDH-organic nanolaminates. However, it usually relies on exfoliation of the layered host materials.21 The thickness of the resulting films is very limited (usually thinner than 100 nm), not enough for many applications. The solvothermal method was also used to fabricate oriented LDHaliphatic dicarboxylate films.22 The resulting films were not crack free, and high temperature was needed to complete the solvothermal reaction. Finally, exfoliated LDH nanosheets dissolved in organic solvents have been used to build thick films by spin coating or drop casting. The methods often involve toxic solvents and may introduce impurities into the hybrids. In an effort to search for low-cost and environmentallyfriendly techniques to produce high-quality LDH-organic nanolaminates for optoelectronic applications, we found that electrodeposition has many advantages over other techniques. It is simple and fast and does not require complicated pretreatment of the substrates. Here we report the electrochemical synthesis of oriented Zn(OH)2/PABA (p-aminobenzoic acid) hybrid nanolaminates. We choose to incorporate PABA into LDH because PABA is a good UV absorbent.5 Intercalating the organic UV absorbents into the LDH matrix not only keeps their UV absorption ability but also avoids direct contact of the organic UV absorbents and skin which may be harmful to human health. The resulting Zn(OH)2/PABA nanolaminates * To whom correspondence should be addressed. Phone: 86-10-82649058. Fax: 86-10-8264-0224. E-mail: [email protected].

exhibit unique optical properties which are not seen in either Zn(OH)2 or PABA crystals. The contact angle measurements reveal that the surface of the nanolaminates is superhydrophobic with a contact angle for water over 151°. The superhydrophobicity endows the surfaces of the nanolaminates with many outstanding properties,24-29 which are very important for their practical applications. Experimental Section PABA (p-aminobenzoic acid) and zinc nitrate hexahydrate (Zn(NO3)2‚6H2O) (Beijing Chemical Reagents Co.) were analytical reagent grade and used without further purification. Pure water (>18 MΩ‚cm) was produced using a Millipore filter system. The electrolyte consists of Zn(NO3)2 and PABA whose pH value was adjusted to 6.3 by a NaOH solution at room temperature. P-type silicon wafers with a resistance of 0.01 Ω‚ cm or ITO glasses were used as the cathodic electrodes. The silicon wafers were cleaned as in our previous work.30 The ITO glasses were sonicated in acetone, ethanol, and pure water for about 20 min. They were then dried in a stream of nitrogen before use. Electrodeposition was performed on a CHI660A electrochemical station using a homemade three-electrode cell with the silicon wafer or the ITO glass as the working electrode, a platinum sheet as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode. Electrical contact with the silicon wafer was realized by painting conducting silver paste on the rear side of it. The depositions were carried out by controlling the cathodic potential at -0.9 V vs SCE for 900 s. The structure of the films was examined by low-angle X-ray diffraction using a Bruker D8-Advance diffractometer equipped with a Goebel mirror to get parallel X-ray beams and suppress the Cu Kβ radiation. The rocking curves around the Bragg points were used to characterize the degree of misorientation of the films. The gracing incidence X-ray diffraction measurements were carried out on a 5-circle Huber diffractometer at the Beijing Synchrotron Radiation Facility (BSRF). A bent triangle silicon crystal was used to select the X-rays of a wavelength of 1.5493 Å. The X-ray beam hits the sample surface at a glancing angle of 0.2°. A scintillation detector was used to collect the in-plane diffraction signals. The surface morphology of the films was

10.1021/jp709978v CCC: $40.75 © 2008 American Chemical Society Published on Web 02/15/2008

Zn(OH)2/PABA Hybrid Nanolaminates

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Figure 1. X-ray diffraction pattern of a Zn(OH)2/PABA nanolaminate deposited on silicon substrate. (Inset) Rocking curve (curve a) around the second Bragg peak of the sample; the rocking curve of the substrate (curve b) is also shown for reference.

Figure 2. Gracing incidence X-ray diffraction pattern of the same sample as in Figure 1. (Inset) Gracing incidence diffraction geometry.

imaged by a Hitachi S-4200 scanning electron microscope (SEM). The contact angle was measured by a commercial contact angle meter (Dataphysics, OCA-20). The optical transmittance was examined on a Hitachi U-3010 spectrophotometer. The reflectance spectra of the nanolaminates on silicon substrates were measured by a GBC UV-vis spectrometer at room temperature in the wavelength range 250-800 nm. Results and Discussion The X-ray diffraction pattern of a typical sample is shown in Figure 1. The period of the nanolaminate is 15.5 ( 0.2 Å. Five Bragg peaks are observed, indicating that the synthesized nanolaminate is of high crystallinity. The fact that the Bragg peaks are observed only in the [001] direction suggests that the nanolaminate is oriented with its sublayers parallel to the surface of the substrate. Indeed, the rocking curve (the inset of Figure 1) around the second Bragg peak displays a very sharp peak whose width is only 0.026°, although the background near the central peak reveals that the surface of the sample is not very flat. The peak width is 2 times the instrument resolution. The widening is due to the mosaicity of the lamellar domains. We performed gracing incidence X-ray diffraction measurements to check the ordering of the molecules in the directions parallel to the surface. The result is shown in Figure 2. Three peaks are clearly visible. The peak at 2θ ) 9.64° is attributed to the two-dimensional PABA arrays. The other two peaks at 2θ ) 33.12° and 59.2° are attributed to the layered zinc hydroxide. It suggests that the c axis of the layered Zn(OH)2 runs along the surface normal of the substrate while the a axis and b axis run parallel to the surface. The appearance of the PABA peak indicates that the PABA molecules in the nanolaminate are organized into two-dimensionally ordered matrices.

Figure 3. Simulation of the low-angle X-ray diffraction pattern in Figure 1 using a (a) 2-slab model and (b) 4-slab model (symbols, experimental data; lines, fittings). Insets in a and b are the corresponding electron density profiles.

To better understand the structure of the nanolaminates, we simulated the low-angle X-ray diffraction pattern in Figure 1 using the matrix method.31,32 In the simulation the thickness (4.8 Å)11 and electron density of the Zn(OH)2 sublayers were known parameters. The thickness (10.7 Å) of the PABA sublayers was also known. We were not able to satisfactorily fit the data with a simple two-slab model for one period of the nanolaminate (Figure 3a). The fitting can be much improved using a four-slab model in which the PABA sublayer is divided into an aromatic slab and two carboxyl slabs (Figure 3b). The electron density of the aromatic slab is higher than that of the carboxyl slabs (Figure 3b, inset), implying an interdigitated arrangement of the PABA molecules. Accordingly, we can propose a structure model for the nanolaminate in which the carboxyl heads of the PABA molecules stick to the inorganic double hydroxide layers via acid-base interaction and the PABA molecules are interdigitally aligned in the spaces between the inorganic layers (cf. Figure 4). The side-to-side distance of two PABA molecules is 9.2 Å according to the diffraction peak of the 2-dimensional PABA array (Figure 2). Assuming the arrangement of the PABA molecules is similar to that in a lattice plane of the PABA crystal, the face-to-face distance between two aromatic rings can be calculated using the molecular density (4.785 × 10-3 Å-3) resulting from the simulation. The aromatic-aromatic distance is estimated to be ∼3.7 Å, which is close enough to ensure a strong π-π interaction between the aromatic rings.33 According to ref 33, the π-π interaction between two adjacent benzene rings is maximum when one ring is slipped relative to the other by ∼1.8 Å. This value, together with the thickness of a PABA bilayer and the length of a PABA molecule, can be used to calculate the tilt angle of the PABA molecules standing on the Zn(OH)2 surface. The tilt angle is about 82°. The SEM image of a nanolaminate is shown in Figure 5a. The films are continuous and compact except for some

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Figure 4. Structure model of the Zn(OH)2/PABA nanolaminates: (a) front view and (b) side view.

Figure 6. X-ray diffraction patterns of the nanolaminates grown under different conditions. (a) Samples grown with 0.02 mol/L Zn(NO3)2 and 0.1 wt % PABA but at different temperatures: (curve 1) 303, (curve 2) 308, (curve 3) 313, (curve 4) 318, (curve 5) 323, and (curve 6) 328 K. (a, inset) Relationship between the period and the growth temperature. (b) Samples grown with 0.1 wt % PABA at 303 K but with different Zn(NO3)2 concentrations: (curve 1) 0.02, (curve 2) 0.04, (curve 3) 0.06, (curve 4) 0.08, and (curve 5) 0.1 mol/L. (b, inset) Relationship between the period and the salt concentration.

Figure 5. SEM images of the (a) Zn(OH)2/PABA nanolaminate and (b) amorphous Zn(OH)2 thin film. (c) Profile of a water droplet on the surfaces of the nanolaminate.

protrudent plate-like structures which explain why the rocking curve in Figure 1b has a strong background. In contrast, the

Zn(OH)2 film grown by electrodeposition under the otherwise same condition in the absence of PABA exhibits porous morphology (Figure 5b). In fact, the Zn(OH)2 film is of bad crystallinity; no diffraction peaks are observable (data not shown). Interestingly, the surface of the nanolaminate is superhydrophobic. The contact angle of water with the asdeposited Zn(OH)2/PABA nanolaminate is 151° (Figure 5c). Two factors render the nanolaminate superhydrophobic: (1) the surface of the nanolaminate is rough, exhibiting many protrudent plate-like structures, and (2) the surface energy is reduced by exposure of the PABA molecules in air. The structure and quality of the Zn(OH)2/PABA nanolaminates can be affected by three factors, namely, the growth temperature, the electrolyte concentration, and the pH value. The period of the nanolaminates varies with both the growth temperature and the electrolyte concentration. The X-ray diffraction patterns of the nanolaminates deposited under different conditions are shown in Figure 6. At a certain electrolyte concentration, the period of the nanolaminate increases from 15.2 ( 0.2 to 15.9 ( 0.2 Å when the temperature is increased from 303 to 318 K. The period then decreases when the temperature is further increased (the inset of Figure 6a). Similarly, as shown by the inset in Figure 6b, the period of the nanolaminates increases rapidly with the concentration of zinc nitrate. It saturates at a concentration of zinc nitrate larger than 0.06 mol/L. The period of the nanolaminates is mainly determined by the density of the PABA arrays. For example, it increases with the increase of the PABA concentration in the electrolyte. It is believed that the crystallinity of the nanolaminates is affected by the growth kinetics so that the quality of the films becomes worse at higher electrolyte concentrations and higher growth temperatures because the deposition rate is

Zn(OH)2/PABA Hybrid Nanolaminates

Figure 7. (a) Comparison of the room-temperature reflectance spectra of a Zn(OH)2/PABA nanolaminate grown on silicon (solid line), a silicon wafer (dashed line), and a PABA crystal (dots). (b) UV-vis transmittance spectrum of a nanolaminate grown on glass (dashed line). The diffuse reflectance spectroscopy signal of the Zn(OH)2/PABA hybrid nanolaminate (solid line) as calculated from the reflectance spectrum according to refs 35 and 36 is also shown for comparison.

faster under these conditions. No lamellar structure can be observed when the concentration of PABA is higher than 0.3 wt %. The pH value of the electrolyte is also crucial for the deposition, although the period of the nanolaminates exhibits little dependence on the pH value. The solution with pH ≈ 6 appears to be most favored for growth of the nanolaminates. No nanolaminate can be formed when the pH value is e5. The room-temperature reflectance spectrum of a hybrid nanolaminate grown on a silicon wafer is shown in Figure 7a. The reflectance spectra of a silicon wafer and a PABA crystal are also shown for comparison. Two absorption peaks at 297 (4.17 eV) and 338 nm (3.67 eV) are observed for the nanolaminates. The absorption peak at 297 nm is related to the singlet-to-singlet (S0 f S1) transitions of the PABA molecules.34 The generated Frenkel excitons are localized to the individual PABA molecules. Compared with the absorption spectrum of the dilute aqueous solution (∼2 µM) of PABA, the absorption peak is red shifted due to the two-dimensional confinement of the electrons by the inorganic Zn(OH)2 dielectric barriers. The corresponding absorption in the PABA crystal is very broad because of the strong exciton-phonon interactions. The absorption peak at 338 nm is attributed to generation of the chargetransfer excitons.34 The π-π orbital overlapping of the very closely stacked PABA molecules in each interspace of the inorganic layers leads to formation of the aggregate state of PABA molecules, resulting in generation of the charge-transfer excitons. The binding energies of the excitons are strong enough to be observed even at room temperature. To the best of our knowledge, it is the first time to observe exitons originating from the organic component in an LDH-like structure. Such multiquantum well effects can be observed only in high-quality

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3803 nanolaminates in which the PABA molecules are well ordered. For example, the UV-vis transmittance spectrum of the nanolaminate becomes very broad (Figure 7b) when the crystallinity of the nanolaminate becomes worse. The broad absorption peak is similar to those reported in the literature, for example, in ZnAl-LDH/PABA hybrids in which the inplane ordering of the PABA molecules did not occur.5 The Zn(OH)2/PABA nanolaminates are formed only when both zinc nitrate and PABA are present in the electrolyte solution. The growth mechanism can be understood as follows. At a potential of -0.9 V vs SCE, the zinc ions in the electrolyte are attracted to the inner Helmholtz plane, while negatively charged NO3- ions and PABA molecules stay at the outer Helmholtz plane. OH- ions are generated in the cathodic reaction NO3- + H2O + 2e- f NO2- + 2OH-. The OH- ions combine immediately with the Zn2+ ions in the Helmholtz layer with the reaction Zn2+ + 2OH- f Zn(OH)2V to form a double hydroxide layer. Meanwhile, as an acid, PABA combines with the double hydroxide layer through the acid-base reaction. The reaction results in a mixture of HO-Zn-OH, RCOO-ZnOH, and/or RCOO-Zn-OCOR bonds. Besides, other PABA molecules could combine with OH- directly to yield water and PABA anions in the reaction OH- + RCOOH f RCOO- + H2O. These organic anions combine with Zn2+ and/or OH- to generate the bonds mentioned above. The inorganic component tends to form layered structures. The PABA molecules tend to line up on the double hydroxide through the face-to-face π-π interaction. A PABA bilayer can thus be constructed atop the double hydroxide layer because of the hydrophobic interactions. A newly formed double hydroxide layer continues to react with the top leaf of the PABA bilayer. The process repeats itself to form the Zn(OH)2/PABA nanolaminates in a layer-by-layer manner. The PABA molecules by themselves do not self-assemble to form a multilamellar structure. This makes the growth mechanism of the Zn(OH)2/PABA nanolaminates different from the electrochemically induced self-assembly of ZnO-surfactant multilayers37 in which the surfactants can aggregate at the electrolyte-electrode interface to form micelles or bilayers at a concentration lower than the critical micelle concentration. The surfactants act as templates to direct the deposition of ZnO in the electrochemical process. In our system, the double hydroxide layers function as a support for the PABA molecules. The growth mechanism is also different from that of the R-zirconium phosphate-polyaniline hybrids grown on platinum electrodes.38 The later is grown by simultaneous electrophoretic and electrolytic deposition in which the exfoliated, negatively charged, nanosheets were transported electrophoretically to the anode. Polar organic solvents, especially noxious acetonitrile, are required to dissolve the exfoliated nanosheets of R-zirconium phosphate. Conclusions We have shown that Zn(OH)2/PABA hybrid nanolaminates can be grown on solid supports through electrochemical synthesis. The growth condition is very mild, involving no organic solvent. The resulting nanolaminates are of high crystallinity and highly oriented. The transparency in the visible light region and strong absorption in the ultraviolet region make the nanolaminates good candidates for window materials and/ or protective coatings. The PABA molecules are well organized between the double hydroxide layers. The strong confinement of the electron-hole pairs by the dielectric barriers leads to formation of stable excitons even at room temperature, sug-

3804 J. Phys. Chem. C, Vol. 112, No. 10, 2008 gesting that the nanolaminates behave like a multiquantum well structure. Moreover, the surface of the nanolaminates is superhydrophobic, making them good candidates for functional coatings because they will have the properties of self-cleaning and antiwetting.29 We expect that the unique mechanisms of electrochemistry-assisted crystal growth will broaden the type of solid-supported hybrid materials with new phases and orientations. Acknowledgment. This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (kjcx3.syw.n8), the National Basic Research Program (2004CB619005), and the National Natural Science Foundation of China (10674158 and 10574159). We thank Prof. L. Jiang for the contact angle measurement and Prof. L. Guo for the room-temperature reflectance measurement. Work in BSRF was supported by the Beijing Synchrotron Radiation Laboratory. References and Notes (1) Kasai, A.; Fujihara, S. Inorg. Chem. 2006, 45, 415. (2) Choy, J. H.; Park, J. S.; Kwak, S. Y.; Jeong, Y. Z.; Han, Y. S. Mol. Cryst. Liq. Cryst. 2000, 341, 425. (3) Evans, D. G.; Duan, X. Chem. Commun. 2006, 5, 485. (4) Xu, Z. P.; Stevenson, G. S.; Lu, C. Q.; Lu, G. Q.; Bartlett, P. F.; Gray, P. P. J. Am. Chem. Soc. 2006, 128, 36. (5) He, Q.; Yin, S.; Sato, T. J. Phys. Chem. Solids 2004, 65, 395. (6) Shi, H. Z.; Lan, T.; Pinnavaia, T. Chem. Mater. 1996, 8, 1584. (7) Guth, U.; Brosda, S.; Schomburg, J. Appl. Clay Sci. 1996, 11, 229. (8) Ruiz-Hitzky, E.; Darder, M.; Zranda, P. J. Mater. Chem. 2005, 15, 3650. (9) Desigaux, L.; Belkacem, M. B.; Richard, P.; Cellier, J.; Leone, P.; Cario, L.; Leroux, F.; Taviot-Gueho, C.; Pitard, B. Nano Lett. 2006, 6, 199. (10) Shukoor, M. I.; Therese, H. A.; Gorgishvili, L.; Glasser, G.; Kolb, U.; Tremel, W. Chem. Mater. 2006, 18, 2144. (11) Wypych, F.; Arizaga, G. G. C.; Gardolinski, J. E. F. C. J. Colloid Interface Sci. 2005, 283, 130. (12) Aisawa, S.; Ishida, E.; Takahashi, S.; Hirahara, H.; Narita, E. Chem. Lett. 2005, 34, 630. (13) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (14) Ogata, S.; Tagaya, H.; Karasu, M.; Kadokawa, J. J. Mater. Chem. 2000, 10, 321.

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