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Oriented Crystallization of KH2PO4 under a Compressed Langmuir Monolayer Bin Li, Yang Liu, Nan Lu, Jihong Yu, Yubai Bai, Wenqin Pang, and Ruren Xu* Department of Chemistry, Jilin University, Changchun 130023, China Received November 20, 1998. In Final Form: April 6, 1999 Oriented growth of KH2PO4 (KDP) under a compressed Langmuir monolayer of arachidic acid (AA) has been studied by using π-A isotherms, X-ray diffraction (XRD), Brewster angle microscope (BAM), and atomic force microscopy (AFM) techniques. All produced crystals are almost connected with the monolayer by the crystal face (100), epitaxial forming multilayer structures along the vertical direction. BAM observation shows that KDP crystals induced by the Langmuir monolayer are nearly homogeneous and uniform in crystal size. Crystallization induction time is reduced evidently. The optical second harmonic generation (SHG) experiment indicates that the SHG signal of the sample of 25 layers deposited in Z type is better than that of the crystals in the absent of a monolayer, which demonstrates that oriented KDP crystals are homogeneous with a uniform orientation.
Introduction A new field, usually named biomineralization, has received considerable attention and has been developed very rapidly in recent years.1-4 A development in this area involves the use of Langmuir monolayers of surfactant molecules as molecular templates for the oriented nucleation of organic and inorganic crystals. Because of ion binding, lattice matching, and structural and stereochemical complementarity between the monolayer and the crystal, very high selectivity in the nucleation and growth of crystal can be observed.5-24 The ability to control (1) Mann, S. Struct. Bonding 1983, 54, 125. (2) Berman, A.; Addadi, L.; Weiner, S. Nature 1988, 331, 546. (3) Berman, A.; Addadi, L.; Kuick, A.; Leiserowitz, L.; Nelson, M. Science 1990, 250, 664. (4) Heuer, A. H.; Frind, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis A.; Caplan, A. I. Science 1992, 255, 1098. (5) Landau, E. M.; Popovitz-Biro, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1986, 134, 323. (6) Mann, S.; Heywood, B. R.; Rajam, S., Birchall, J. D. Nature 1988, 334, 692. (7) Landau, E. M.; Wolf, S. G.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 1436. (8) Mann, S.; Heywood, B. R.; Rajam, S., Birchall, J. D. Proc. R. Soc. London, Ser. A 1989, 423, 457. (9) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. W. Surface Reactive Peptides and Polymers; American Chemical Society: Washington, DC, 1989. (10) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. W.; Davey, R. J.; Birchall, J. D. Adv. Mater. 1990, 2 (05), 257. (11) Gavish, M.; Popovitz-Bior, R.; Lahav, M.; Leiserowitz, L. Science 1990, 250, 973. (12) Mann, S. J. Chem. Soc., Dalton Trans. 1990, 1873. (13) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. W. J. Phys. D: Appl. Phys. 1991, 3, 154. (14) Rajam, S.; Heywood, B. R.; Walker, J. B. W.; Davey, R. J.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 87 (5), 727. (15) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87 (5), 735. (16) Zhao, X. K.; Xu, S.; Fendler, J. H. Langmuir 1991, 7, 520. (17) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (18) Heuer, A. H.; Frind, D. J.; Laraia, V. J.;.Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098. (19) Zhao, K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (20) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 1266. (21) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (22) Tang, R.; Tai, Z.l Chao, Y. J. Chem. Soc., Dalton Trans. 1996, 4439.
the crystallization of inorganic solids is critical to improve technological use of materials currently and in the future.15 Due to crystal anisotropy, only a special crystal face can be employed to realize crystal function. This will greatly improved the utilization ratio of the element if the surface of the crystal is a functional crystal face. It is possible to obtain the desired crystal face by oriented growth of crystal induced by a Langmuir monolayer. Moreover, modification of some features, such as structural perfection, crystal size, and orientation, is important in optimizing and determining electrical, optical, magnetic, and catalytic properties.15 However, there still exists a research blank in the optical field, especially in the study of the nonlinear optics (NLO) of crystals. As we have known, the crystal structure has a remarkable effect on nonlinear coefficients. And a high orientation of ions is necessary for crystals with perfect SHG signals. Inducing oriented growth of crystals by LB techniques provides not only identical orientation but also multilayer assembly, which undoubtedly improves the frequency doubling effect of the material. KDP is an excellent NLO material and finds widespread employment as a frequency doubler in laser application.25 For several decades extensive research has been concentrated on its crystal morphology, structure, and growth process. One aim of these studies is to understand thoroughly the influences of various factors on the nonlinear optical effect, which will provide reliable evidence in the search the new NLO materials.25-30 Therefore, it is important to investigate the KDP crystal with a special crystal face. Here we report, for the first time, epitaxial formation of KDP crystal. (23) Tang, R.; Jiang, C.; Tai, Z. J. Chem. Soc., Faraday Trans. 1997, 3371. (24) Ma, C. L.; Lu, H. B.; Wang, R. Z.; Zhou, L. F.; Chi, F. Z.; Qian, F. J. Crystal Growth 1997, 173, 141. (25) Rashkovich, L. N. KDP-Family Single Crystals; IPO Publishing Ltd.: Bristol, U.K., 1991. (26) Blinc, R.; Dimic, V., Kolar, D.; Lahajnar, G.; Stepisnik, J.; Zumer, S.; Vene N.; Hadzi, D. J. Chem. Phys. 1968, 49, 4996. (27) Itoh, K.; Matsubayashi, T.; Nakamura, E.; Motegi, H. J. Phys. Soc. Jpn. 1975, 39, 843. (28) Mathew, M.; Wong-Ng, W. J. Solid State Chem. 1995, 114, 219. (29) Xue, D.; Zhang, S. J. Phys. Chem. Solids 1996, 57 (9), 1321. (30) Vries, S. A.; Goedtkindt P.; Benntt, S. L.; Huisman, W. J.; Zwanenburg, M. J.; Smilgies, D. M.; De Yoreo, J. J.; van Enckevort, W. J. P.; Bennema, P.; Vlieg, E. Phys. Rev. Lett. 1998, 80, 2229.
10.1021/la9816297 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/12/1999
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Figure 1. Surface pressure-area isotherms of AA on pure water (a) and on KDP solution (b).
Experimental Section Materials. Arachidic acid (AA) (Fluka Chemical Co.) was employed without any further purification. Chloroform solvent was purified by standard procedures. KDP (AR grade) was purchased from Beijing Hongxing Chemical Reagents Co. The preparation of KDP supersaturated solution (1.8 mmol dm-3) is as follows: KDP (300 g) was added to water (1.2 L) at 90 °C. After the solutes had dissolved, the temperature was reduced to room temperature and then the solution was filtered. The filtrate, a supersaturated solution, was transferred to an LB system. Apparatus. The Langmuir trough system was a NIMA. The crystal face of the sample was examined by X-ray diffractometer with Ni-filtered Cu KR radiation at room temperature (D/MAXIIIA, Rigaku). BAM-II (China) is used to image the morphology at the interface. The light source of the BAM was a polarized 5 mW He-Ne laser. The reflected later is focused by a focal length lens into a CCD magnified 160 times and recorded on videotape; the pictures are obtained from the screen by a camera. The AFM image was taken with a Model AP-0190 (Park Scientific Instruments) in noncontact mode. The SHG experiments were performed using a high-power active mode locked Nd:YAG laser with 1.06 µm wavelength and 250 ps pulses (M2000, JK laser). Isotherm Measurement and Crystal Growth of KDP under an Arachidic acid (AA) Monolayer. AA was dissolved in chloroform at a concentration of 1.0 × 10-3 mol dm-3, and the solution was spread on the subphases. Two subphases were used in the experiment: pure water with a conductivity of 18 MΩ cm-1 and a KDP supersaturated solution at room temperature (1.8 mmol dm-3). After vaporization of chloroform (15 min for pure water, 40 min for KDP solution), the surface pressurearea (π-A) isotherms were recorded automatically. For the latter subphase, crystals grown on the monolayer were collected on a hydrophilic glass substrate in Z type at a π value of 30 mN/m at room temperature. The dipping speed was 3 mm/min. The sample was used for XRD, AFM, and NLO measurement, respectively.
Results and Discussion Surface Pressure-Molecular Area Isotherm. Figure 1 shows surface pressure-molecular area (π-A) isotherms of AA monolayer on pure water and on the KDP aqueous solution. Evidently, when a KDP supersaturated solution is employed as the subphase, the features of the π-A isotherm change remarkably. It can be observed that the π-A curve on pure water only has a solid-phase segment, while the π-A curve on a KDP solution shows clear changes of gas, liquid, and solid states, and there is
Figure 2. X-ray diffraction patterns of (a) crystals in the absence of the monolayer and (b) crystals induced by the monolayer, revealing the (100) face.
a coexistence region of liquid condensed-liquid expanded. This indicates that there is very strong interaction between AA molecules and KDP molecules. Because AA is a typical surfactant and its headgroup can interact with K ions forming ion binding, this results in a local concentration of KDP under the monolayer that is much higher than that in solution and makes it possible for LB film to induce orientated growth of KDP crystals. XRD Studies. A KDP supersaturated solution was placed for 2 h in the absence of an AA monolayer, and then the KDP solution surface was deposited on a glass substrate by the LB technique. XRD measurement of the substrate shows that few diffraction peak can be observed, which indicates that, in the absence of a monolayer, nucleation and growth of KDP crystals under the surface is difficult and slow, and the majority of crystals were located at the bottom of the crystallization vessel as a result of sedimentation. The crystal powder XRD of the crystals shows several groups of diffraction peaks corresponding to each crystal face of KDP (Figure 2a), respectively. While in the presence of an AA monolayer the induction time of crystallization was rapidly reduced. After 10 min at a given target pressure, one strong diffraction peak appears. With increasing time, the intensity of the diffraction peak gradually increases. This phenomenon can be explained as following as follows: On one hand, there is the formula GN ) 16G13/(3GB2),31 where GN is the activation energy for nucleation, G1 is the (31) Nancollas, G. H. Biological Mineralization and Demineralization; Springer: Berlin/Heidelberg, 1982.
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energy required to form the new interface as a new phase (crystal) grown from the substable phase (solution), and GB is the energy released in the formation of bonds in the bulk of the aggregate and is a function of the supersaturation ratio S. It is well-known that the surface energy is decreased when the monolayer forms. The monolayer itself can also be considered as resulting from heterogeneous nucleation, so that the value of G1 is lowered. But this nonspecific activation energy leads to no change in the outcome of mineralization.20 On the other hand, the monolayer is an organized organic aggregation and has a definite lattice, with the parameters a, b, and θ, like a crystal face. When it is formed on the surface of the solution, the local environment under it also becomes organized, undergoing changes in physical functions such as the electric field, energy, and mass transmission, etc., and some other chemical functions which are related to the crystallization. Under given conditions there exists complementarity between the monolayer and the crystal, and structure-specific nucleation occurs, which further decreases the activation energy for nucleation. Therefore, the nucleation of KDP becomes much easier.20 Figure 2b shows one single diffraction peak corresponding to crystal face (100) of KDP. This suggests that the (100) face was recognized by the AA monolayer and the crystallization was oriented nearly neatly. KDP belongs to the tetragonal space group I4 h 2d with unit cell dimensions a ) 7.4532 Å, and c ) 6.9742 Å. The unit cell contains four molecules. Atomic coordinates are (0, 0, 0), (1/2, 0, 1/4), (1/2, 1/2, 1/2), and (0, 1/2, 3/4) for P and (0, 0, 1/2), (1/2, 0, 3/4), (1/2, 1/2, 0), and (0, 1/2, 1/4) for K.32 After simulation by computer techniques, the distribution of atoms in crystal face (100) is shown in Figure 3a. It is found that the distances between the closest K-K of 4.16 Å fits the d(100) network spacing of the AA monolayer (4.20 Å),33,34 which implies that the alignment of KDP is along its (100) plane to the plane of the AA monolayer, (Figure 3b). In previous reports, it has been found that there exists the geometrical complementarity between inorganic solids and the AA headgroups.15,19,23 In our study K+ ions of two crystal edges are not completely coincident with AA molecules but are near at the middle site of two AA molecules. So, this explanation needs further supporting evidence. In principle, complementarity between the surface lattice geometries, spatial charge distribution, polarity of hydration layers, defect sites, and stereochemistries of the inorganic and organic surfaces are all possible reasons that inorganic nuclei can be recognized by the organic matrix.20 Unfortunately, the explanation about oriented crystallization of crystals under a monolayer mostly now is restricted in the lattice matching, while the other possible reasons are scarcely inferred for the limited conditions. In a word, certain features of the KDP nuclei can be recognized by AA, which results in oriented crystal growth that is different from the growth of KDP crystals from aqueous solution.30 In our investigation, we employ BAM to carry out in situ observation for further study on the process of KDP crystallization induced by AA film. In the absence of a monolayer, the surface of supersaturated KDP solution has not changed obviously after staying for 1 h; see Figure 4a. However, the image was obviously changed after AA in chloroform was spread on the surface of the subphase (32) Avrami, M. Phys. Rev. 1938, 54, 300. (33) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Krayer, P.; Mohwald, H. Thin Solid Films 1988, 159, 17. (34) Garoff, S.; Deckman, H. W.; Dunsmuir, J. H.; Alvarez, M. S.; Bloch. J. M. J. Phys. (Paris) 1986, 47, 701.
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Figure 3. (a) Two-dimensional representation of (100) face in KDP crystal: block sphere ) K; tetrahedron ) PO4. (b) Schematic two-dimensional representation: b ) K; O ) AA headgroup; . ) K and AA headgroup.
followed by compression. The aggregation of AA molecules can be observed with enlarged pressure. The small light spots in Figure 4a represent the state of aggregation of AA. When the pressure approached 20 mN/m, several microcrystals appeared in Figure 4b as the larger light spots with similar size appeared under the AA monolayer. Consequently the amounts of microcrystals are continuously increased with increasing pressure. When reaching a given target pressure at 30 mN/m, the amount of microcrystals approached the maximum and their distribution was fairly uniform. Then the size of the KDP crystal increased with the increasing time under constant pressure. But after rapid growth for 50 min, the speed of further growth clearly slowed and the size of the KDP crystal only slightly increased, which indicates that the crystallization induced by the monolayer was nearly completed and the initial specific nucleation catalysis provided by LB films no longer played a role. The growth of KDP then reaches its natural and slow stage. Since the monolayer cannot bear a tremendous weight, when further growth occurs, the crystal will separate from the monolayer and drop to the bottom of the container. This is also one of reasons that the size of the crystal at the surface no longer increases. Some information about the growth of the crystal can be obtained by the in situ observation of BAM. However, the employment of BAM is limited because its magnifying power is not large enough to give a satisfactory image about the crystal microstructure. AFM is an effective probe of the overall topological landscape and structural detail of the surface.35 Here AFM was employed to observe the surface image and structural details of oriented crystals (35) Yang, H.; Coombs, N.; Sokolov, I.; Ozin G. A. Nature 1996, 381, 589.
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Figure 5. AFM image of oriented KDP crystals.
Figure 4. BAM images showing growth stages of oriented KDP under AA monolayers: (a) absence of AA; (b) surface pressure (sp/mN m-1) ) 20; (c) sp ) 25; (d) sp ) 30, t ) 0 min; (e) sp ) 30, t ) 50 min; (f) sp ) 30, t ) 120 min.
deposited on glass slides. Figure 5 shows the topological landscape of oriented KDP crystals. We can observe the multilayer structures at the edge of KDP crystals. Under a compressed AA monolayer, K+ ions are gradually enriched to form the first layer by the bonding force with the carboxylic group of an AA molecule and, thus, are extended into a multilayer structure, where the elements are all similar in size and shape. The obtuse angle of the produced crystal, about 130°, is in agreement according with the angle of the (100) crystal face lattice structure of K ions in KDP, proving that the orientation of the produced crystal face is the exact (100) direction. In this way, the crystal nuclei form and then further grow in orientation. The crystal of oriented growth also exhibits a strong nonlinear optical effect. The SHG measurement results are shown in Figure 5. They indicate the spectrum for a sample of 25 layers deposited in Z type shows a better SHG signal than that for the KDP thin film coated on glass (in the absence of a monolayer). This confirms our conclusions that the crystals of oriented growth are more homogeneous and uniformly oriented than the polycrystal grown in the absence of a monolayer.
Figure 6. SHG intensity of crystals (a) in the presence of the monolayer and (b) in the absence of the monolayer.
Conclusions This work has demonstrated the oriented growth of SHG materials under a monolayer. We have shown that an LB film of AA acting as a template can recognize the (100) crystal face of KDP and induce KDP perfect oriented growth. The LB film of AA causes obvious decrease of crystal growth time. The produced crystals distribute homogeneously and are nearly uniform in crystal size. Furthermore, the crystal morphology has an obvious change. It is found that the SHG effect is greater than that in the absence of monolayer. These results will assist
Oriented Crystallization of KH2PO4
in the further understanding about oriented crystallization and in the design of novel synthetic strategies for the tailored formation of advanced materials. This approach opens the door to the generation of NLO materials in the unusual crystal structures with unique properties. Monolayer-directed generation of NLO crystals will be the subject of subsequent communications from our laboratories.
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Acknowledgment. The authors thank the NSFC, Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jinlin University, State Key Laboratory of Crystal Materials, and Key Laboratory Colloid and Interface Chemistry of Shandong University for financial support. LA9816297
