Crystallization of Prussian Blue Analogues at the Air−Water Interface

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Crystallization of Prussian Blue Analogues at the Air-Water Interface Using an Octadecylamine Monolayer as a Template Sipra Choudhury,† Nitin Bagkar,† G. K. Dey,‡ H. Subramanian,§ and J. V. Yakhmi*,| Novel Materials and Structural Chemistry Division, Materials Science Division, and Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India, and Water and Steam Chemistry Laboratory (B.A.R.C), Kalpakkam 603 102, India Received February 20, 2002. In Final Form: May 31, 2002 We demonstrate the use of a Langmuir monolayer as a templating agent for growth of oriented crystals of metal(II) -hexacyanoferrate(III) (where metal can be Ni, Co, or Cu), popularly known as the Prussian Blue analogues, crystals of which are difficult to grow by conventional means. Crystalline films of metal(II) -hexacyanoferrate(III) are deposited at the air-water interface, under an octadecylamine (ODA) monolayer in a Langmuir trough, upon addition of K3Fe(CN)6 solution into the subphase containing aqueous metal chloride solution. Template action to promote crystallization at the air-water interface appears to be provided by the protonated ODA molecules. In the absence of the template (without ODA), no crystals are deposited and instead a precipitation reaction occurs resulting in the formation of a nonoriented cubic M3[Fe(CN)6]2‚nH2O (M ) Ni, Co, or Cu) precipitate. Crystalline Ni-hexacyanoferrate films formed through the template action were characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy, and so forth. We also report initial results on the formation of crystalline Co(II)- and Cu(II)-hexacyanoferrate(III) films by XRD results.

Introduction Controlled growth of inorganic crystals is known to be promoted by an inorganic-organic interface such as a Langmuir monolayer, self-assembled monolayer, and so forth,1 which can be termed as templating agents due to their simple two-dimensional nature. Recently, there have been numerous studies in which Langmuir monolayers of surfactant molecules were used as templates that promote crystallization of organic2 and inorganic materials.3-10 In these studies, functional groups of amphiphilic molecules, * To whom correspondence should be addressed. E-mail: yakhmi@ magnum.barc.ernet.in. † Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre. ‡ Materials Science Division, Bhabha Atomic Research Centre. § Water and Steam Chemistry Laboratory. | Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre. (1) (a) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (b) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E. Langmuir 1994, 10, 619. (c) Agarwal, M.; DeGuire, M. R.; Heuer, A. H. J. Am. Ceram. Soc. 1997, 80, 2967. (2) (a) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353. (b) Landau, E. M.; Grayer Wolf, S.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 1436. (3) Landau, E. M.; Popovitz-Biro, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1986, 134, 323. (4) (a) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 1286. (b) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (c) Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R. J.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 87, 727. (5) (a) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (b) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (6) Hughes, N. P.; Heard, D.; Perry, C. C.; Williams, R. J. P. J. Phys. D 1991, 24, 146. (7) Popovitz-Biro, R.; Wang, J. L.; Majewski, J.; Shavit, E.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1994, 116, 1179. (8) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500.

constituting the Langmuir monolayer, serve as nucleation and/or growth sites for crystallization, while controlling the morphology and orientation of the crystal axis. A profound correlation has been established between the structure of Langmuir monolayers employed and that of the crystals to be formed,2-10 which provides a valuable insight into the mechanism of nucleation and crystal growth. Using a Langmuir monolayer as a template, not only the oriented crystals of inorganic salts such as NaCl,3 CaCO3,4 BaSO4,5 SrSO4,6 ice,7 CdS,8 PbS,9 and silver propionate10 have been successfully grown at the airwater interface, but macroscopic-scale continuous calcium carbonate thin films have also been deposited under the amphiphilic template in the presence of a soluble inhibitor.11 Prussian Blue, having the formula [FeIII4FeII(CN)6]3‚ nH2O (n ) 14-16)], is a blue pigment used in industry for several decades. Prussian Blue (PB) and related metal hexacyanates are of renewed interest because of their potential as molecular magnets. Critical temperatures (Tc) as high as room temperature12a,b and more12c,d have been reported for some of the Prussian Blue analogues (PBAs). Structural information on these PBAs is indispensable for the understanding of their magnetic behavior, which is crucial for the development of new molecule-based (9) (a) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (b) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505. (10) Weissbuch, I.; Majewski, J.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. 1993, 97, 12848. (11) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (12) (a) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (b) Dujardin, E.; Ferlay, S.; Phan, X.; Desplanches, C.; Cartier dit Moulin, C.; Saintavit, P.; Baudelet, F.; Dartyge, E.; Veillet, P.; Verdaguer, M. J. Am. Chem. Soc. 1998, 120, 11347. (c) Holmes, H. S.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (d) Hatlevik, O.; Buschmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. Adv. Mater. 1999, 11, 914.

10.1021/la0201873 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002

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magnetic materials and photomagnetic devices based on them. PBAs are known to have face-centered cubic (fcc) structure13 but are often structurally disordered with a certain degree of exchange within different cationic sites, and therefore it is difficult to grow them as single crystals. Recently, synthesis of Prussian Blue nanoparticles in a microemulsion medium14 and deposition of ferromagnetic Langmuir-Blodgett films where Prussian Blue15a,b and one of its analogues15c were incorporated into the dimethyldioctadecylammonium multilayer have been reported. In the present study, we describe, in detail, our successful efforts to grow thin crystalline films of a PBA, namely, nickel(II)-hexacyanoferrate(III), under an octadecylamine (ODA) monolayer at the air-water interface. These crystalline films were characterized using several techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, and cyclic voltammetry. Experimental Section Materials. ODA, octadecyl alcohol, and stearic acid were purchased from Fluka, and nickel chloride (NiCl2), cobalt chloride (CoCl2), copper chloride (CuCl2), and potassium ferricyanide [K3Fe(CN)6], from S. D. Fine Chemicals. All these chemicals were used as received. Chloroform (CHCl3), which was used as a spreading solvent in this study, was HPLC grade. Water used throughout this study was from a Millipore Milli-Q filter system and had a resistivity of 18 MΩ cm. Instrumentation. Crystallization experiments were carried out on a KSV 5000 Langmuir double-barrier Teflon trough where surface pressure was measured with a platinum Wilhelmy plate microbalance. XRD was recorded with Phillips PW1710 diffractometer using Cu KR radiation at room temperature. TEM was carried out with a JEOL-2000FX microscope operating at 160 KV. XPS spectra were recorded on a VG Scientific Escalab MK2 unit with an Al KR (1486.6 eV) source. A Bomem-MB102 Fourier transform infrared spectrometer, providing a resolution of (4 cm-1, was used for recording IR spectra with 100 scans. Cyclic voltammetric measurements were performed with an EcoChemie Autolab PGStat20 potentiostat/galvanostat, controlled by GPES software, using Ag/AgCl as a reference electrode. Crystallization Experiments. The crystallization process of the metal-hexacyanoferrate was carried out under a surfactant monolayer at the air-water interface in the Langmuir trough. Surfactant molecules such as ODA, octadecyl alcohol, and stearic acid dissolved in CHCl3 (1 mg/mL) were spread onto the subphase containing a 1.5 × 10-4 M solution of MCl2 (where M ) Ni, Co, or Cu) through a Hamilton microsyringe to form the Langmuir monolayer. After allowing sufficient time for solvent evaporation and equilibration of the monolayer, it was compressed on the water surface at a linear rate of 1 (mN/m)/min. A barrier speed of 5 mm/min was used for recording the surface pressure-area isotherm. Using a pipet, a few drops of concentrated K3Fe(CN)6 solution were then added into the region outside the barrier in the trough, taking care so as not to disturb the monolayer. These drops had a calculated amount of K3Fe(CN)6 to make the final concentration of the solution to be ∼1.0 × 10-4 M. This concentrated K3Fe(CN)6 solution mixed gradually with MCl2 solution, allowing Fe[CN]63- anions to react with M2+ cations to yield the metal-hexacyanoferrate film below the ODA monolayer surface. The whole process was gradual for which the compressed monolayer (at 30 mN/m surface pressure) was kept undisturbed (13) (a) Herren, F.; Fischer, P.; Ludi, A.; Halg, W. Inorg. Chem. 1980, 19, 956. (b) Buser, H. J.; Schwarzenbach, D.; Petter, W.; Ludi, A. Inorg. Chem. 1977, 16, 2704. (14) Vaucher, S.; Li, M.; Mann, S. Angew. Chem., Int. Ed. 2000, 39, 1793. (15) (a) Mingotaud, C.; Lafuente, C.; Amiell, J.; Delhaes, P. Langmuir 1999, 15, 289. (b) Ravaine, S.; Lafuente, C.; Mingotaud, C. Langmuir 1998, 14, 6347. (c) Mingotaud, C.; Lafuente, C.; Gomez-Garcia, C.; Ravaine, S.; Delhaes, P. Mol. Cryst. Liq. Cryst. 1999, 335, 349.

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Figure 1. Surface pressure-area (π-A) isotherm of the ODA monolayer: (a) on pure water (dashed line) and on K3Fe(CN)6 solution (solid line); (b) on NiCl2 solution (pH ) 5.8) (dashed line) and 16 h after the addition of K3Fe(CN)6 solution to NiCl2 solution (solid line). for ∼15-16 h. Experiments were also repeated with the monolayers in uncompressed state (at zero surface pressure) in order to compare the influence of the template action on the growth of the crystals. The onset of the growth of a Ni-hexacyanoferrate crystalline film was signaled by the appearance of greenishyellow patches floating on the surface of the subphase, under the ODA monolayer. The film grown under the compressed monolayer appeared to be more uniform to the naked eye vis-a`-vis that generated under the uncompressed monolayer. The Ni-hexacyanoferrate film deposited thus was then collected by transferring it onto a substrate, either by inserting the latter through the surface film and lifting the substrate horizontally or by touching the film laterally from above with a substrate. The former dipping technique enabled a more convenient transfer of the deposited film since the template ODA layer happened to be on top of this film. Films were transferred at a surface pressure of 30 mN/m on various suitable substrates, such as glass, quartz, mica, Si wafer, and calcium fluoride. Ni-hexacyanoferrate films were examined by XRD, TEM, XPS, FTIR and UV-visible spectroscopy, and cyclic voltammetry. Ellipsometric measurements were performed on the Ni-hexacyanoferrate film transferred on the Si wafer. No crystals could grow at the air-water interface under the monolayers formed from octadecyl alcohol or stearic acid, under similar conditions. Bulk Ni3[Fe(CN)6]2‚nH2O powder was also synthesized (without the use of the Langmuir trough) according to the standard chemical procedure in which a 0.1 M aqueous solution of K3Fe(CN)6 was slowly added to a solution of 0.1 M NiCl2 under vigorous stirring. The greenish-brown precipitate was dried after washing with water and analyzed by XRD and FTIR spectroscopy.

Results and Discussion Formation of the Langmuir monolayer was observed in situ by measuring the surface pressure as a function of the surface area. The long-chain amphiphilic molecule, octadecylamine, is known to form relatively stable monolayers at the air-water interface, whereas alkylamines with shorter chain lengths are known to yield expanded monolayers which dissolve slowly into the acidic subphase. Multivalent anions when dissolved in the subphase are reported to stabilize the ODA monolayer.16,17 The plots of surface pressure-area isotherms of ODA on various subphases are shown in Figure 1. The limiting area per ODA molecule on pure water obtained from the surface pressure-area curves shown in Figure 1a is found to be (16) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (17) (a) Gaines, G. L., Jr. Nature 1992, 298, 544. (b) Ganguly, P.; Paranjape, D. V.; Sastry, M. Langmuir 1993, 9, 571. (c) Ganguly, P.; Paranjape, D. V.; Sastry, M. J. Am. Chem. Soc. 1993, 115, 793. (d) Ganguly, P.; Paranjape, D. V.; Rondelez, F. Langmuir 1997, 13, 5433.

Crystallization of Prussian Blue Analogues

Figure 2. XRD patterns of (a) bulk randomly oriented Ni3[Fe(CN)6]2‚nH2O powder (obtained without using a template) and (b) Ni-hexacyanoferrate film transferred on glass substrate from the air-water interface.

∼20 Å2. Occasionally, values as low as 20 Å2 or less18 are obtained for the limiting area per molecule of ODA on pure water (pH ) 5.8). This is generally attributed to partial dissolution of the ODA molecule upon uptake of atmospheric CO2 in pure water. The surface pressurearea plot for ODA spread on a 1.5 × 10-4 M potassium ferricyanide solution showed an expanded monolayer (limiting area per molecule ∼ 50 Å2) (Figure 1a), indicating a strong interaction between the ferricyanide anion present in solution and the -NH3+ cation of the ODA molecule. The pressure-area isotherm of ODA using a 1.5 × 10-4 M NiCl2 solution as the subphase at pH ) 5.8 indicates a limiting area of 18 Å2 (Figure 1b), a minor drop from the area on pure water (Figure 1a). At this acidic pH, the amine (-NH2) groups are mostly in the protonated (-NH3+) form and, therefore, one can ignore the formation of a Ni-amine (NirNH2) coordination complex. Sixteen hours after the addition of a few drops of concentrated K3Fe(CN)6 solution to the NiCl2 solution, greenish-yellow patches were observed and the limiting area per molecule showed an increase to ∼25 Å2 (Figure 1b). This increase in area is not substantial, but it does confirm the association of the metal complex Ni-hexacyanoferrate with the monolayer. No crystal formation was observed under the negatively charged stearic acid monolayer or neutral octadecyl alcohol monolayers. PBAs, having a face-centered cubic (fcc) structure,13 are reported to have Fe(CN)6 vacancies, whereby six water molecules can fill the empty nitrogen (from cyano group) sites. For Prussian Blue, the average composition of the Fe(III) coordination unit is FeN4.5O1.5 where N and O represent the terminal nitrogen of CN and the oxygen of the H2O molecule, respectively. The powder XRD pattern of the bulk Ni3[Fe(CN)6]2‚nH2O powder shown in Figure 2a is consistent with the reported13 fcc structure having a unit cell lattice parameter a ) 10.2 Å. The XRD pattern for a Ni-hexacyanoferrate film deposited under the ODA monolayer, transferred on glass, showed sharp, intense peaks only at 2θ values 17.34°, 35.1°, and 53.8°, corresponding to the {200}, {400}, and {600} reflections (Figure 2b), implying a preferential orientation of Ni-hexacyanoferrate fcc crystals under the ODA monolayer. The crystallization of other PBAs such as Co-hexacyanoferrate and Cu-hexacyanoferrate were also carried out successfully at the air-water interface under the ODA monolayer. (18) (a) Didymus, J. M.; Mann, S.; Benton, W. J.; Collins, I. R. Langmuir 1995, 11, 3130. (b) Mayya, K. S.; Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 3377.

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Figure 3. XRD patterns of (a) Co-hexacyanoferrate and (b) Cu-hexacyanoferrate crystalline films transferred from the air-water interface onto glass substrate.

Figure 4. Transmission electron micrograph of Ni-hexacyanoferrate film (∼1300 Å thick) grown under an ODA monolayer for 3 days and transferred on a carbon-coated copper grid.

The XRD patterns (Figure 3) of these crystalline films collected from the air-water interface also showed intense {200}, {400}, and {600} reflections of fcc structure having a lattice parameter of ∼10.2 Å. The XRD pattern of the Ni-hexacyanoferrate film deposited under the compressed ODA monolayer (at 30 mN/m surface pressure) showed more intense peaks as compared to that grown under the uncompressed monolayer, implying a larger number of oriented crystallites in the former case. The morphology and the crystalline nature of the Nihexacyanoferrate film were further analyzed using TEM. The films grown over a period of 3 days under the ODA monolayer at the air-water interface were transferred on a holey carbon-coated copper grid by touching it horizontally on the water surface. The TEM photograph, shown in Figure 4, indicates the formation of a thin sheet of Ni-hexacyanoferrate crystallites, which appear to come from the two-dimensional growth habit of the material. The thickness of the film as obtained from ellipsometric measurements was approximately 1300 Å. The electron diffraction (ED) pattern (Figure 5a) for the film transferred on the TEM grid indicates the formation of a single crystalline Ni-hexacyanoferrate film at the air-water interface. It corresponds to the [001] zone19 for a fcc (19) Edington, J. W. Practical Electron Microscopy in Materials Science; Philips Technical Library; MacMillan: London, 1976; Vol 2.

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Figure 5. ED patterns from the Ni-hexacyanoferrate film (shown in Figure 4). The pattern corresponds to (a) the [001] crystallographic zone of the fcc lattice having a lattice parameter of ∼10.2 Å and (b) the overlapping of (a) by rotating in 0°, 60°, and 120° directions (see text).

structure with a lattice constant of a ) 10.2 Å, typical of PBAs, indicating the parallel alignment of the {100} crystal face of Ni-hexacyanoferrate to the ODA monolayer surface. Figure 5b is the representative electron diffraction pattern observed on a few locations of the crystalline film. It exhibits a well-defined, 12-fold symmetric pattern with sharp spots which represents the planes {200}, {220}, {400}, {420}, and so forth of the Prussian Blue type cubic structure. An exact 12-fold diffraction pattern (Figure 5b) is expected to originate when the three 4-fold diffraction patterns (Figure 5a) are overlapped by rotating 0°, 60°, and 120°. This implies that few locations of the as-grown film are not uniform, at least at the spots where a characteristic pattern as given in Figure 5b is obtained. Instead, in these locations small crystallites of Nihexacyanoferrate are lying in the 0°, 60°, and 120° directions and {100} planes of all these crystallites are parallel to the ODA monolayer surface. The diffraction pattern is changed slowly after prolonged exposure of the film to the electron beam inasmuch as the spots are gradually transformed into continuous rings indicating a transformation of the single crystalline film into polycrystalline form. This transformation occurs possibly due to the damage of the ODA monolayer under the electron beam, which in turn destroys the crystalline nature of the Ni-hexacyanoferrate films. Further studies are required to understand this phenomenon clearly. The square particles (∼500 nm) depicted in the TEM photograph (Figure 4) were quite stable under the electron beam and mostly opaque to the electron beam as they were quite thick. Electron diffraction patterns could be obtained from a very few particles, and these showed a diffuse halo similar to that obtained from an amorphous material. The composition of these particles is, however, not known. The unit cell of Ni-hexacyanoferrate is schematically shown in Figure 6a. Our main observation is that the ODA monolayer, regardless of the surface pressure, is able to direct the growth of the Ni-hexacyanoferrate film along its {100} planes (Figure 6b).

Figure 6. (a) Unit cell of Ni-hexacyanoferrate having a cell parameter of ∼10.2 Å. (b) Schematic diagram of the {100} plane of the fcc lattice. The open circle corresponds to Ni, and the dark filled circle represents Fe. The cyanide group (-CN) is bridged (small double circle, shaded circle as N) between Ni and Fe.

Figure 7. XPS spectra of a Ni-hexacyanoferrate crystalline film deposited under an ODA monolayer and transferred on a Si wafer.

XPS studies on the crystalline Ni-hexacyanoferrate film transferred on the Si wafer provide evidence for the presence of Ni, Fe, C, N, and O (Figure 7). The absence of any signal due to K implies that the potassium ion did

Crystallization of Prussian Blue Analogues Table 1. Binding Energies of the Elements Obtained from XPS Spectra of the Ni-Hexacyanoferrate Crystalline Film on Si elements

BE (eV) observeda

BE (eV) from literatureb

Ni2p Fe2p C1s N1s O1s

856.9 710.2 285c 398.2 533

857.5 709.6 286.4 398.8

a The BE calibrated with the C1s peak of the hydrocarbon chain at 285 eV. b The BE calibrated with the graphitic C1s peak taken at 284.6 eV. c C1s signal mostly due to the presence of outermost hydrocarbon layer.

Figure 8. Cyclic voltammogram of a Ni-hexacyanoferrate film (transferred on a gold-coated glass electrode) in 0.1 M KCl solution (scan rate ) 50 mV/s, reference electrode ) Ag/AgCl).

not get itself positioned in the interstitial position. The strong signals from C1s and N1s are from the cyanide group and from the ODA (C18H35NH2) molecule. The strong O1s signal is mostly due to oxygen from the coordinated water molecules in Ni-hexacyanoferrate crystals. The binding energies (BE) of Ni2p, Fe2p, C1s, and N1s are compared in Table 1 with that of a Ni-hexacyanoferrate film20 electrochemically deposited on a graphitic carbon electrode. The observed values of the binding energies of the elements present in Ni-hexacyanoferrate crystals are in overall agreement with the corresponding literature values20 and thus confirm the formation of this compound. FTIR spectra from the Ni-hexacyanoferrate crystalline films transferred on Si or CaF2 substrates show two intense peaks in the region of 2200-2000 cm-1, which are similar to those obtained for bulk Ni3[Fe(CN)6]2‚nH2O. The peaks at 2167 and 2098 cm-1 correspond to the bridging cyanide (Fe-CN-Ni) and the terminal cyanide group,21 respectively. The OH stretching and bending modes of H2O molecules have also been observed around 3400 and 1600 cm-1, respectively. The optical absorption spectra of the Ni-hexacyanoferrate film transferred on quartz had a broad absorption peak at ∼390 nm indicating the chargetransfer band of Fe(CN)63-, whereas the absorption band for pure K3Fe(CN)6 appeared at ∼420 nm. Cyclic voltammetry has been performed on a Nihexacyanoferrate film transferred on a gold-coated glass electrode in 0.1 M KCl solution. A platinum wire was used as a counter electrode, and the reference electrode was a saturated Ag/AgCl electrode with a salt bridge containing 3 M KCl aqueous solution. The voltammogram shown in Figure 8 was recorded at a scan rate of 50 mV/s, in the lower potential range of 0 to +0.6 V because the amine (20) Cataldi, T. R. I.; Guascito, R.; Salvi, A. M. J. Electroanal. Chem. 1996, 417, 83. (21) Lafuente, C.; Mingotaud, C.; Delhaes, P. Chem. Phys. Lett. 1999, 302, 523.

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group from the ODA monolayer is oxidizable at a higher potential. Since the Ni(II) ion in the film cannot be reduced in this potential range, the redox peak in Figure 8 corresponds to the hexacyanoferrate couple (III/II),22 the midpoint potential (E1/2) for which is estimated to be 0.43 V versus the Ag/AgCl electrode. The peak current increases linearly with the potential scan rate up to 200 mV/s indicating an electrode process involving a surfaceattached redox species. Metal-hexacyanoferrate films can be used as biosensors in view of their novel optical and electrochemical properties.23 Studies on nucleation and growth of crystals under organized monolayers are known to elucidate different factors responsible for controlled crystallization, such as the crystallographic matching of a close-packed monolayer with the face of the nucleating crystal, stereochemistry,24 and electrostatic interaction,4a,25 which can influence the preferential nucleation of crystals. A compressed monolayer is an organized well-defined structure that is likely to provide geometrical and stereochemical fit with a specific face of the growing crystal. Although geometric and stereochemical factors influence the structural compatibility of the 2D domains in the monolayer with the 2D lattice of specific crystal faces in several cases, electrostatic interaction between the ionized monolayer and the oppositely charged ion present in the subphase can also promote the recognition process for oriented crystallization. In several cases, it has been found that the strong binding of the ions to the charged monolayer can override geometric and stereochemical matching. As shown in Figure 1a, an expanded limiting area for the ODA monolayer in the presence of the potassium ferricyanide ion in solution, as compared to that on pure water, clearly indicates strong electrostatic interaction between the [Fe(CN)6]3- ion and the protonated amine headgroup of ODA. The possibility of Ni2+ ion binding to the amine headgroup of the ODA monolayer as an intermediate step for the crystallization process can be ruled out, as most of the amine groups are protonated at pH ) 5.8. The fact that no crystals have been found to grow under the stearic acid monolayer in our experiments supports the conjecture that the binding of the Ni2+ ion to the amine group is not a prerequisite for stabilization of the metastable phase for oriented crystallization. No crystal formation under the neutral octadecyl alcohol monolayer was observed, which further emphasizes the role of electrostatic interaction in the present case. One can delineate the structural compatibility factor in the present crystallization process based on the following considerations. The value of 20 Å2 for the area per molecule of the ODA, in the compressed state, obtained from the surface pressure-area isotherm (Figure 1a) yields an interheadgroup spacing of ∼4.8 Å, assuming a hexagonally close-packed monolayer. This spacing is slightly smaller than the reported M-Fe distance of ∼5.1 Å across the cyano group in the case of metal-hexacyanoferrate crystals. It has been reported26 that at low surface pressure, close-packed two-dimensional domains of surfactant molecules coexisting with the more expanded molecules can also induce oriented nucleation. (22) Pournaghi-Azar, M. H.; Razmi-Nerbin, H. J. Electroanal. Chem. 1998, 456, 83. (23) Koncki, R.; Lenarczuk, T.; Radomska, A.; Glab, S. Analyst 2001, 126, 1080. (24) (a) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. J. Phys. D: Appl. Phys. 1991, 24, 154. (b) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 735. (c) Geva, M.; Izhaky, D.; Mickus, D. E.; Rychnovsky, S. D.; Addadi, L. Chembiochem 2001, 2, 265. (d) Izhaky, D.; Addadi, L. Chem.sEur. J. 2000, 6, 869. (25) (a) Archibald, D. D.; Qadri, S. B.; Gaber, B. P. Langmuir 1996, 12, 538. (b) Ma, C. L.; Lu, H. B.; Wang, R. Z.; Zhou, L. F.; Cui, F. Z.; Qian, F. J. Cryst. Growth 1997, 173, 141.

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We, too, observed the crystallization under the uncompressed monolayer. We measured the limiting area per molecule of the ODA monolayer, after the addition of K3Fe(CN)6 solution to the NiCl2 subphase, to be 25 Å2 which leads to an interheadgroup spacing of ∼5.4 Å, which turns out to be slightly larger than the M-Fe distance, 5.1 Å. As the interheadgroup spacing in the monolayer structure, in both the compressed and uncompressed states, differs from the M-Fe distance of metal-hexacyanoferrate, it can be inferred that crystal matching may not be the main driving force for the crystallization process. In fact, the lack of crystal matching was also reported27 in the case of the oriented nucleation of NaCl by R-amino acid monolayers. From these considerations, we conclude that it is the electrostatic interaction between the ferricyanide ion, [Fe(CN)6]3-, present in the solution and the -NH3+ cation (the amine group is protonated at pH ) 5.8) of the ODA monolayer that is mainly responsible for the growth (26) (a) Weissbuch, I.; Berkovic, G.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1990, 112, 5874. (b) Weissbuch, I.; Berkovic, G.; Yam, R.; Als-Nielson, J.; Kjaer, K.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. 1995, 99, 6036. (c) Buijnsters, P. J. J. A.; Donners, J. J. J. M.; Hill, S. J.; Heywood, B. R.; Nolte, R. J. M.; Zwanenburg, B.; Sommerdijk, N. A. J. M. Langmuir 2001, 17, 3623. (27) Jacquemain, D.; Grayer Wolf, S.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. J. Am. Chem. Soc. 1990, 112, 7724.

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of the Ni-hexacyanoferrate crystalline film at the airwater interface. Conclusions It is observed that compressed and uncompressed ODA monolayers promote {100} oriented crystallization of Nihexacyanoferrate at the air-water interface. ODA molecules, which are two-dimensionally organized at the airwater interface, act as a template for the oriented crystallization where protonated amine headgroups of the ODA monolayer serve as the sites for the growth of oriented Ni-hexacyanoferrate crystalline film. The oriented crystallization appears to be due to the electrostatic interaction of the ferricyanide anion with the positively charged ODA monolayer. It has been observed that the {100} crystal plane of metal-hexacyanoferrate is aligned parallel to the ODA surface. The ODA monolayer can also be used as a template for the growth of other crystals of the Prussian Blue family which are generally difficult to grow in the bulk. Studies in this direction are in progress. Acknowledgment. We thank Dr. S. V. Narasimhan, Water and Steam Chemistry Laboratory, Kalpakkam, for use of the XPS instrument. We thank Dr. S. Banerjee and Dr. J. P. Mittal for encouragement. LA0201873