Langmuir 1995,II,1412-1414
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Atomic Force Microscopy Studies of Iron Oxychloride M. E. Pieczko and J. J. Breen* Department of Chemistry, Indiana University Purdue University-Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202-3274 Received October 13, 1994. In Final Form: January 19,1995
Iron oxychloride1 is a member of a class of metal oxyhalides (MOX)where M = Al,Cr, Fe, In, Ti, or V and X = Br or C1. As illustrated in Figure 1,these materials crystallize with a layered structure in which adjacent layers of halide ions are primarily bonded by van der Waals forces. A number of these materials have been shown to participate in intercalation reactions similarlyto both the layered transition metal dichalcogenides and graphite. In particular and owing to the often interesting structural and electroniceffectsoccurringupon intercalation, FeOCl has been the subject of numerous studies of intercalation chemistry involving a variety of species including alkali ions,2crown ether compound~,~ Fe and Co metallocenes and substituted metall~cenes,~ substituted amines: pyridine and substituted triethylphosphine, trimethyl phosphite, and organophosphate^.^*^ More recently a new class of layered,conductingpolymer inorganic hybrid materials has been reported in which FeOCl is the host material. Conducting polymers of aniline, pyrroles, and thiophenes are formed in situ following intercalationinto FeOCl and via a redox reaction involvingthe host material.1°-14 It is postulated that these new hybrid materials in which the conducting polymers are confined in a well-definedenvironmentmight display new and interesting chemical, physical, and charge transport properties or improve existing properties associated with the conductingpolymers themselves. In all reported cases of intercalation chemistry, with Li ions as an exception,the structure of FeOCl lattice is significantly altered upon the intercalation of the guest molecules. In our study we used the atomic force microscope (AFM)15to examinethe surface structure of single crystals of FeOC1. Imaging experiments were performed in air (1) Goldstaub, S. Bull. SOC.Fr. Mineral. 1935,58, 49-69. (2) Armand, M.; Coic, L.; Palvadeau, P.; Rouxel, J. J. Power Sources 1978,3, 137-144. ( 3 )Herber, R. H.; Cassell,R. A. J. Chem Phys. 1981,75,4669-4678. (4) Halbert, T. R.; Johnston, D. C.;McCandlish, L. E.; Thompson,A. H.; Scanlon, J. C.; Dumesic, J. A. Physica 1980,99B, 128-132. (5) Hagenmuller,V. P.; Portier, J.; Barbe, B.; Bouclier, P. 2.Anorg. Allg. Chem. 1967,355,209-218. (6) Kanamaru,F.; Yamanaka, S.;Koizumi, M.; Nagi, S. Chem. Lett. 1974,373-376. (7) Kanamaru, F.; Shimada, M.; Koizumi, M.; Takano, M.; Kakada, T. J . Solid State Chem. 1973, 7 , 297-299. ( 8 ) Herber, R. H.; Maeda, Y. Physica l981,105B, 243-248. (9) Palvadeau, P.; Rouxel, J.; Queignec, M.; Bujoli, B. In Supramo1ecularArchitecture: Synthetic Control in ThinFilms and Solids;Bein, T., Ed.; American Chemical Society: Washington, DC, 1992; Vol. 499; pp 114-127. (10) Kanatzidis, M. G.; Tonge, L. M.; Marks, T. J.; Marcy, H. 0.; Kannewurf, C. R. J. Am. Chem. Soc. 1987,109,3797-3799. (11) Kanatzidis, M. G.; Marcy, H. 0.;McCarthy, W. J.; Kannewurf, C. R.; Marks, T. J. Solid State Ionics 1989,32133, 594-608. (12) Kanatzidis, M. G.; Wu, C.-G.; Marcy, H. 0.;DeGroot, D. C.; Kannewurf,C. R.; Kositakas, A.; Papaefthymiou, V. Adu. Mater. 1990, 2,364-366. (13) Kanatzidis, M. G.; Wu, C.-G.; Marcy, H. 0.;DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R.; Benz, M.; Legroff, E. In Supramolecular Architecture: Synthetic Contol in Thin Films and Solids; Bein, T., Ed.; American Chemical Society: Washington, DC, 1992;Vol. 499; pp 194-219. (14) Kanatzidis, M. G.; Bissessur, R.; DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R. Chem. Mater. 1993,5, 595-596.
Figure 1. Illustration of the structure of FeOCl.
using a Nanoscope I11 AFM in the contact mode of operation and equipped with a 100pm long (0.58nN/m) gold coated Si3N4 cantilevers (DigitalInstruments, Santa Barbara, CA). Images were acquired by maintaining a constant cantilever deflection (Height Mode) using a 12pm (Model D) scanner. Real-time and postacquisition filtering (corrections for image bowing and low pass filtering)was sparingly used. The vertical lines appearing in Figures 2 and 4 are image artifacts and are not representative of the FeOCl surface structure. Measurements reported are the averages of approximately 100 measured distances and heights taken from a number of different images. FeOCl single crystals were prepared by heating equimolar amounts of iron(111)chloride (FeC13)and iron(111)oxide (FezOs),both from Strem Chemicals, Inc. (Newburyport, MA), a t 350 "C in an evacuated and sealed Pyrex tube ( I = 10 cm, d = 2.6 cm) for 2 weeks. Approximately 0.5 g of material was used as received for each batch of crystals. Following gradual coolingto room temperature, the tubes were opened and the crystals washed in acetone and hexane and stored when necessary in mineral oil. From each batch numerous crystals measuring as large as 5 mm x 5 mm x 1 mm on a side were produced. The FeOCl crystals have a metallic purplehlack luster and cleave easily using a piece of adhesive tape. X-ray powder diffractionof a sample of ground crystals agreed well with the pattern on ASTM Data Card 24-1005. The FeOCl host material is stable for imaging but does show signs of corrosion following extended exposure to the atmosphere. Imaging studies of cleaved FeOCl crystals reveal atomically smooth surfaces with planes as large as 10 pm. Figure 2 is a 10 pm x 10 pm image representative of the observed surface structure following cleaving. (15) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986,56, 930-933.
0743-7463/95/24 11-1412$09.00/0 0 1995 American Chemical Societv
Notes
Figure 2. A 10 pm
x 10 pm AFM image of the large scale surface structure of a freshly cleaved sample of FeOCl. Clearly evident in the image is a step in the surface which corresponds to the difference of a single layer (0.7 nm). Note: The series of vertical lines present in the image is only an artifact and not representative of the structure of FeOCl.
Figure 3. A 5 nm
x 5 nm atomic resolution AFM image of the surface structure of a freshly cleaved sample of FeOCl. The surface periodicities obtained from this and other similar images are 0.39 f 0.01 nm and 0.33 f 0.01 nm.
Clearly evident in the image is the flatness of the cleaved crystal surface and the layered nature of the material. Height difference measurements between differentsurface planes in this and other images were distributed about 0.70 f 0.02 nm and 1.31 f 0.02 nm. The layer spacing in FeOCl reported from X-ray diffraction measurements of single crystals of FeOCl is 0.7917 nm (lattice constant b).16 Consequently our measurements correspond to single and double layer distances with the discrepancies in our measurements attributed to the calibrationof the scanner. Pictured in Figure 3 is 5 nm x 5 nm atomic resolution image of the cleaved FeOCl crystal surface. The crystals cleave easily along the van der Waals gap formed by the bridging chloride ions and yield atomic resolution images with relative ease, even as compared to mica. The surface periodicities as measured from this and other high resolution images are 0.39 f 0.01 nm and 0.33 f0.01 nm. These measurements compare well to the a and c lattice constants reported in the X-ray diffraction study where a = 0.3780 nm andc = 0.3302 nm.16 The crystal structure of FeOCl consistsof stacks of a double layer sheet of oxygen octahedra linked together with shared edges with the outermost atoms on each of the layers the chloride ions. (16)Lind, M. D. Acta Crystallogr. 1970, B26, 1058-1062.
Langmuir, Vol. 11, No. 4, 1995 1413
Thus the atoms revealed in the image are the chlorides in the ac plane and which form the boundaries of the constrained van der Waals gap in FeOC1. Atomic scale images obtained with a scanning tunneling microscope reveal the same structure and periodicitiesof the chloride layers; however, extensive damage to the crystal surface is induced to the sample during the imaging experiment by the tunneling microscope tip as a result of the low conductivity of FeOC1. On a number of occasions well-structured (rectangular) surface defects on the order of 100 nm were observed in micrometer scale images of FeOCl crystals following cleaving. The majority of these defect sites are only a single lattice layer deep, but substantially deeper holes (up to 7 nm) were observed. Attempts to create similar defects in the surface layer by pressing the AFM tip into the surface were unsuccessfhl. We suspect that the inability to damage the surface with the probe tip and the ease with which atomic resolution images are obtained are a manifestation of the hardness of the FeOCl material. Repeated scanning with the force microscope does cause these defects to increase in size at a rate proportional to the number of scans. Figure 4 is a composite of six topographic images taken of the same region every 90 min. Initially the surface defects in this experiment were -100 nm wide and at the end of the 11 h the holes have doubled in size to.-200 nm wide. The images were obtained at a slow scan rate (six passes per hour) and consequently the defects appear rounded in the images due to thermal drift. The shape of the defects appear to be maintained as the defects enlarge. The applied force a t the end of this experiment as measured from a force curve was -2 x N. The depth of these defects, -0.7 nm corresponds to the single layer thickness (lattice constant b ) of FeOC1. Material is removed at a rate of -2 n d s c a n pass and only from the surface layer directly in contact with the cantilever probe tip. As evidenced by the images in Figure 4, material is removed preferentially from the less supported sides of the defectsand the defects appear to merge as they increase in size due to continued scanning. This abrasive wear induced by the cantilever probe tip which damages or modifies the surface of FeOCl on the nanometer scale is similar to that reported in other imaging experiments with layered materials such as metal chalcogenides,17 surfaces of Moo3 deposited on MoS2,18 and a collection of scanning tunneling microscope experiments which have recently been reviewed.lg Additional, experiments were conducted in an attempt to examine the surface structure of the hybrid materials formed with FeOCl following the intercalation of aniline and trimethyl phosphite. For these experiments several of the larger crystals were chosen from a freshly prepared batch and immersed for various periods of time in an appropriate dilute solution of either aniline or trimethyl phosphite. . More specifically, in the case of analine intercalation crystals were immersed for as long as 6 weeks in a 5-10% uncovered solution of aniline in acetonitrile at room temperature.14 For the case of trimethyl phosphite intercalation, crystals were immersed in a dry 0.1-0.2 M solution of trimethyl phosphite in hexane at 50 "C for times as long as 7 days under an argon purge.8 All organic chemicals were obtained from Aldrich (Milwaukee, WI) and were used as received. When removed from the dilute reactionsolutions,all crystals immersed appeared a duller, (17)Delawski, E.; Parkinson, B. A. J. Am. Chem. Soc. 1992,114, 1661-1667. (18) Kim, Y.; Lieber, C. M. Science 1992,257,375-377. (19)Parkinson,B. In SupmmolecularArchitectul.e:Synthetic Control in Thin Films and Solids;Bein, T., Ed.; American Chemical Society: Washington, DC, 1992; Vol. 499; pp 76-86.
1414 Langmuir, Vol. 11, No. 4, 1995
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x 5 pm, AFM images taken of the same surface region over a period of 11 h of scanning with the force microscope. The images pictured were obtained in the sequence, feocl.1 feocl.6, and taken at 90-min intervals. The applied force measured at the end of the experiment was 2 x lo-* N. Note: The image artifact visible in Figure 2 is also visible in each of the images in this series,.
Figure 4. A composite figure containing six, 5 pm
blacker color and were both fragile and difficult to cleave cleanly. Millimeter sized crystals which remained intact after intercalation were used in the imaging experiments which were conducted on representativeareas of a number of freshly cleaved surfaces. Atomic or molecular resolution imaging experiments following the reaction of aniline and trimethyl phosphite with FeOCl crystals were unsuccessful. Micrometer sized images of these fragile reacted crystals revealed a significant change in the flat surface morphology of the FeOCl host material to a rougher surface with surface features as tall as 50 nm. This result was true for both FeOCl intercalation systems investigated. The extent of intercalation was readily discernible from gold-colored regions visible upon cleavage of the crystals. Attempts to achieve atomic or molecular resolution images of the intercalated molecules and polymers were unsuccessful. On occasion and for both intercalates, clear atomic resolution images of the van der Waals layer of chloride ions were observed. These images are presumably obtained from unreacted regions in the crystal. We attribute our failure to observe with molecular resolution the intercalated species to the loss of crystal integrity which results from the materials response to accommodate the intercalated species. These structural changes cause an increase in the elastic properties of the reacted material which facilitates damage to the material by the scanning AFM probe. The extent by which the crystal lattice is altered is best exhibited by the differences in the b lattice parameter which is 0.7917 for FeOCl and
followingthe intercalation of trimethyl phospite increases by 0.647 nm8and for the intercalation of aniline increases by 0.654 nm.I2 The ability to obtain atomic resolution images of the layers of chloride ions while examining reacted crystals indicates that the failure to observe the intercalated species is not due to catastrophic damage to cantilever probe tip. In summary, AFM measurements of the FeOCl compare very well with literature values of the a,b, and c lattice constants. The material which can be cleaved in air yields micrometer sized atomically flat regions which can be imaged easily with atomic resolution. The surface is susceptible to damage or can be modified by the scanning probe used in the contact mode in a manner similar to other layered materials provided when defectsare present in the crystal surface structure. The intercalation of aniline and trimethyl phosphite into the FeOCl crystals is evident upon visual inspectionof the crystals consistent with previous reports. Micrometer-scale AFM images show a significant change in the crystal surface structure revealing rough features on the order of 50 nm.
Acknowledgment. We thank G. D. Rosenberg in the Department of Geology at IUPUI for the X-ray powder diffraction measurements. This work was partially supported by the Purdue Research Foundation, the IUPUI . Faculty Development Office, and the Loren T. Jones Undergraduate Scholarship Fund at IUPUI. LA9407976
