0 Copyright 1994 American Chemical Society
The ACS Joumal of
Surfaces and Colloids NOVEMBER 1994 VOLUME 10, NUMBER 11
Letters Self-Assembled Monolayer Coatings on Amorphous Iron 0. Rozenfeld, Y. Koltypin, H. Bamnolker, S. Margel,* and A. Gedanken* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received April 28, 1994. In Final Form: September 12, 1994@ Amorphous iron nanoparticleswere self-assembledcoated with octadecyltrichlorosilane and with sodium dodecyl sulfate. Characterization of the coatings was accomplished by Fourier transform infrared spectroscopy, elemental analysis, and floatability measurements. Under the experimental conditions used, the coatings prepared with sodium dodecyl sulfate were superior t o coatings prepared with octadecyltrichlorosilane.
The synthesis and characterization of self-assembled (SA) monolayer coatings of various organic surfactants on flat metals or metal oxides have been reported in a significant number of publications. These include aIkylsilane surfactants on hydroxylated surfaces, such as silica and aluminum oxide, alkanethiolates on gold, silver, and copper, alcohols and amines on platinum, and carboxylic acids on aluminum oxide and silver oxide.1-6 On the other hand, very few publications have described the synthesis and characterization of SA coatings on metals or metal oxide powders, particularly ofnanometer size In this communication, preliminary studies describing the SA coating of amorphous iron nanoparticles with octadecyltrichlorosilane(OTS)and with sodium dodecyl sulfate (SDS)are reported. We are not aware of previous
* Authors to whom correspondence should be addressed. Abstract published inAdvance ACSAbstracts, October 15,1994. (1)(a)Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1986,132,153. (b) Nezer, L.;Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99,235. (2) Wasserman, S.R.; Tao,Y. T.; Whitesides, G. M. Langmuir 1989, 5,1074. (3)Balachander, N.; Sukenik, C. N. Langmuir 1990,6 (111,1621. Ulman, A.; Shnidman, Y.; Eilers, J. E. J.Am. Chem. (4)Sellers, H,; SOC.1993,115, 9389. (5) Ulman, A. In An Introduction to Ultrathin Organic Films From Langmuir-Blo&ett to Self-Assembly; Academic Press: Boston, MA, 1991. (6)Margel, S.;Dolitzky, Y.; Sivan, 0. Colloids Surf. 1992,62, 215. (7)Brandriss, S.;Margel, S. Langmuir 1993,9, 1232. (8)Badelev. R.D.: Ford, W. T.: McEnroe, F. J.;Assink, R. A. Langmuir 1990,6,792: @
0743-7463/94/2410-3919$04.50/0
published studies describing the self-assembled coating of iron, particularly iron nanoparticles. In recent publications, Suslick et al. described the preparation and characterization of amorphous iron through the sonolysis of iron penta~arbonyl.~ Scanning electron micrograph pictures of the formed amorphous iron demonstrated that the bulk material is a composite of iron nanoparticles of approximately 10 nm diameter. The surface area of these amorphous iron as determined by BET gas adsorption was found to be 120 m2 g-l, approximately 150 times greater than the surface area of the commercially available ultrafine, crystalline iron powder (5 pm average diameter, Aldrich Chemicals). Elemental analysis of the amorphous iron showed that this material is composed of ca. 96%-97% (by weight) iron and trace amounts of carbon (ca. 2%) and oxygen (ca. 1%-2%). The potential use of the amorphous iron as a highly active catalyst has been demonstrated previo~sly.~ Amorphous iron nanoparticles were prepared according to the procedure described by Suslick et aL9 Briefly, pure iron pentacarbonyl(20 mL) in a glass flask was irradiated for 3 h at 0 "C under 1.5 atm of argon with a high intensity ultrasonic probe (Sonics and Materials, Model VC-600, Ti horn, 20 kHz, 100 W cm-2). The closed flask suspension containing the amorphous iron (152 mg) in iron penta(9)(a) Suslick, K.S.; Choe, S. B.; Cichowlas, A.; Grinstaff, M. W. Nature 1991,353,414.(b) Grinstaff, M.W.; Cichowlas, A.; Choe, S. B.; Suslick, K. S. Ultrasonics 1992,30 (3),168.
0 1994 American Chemical Society
3920 Langmuir, Vol. 10, No. 11, 1994
Letters Table 1. Characterization by Elemental Analysis of Coatings Prepared from SDS and OTS onto Amorphous
9 1
0'25 0.20
coatings SDS
/J 3070
2990
%C
%S
calculated (%) surface coverageb
12.1 6.7
2.2
77c
OTS 31 The reported values were obtained after substraction of the % C and % S of the noncoated amorphous iron. For the calculations, the density of amorphous iron was taken as 7.86g/cm, and the area per each bound surfactant molecule was estimated to be 20 k ,as was previously indicated.2 Average value calculated from the
- \ E 3
-A
0.00
Iron elemental analysisa
measured % C and % S. 2910
2830
2750
WAVENUMBER (cm.')
Figure 1. FTIR spectra of noncoated amorphous iron (A) and of OTS (B) and SDS (C)coatings on amorphous iron.
carbonyl was then transferred to an inert atmosphere box (home-made, 72.5= 35 35
.
>72.5= 23 27
12 8
a In water (surface tension 72.5 dydcm), all of the noncoated amorphous iron nanoparticles sink.
demonstrated that the CST of coatings on particles closely resembles the total sinking surface tensions rather than the total floating surface tensions. Table 3 therefore estimates that the CST of the SDS coating on the amorphous iron is 23 dyn/cm and that of OTS is 27 dyn/ cm. The relative broad surface tension transition regions and the CST values of these coatings on amorphous iron compared to that reported for close-packed monolayer coatings of the same, or similar, surfactants, i.e. OTS, on flat solid surface^^^^^^ illustrate the decreasing order and increasing liquidity of the SDS and OTS coatings on amorphous iron. The nature of OTS and SDS coatings on amorphous iron is not yet understood. Preliminary XANES (X-ray absorption near-edge structure) studied3 have demonstrated that the valency of the iron atoms involved in the surfactant coatings, for both OTS and SDS, is +3. These data, together with the elemental analysis results that showed the presence of ca. 2%oxygen, may indicate the presence of some iron ions and/or iron oxide layer on the iron nanoparticle surfaces. The formation of SDS and OTS coatings on the iron particle surfaces may be therefore explained by the formation of ionic bonds between the sulfate groups of SDS and the iron +3 ions on the particle surfaces and/or between the -Sic13 groups of OTS and the oxide layer on the iron particles, similar to the explanation that described the formationof alkylthiol and alkylsilane coatings on silica and gold substrates, respectively. 1-5 These preliminary studies with amorphous iron nanoparticles may be of a special interest because of its magnetic properties, its high surface area, and the potential applications, i.e. catalysis, protection against corrosion, magnetic separation, etc. Further studies are ongoing in our laboratories.
Acknowledgment. This work was partially supported by Minerva (Otto Meyerhoff Center for the study of drugreceptor interactions). 0.Rozenfeld and Y. Koltypin thank The Ministry of Absorption, The Center for Absorption in Science. Y. Koltypin is specially thankful for the financial support from Dr. Irving and The Cherna Moskowitz Program for the Absorption of Scientists in Israel.
