318
Langmuir 1996,11, 318-326
Aqueous Solution Deposition Kinetics of Iron Oxyhydroxide on Sulfonic Acid Terminated Self-Assembled Monolayers Peter C. Rieke,* Brian D. Marsh, Laurie L Wood, Barbara J. Tarasevich, Jun Liu, Lin Song, and Glen E. Fryxell Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352 Received April I , 1994. I n Final Form: September 22, 1994@ The deposition kinetics of iron oxyhydroxide on sulfonic acid terminated self-assembled monolayers were studied. The thin films of FeOOH were formed on the substrates by thermal hydrolysis of millimolar aqueous solutions of Fe(N03)3at a pH of approximately 2.0. The thickness of the films was measured ellipsometrically at various times. Both Fe(N03)3 and HN03 concentrations were independently varied to provide varying degrees of solution supersaturation. Depending on these concentrations, an induction time was observed before film growth commenced. The correlationbetween supersaturation and induction time was modeled using classical nucleation theory. Very good agreement was observed regardless of whether supersaturation was varied via the concentration of Fe(N03)3or HN03. From these results an interfacialfree energy for nucleation of 148 mJ/m2was calculated. The critical nucleus species was identified as a tetrameric iron species by considering the order of nucleation.
Introduction Substrates derivatized with organic functional groups can be used to control the deposition of inorganiclmineral thin films from aqueous solution. The dramatic influence of functionalized organic interfaces on nucleation and growth has been illustrated in a number of studies.l-13 Substrates such as Langmuir-Blodgett films, functionalized polymer substrates, and inorganic substrates derivatized with organic monolayers have been used for this purpose. Many of these studies have purported to show that nucleation occurs at the interface. While this may well be the case, it has not been shown that classical nucleation theory adapted to heterogeneous nucleation a t a surface can be used to quantitatively describe deposition on macroscopic substrates. In those instances where heterogeneous nucleation does occur, nucleation theory allows interpretation of film growth data in terms of the interfacial free energy for n u ~ l e a t i o n . ' ~ -This ~~ thermodynamic parameter provides a quantitative means of comparing the efficacy of various surfaces toward Abstract published in Advance A C S Abstracts, November 15, 1994. (1)Mann, S.;Archibald, D. D.; Didymus,J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993,261, 1286. (2)Campbell, A. A.; Fryxell, G. E.; Gr&, G. L.; Rieke, P. C.; Tarasevich, B. J . Scanning Microsc. 1993, 7, 423. (3) Rieke, P. C.; Bentjen, S. B. Chem. Mater. 1993, 5, 43. (4) Heywood, B. R.; Mann, S. Langmuir 1992,8, 1492. (5) Zhao, X. K.; Fendler, J. H. J . Phys. Chem 1991, 95, 3716. (6) 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. (7) Hughes, N. P.; Heard, D.; Perry, C. C.; Williams, R. J. P. J.Phys. D: Appl. Phys. 1991,24, 146. (8) Linde, A.; Lussi, A.; Crenshaw, M. A. Calcif. Tissue lnt. 1989,44, 286. (9) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Proc. R. Soc. London, A 1989,423,457. (10)Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.; Weiner, S. Proc. Natl. Acad. Sci. U S A . 1987, 84, 2732. (11) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353. (12)Addadi, L.; Weiner, S. Proc. Natl. Acad. Sei. U S A . 1986, 82, 4110. (13) Greenfield, E. M.; Wilson, D. C.; Crenshaw, M. A. Am. 2001. 1984,24, 925. (14) Nielsen, A. E. Kinetics of Precipitation; Pergamon Press: New York, 1964. (15) Nielsen, A. E. In Crystal Growth; Peiser, H. S., Ed., Pergamon: Oxford, 1967; 419 pp. (16) Becker, R.; Doring, W. Ann. Phys. 1935,24, 719. (17) Zeldovich, J. J . Exptl. Theoret. Phys. (U.S.S.R.) 1942,12,525. @
induction of nucleation. Further, this value may be compared with other measures of surface energy, for example, adhesion ofliquids to the substrate a s measured by contact angles or adsorption isotherms of cations involved in mineral growth. Such comparisons should allow rational design of interfaces with enhanced ability to promote nucleation. Our primary interest is in developing aqueous phase, heterogeneous nucleation as a novel and, in many ways, superior thin-film deposition t e ~ h n i q u e . ~ Both l - ~ ~slightly soluble salts and metal oxide/oxyhydroxide materials may be deposited provided a supersaturated aqueous solution can be prepared and a suitably active substrate identified. A significant advantage resulting from the surfacecontrolled deposition is that complex and convoluted shapes may be uniformly coated. We have previously demonstrated that mineral deposition can be controlled to dimensions on the micrometer scale by patterning the organic functional group^.^^,^^ These features are in contrast to more conventional vapor phase, sol-gel, and slurry deposition techniques in which the rate of film deposition is controlled by mass transfer to the surface and the substrate often plays little role except in promoting adhesion of the thin-film material.27 We report on the kinetics of iron oxyhydroxide thinfilm deposition. FeOOH, in the form of goethite, is a precursor for the industrially important magnetic material, maghemite, and its deposition as a thin film may (18) Frenkel, J . Kinetic Theory of Liquids; Oxford University Press: London, 1946; 366 pp. (19)Turnbull, D.; Fisher, J. C. J . Chem. Phys. 1949, 17, 71. (20) Reiss, H. Ind. Eng. Chem. 1952, 44, 1284. (21) Rieke, P. C.; Tarasevich, B. J.; Fryxell, G. E.; Bentjen S. B.; Campbell, A. A. In Supramolecular Architecture: Synthetic Control in Thin Films and Solids; Bein, T., Ed.; ACS Symposium Series 499; American Chemical Society: Washington, DC, 1992. (22) Heuer, A.; et al. Science 1992,255, 1098. (23) Tarasevich, B. J.; Rieke, P. C.; McVay, G. L.; Fryxell, G. E.; Campbell, A. A. In ChemicalProcessingofAdvanced Materials;J. Wiley & Sons: New York, 1992. (24) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J. Science 1994, 264, 48. (25) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. L.; Baer, D. R.; Fryxell, G. E.; John, C . M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619. (26) Rieke,P. C.;Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vac. Sci. Technol. A 1993, 11, 2292. (27) Ohring, M. The Materials Science of Thin Films; Academic Press: San Diego, CA, 1992.
0743-746319512411-0318$09.00/00 1995 American Chemical Society
FeOOH Deposition on SAMs provide a route t o very high-density magnetic storage devices. However, in this work the focus is on FeOOH as a model compound by which to understand the mechanisms of surface-promoted mineral deposition. The active substrate was a self-assembled monolayer (SAM)terminated with sulfonic acid groups. The induction times and growth rates for film deposition were measured and compared with various models of growth. Classical nucleation theory was found to give an excellent description of growth and, from the data, a quantitative value for the interfacial energy for nucleus formationwas obtained. We show that classical nucleation theory adapted to heterogeneous nucleation a t a surface can be used to quantitatively describe deposition from aqueous solution on macroscopic substrates.
Experimental Section The substrates used in this work were SAMs prepared from the vinyl-terminated monomer 1-heptadecenyltrichlorosilane (vinyl SAMs). A few SAMs were also prepared from octadecyltrichlorosilane (methylSAMs). Polished, 100cut,p-doped, silicon wafers (Silica-SourceTechnologyCorp.)were used as substrates. The substrates were cut into 2.5 x 0.5 cm pieces, sonicated in chloroform,and treated in a air plasma (Hanick). Sampleswere handled using 2.5-cm wafer racks (Fluoroware). The clean substrates were given a 2.0-min wash in 0.1 M KOH, followed bya5.O-minwashin0.1MHN03anda5.0-minrinseindeionized water, and were blown dry. The substrates were further dried under flowing nitrogen for a minimum of 2 h prior to immersion in the silane solution. Commercial octadecyltrichlorosilane (Aldrich) was distilled once prior to use. 1-Heptadecenyltrichlomsilanewas prepared in-house via a procedure adapted from the literature.% Cyclohexane (Aldrich, Sure-Seal) was further purified by distillation from calcium hydride. The 0.1 M solutions were prepared by weight and placed in oven-dried,8-oz canningjars equippedwith an air-tight lid. The rack of substrates was immersed in this solution for 1.0 h and subsequently rinsed twice in clean chloroform and blown dry in nitrogen. "he vinyl-terminated SAMs as well as a few methyl-terminated SAMs were sulfonated by exposure to so3 gas. A glass vacuum apparatus constructed from two 75-mm O-ring joints and equipped with a dry nitrogen purge was used for the sulfonation. A glass plate with 2-mm slits held the samples upright within the apparatus. After pumping to rough vacuum, the apparatus was dried using a heat gun and the chamber was closed off from the rough pump. SO3 gas from either solid SO3 or from fuming HzS04(Aldrich)was introducedfrom a valved round-bottomflask attached t o the chamber. The samples were left exposedto the gas for the requisite time, usually about 60 s, and then the entire system was purged with nitrogen gas. The samples were removed, rinsed thoroughly in deionized water, blown dry, and stored in clean plasticware. To compare the reactivity of the vinyl group with the alkyl chain of the S A M , a few methylterminated SAMs were also sulfonated by this procedure. The methyl-terminated SAMs were not significantly affected by this process. The compositions of the SAMs were monitored by XPS, and quantitative data were obtained by analyzing relative peak heights and/or areas of the S 1s and CB peaks. These two peaks are small and overlapslightly, and it was necessaryto deconvolute them to obtain the desired S/C ratio. Atomic percentages for the SAM were obtained by correcting the data using tabulated XPS sensitivity factors.29 The data suggested that on average approximately 90% of the vinyl groups were sulfonated. The possibility of adsorbed C influencing the result exists. A few vinyl SAMs were prepared with differing hydrocarbon chain lengths (Ca, (211, and (217) and analyzed by XPS. The SIC peak ratio was approximately constant when normalized for the various hydrocarbon chain lengths. This suggest that hydrocarbon contamination was minimal if care was taken to keep the substrates clean. (28)Wasserman,S. R.;Tao,Y.T.; Whitesides,G.M.Lang?nuir 1989, 5,1074. (29)Seah, M.P.;Dench, W. A. Su$. Interface Anal. 1979,1, 2.
Langmuir, Vol. 11, No. 1, 1995 319 Depositionof the FeOOH films occurredby thermal hydrolysis of acidified Fe(N03)~solutions. A 0.1 M HNo3 stock solution was prepared from concentrated nitric acid (Ultrex). The exact concentration of this solution was determined by titration with a NaOH solution standardized against potassium hydrogen phthalate. This stock solution was further diluted to prepare solutions of 7.5-15.0 mM mas. Dilute solutions were prepared in 2.0-L lots to assure more than sufficient quantity to complete aa series of experiments. Deionized water (18MQ) (Millipore) was used to prepare all solutions and rinse samples. Fe(NO& solutionswere prepared immediatelybeforeuse by direct addition of the weighed reagent, Fe(N03)39H20, to the stock solution with rapid stirring. The Fe(N03)39H20 (Aldrich)was recently purchased, used as received, kept tightly closed, and frequently inspectedto assurenone of the red hydrolysisproductthat formed on the rim ofthe container contaminated the bulk of the reagent. A 15-mLportion of this solution and the substrate were placed in 22-mL glass scintillation vials (Wheaton) equipped with polyethylene "poly-seal" caps. The vials were placed in racks and the rack immersed in a 70 f 0.5 "C water bath so that the vial caps werejust above water level but the solution inside was completelybelow the external water bath level. The temperature inside the vial, as measured by a probe inserted through the cap, came to equilibrium 10-15 min after immersion. At the appropriate time, vials were removed from the bath and the substrates were rinsed in deionizedwater and blown dry with nitrogen. The thickness of the films was measured with a Gaertner ellipsometer. The films consist of three layers on the silicon wafer substrate: a 20 A thick Si02 layer, a 25 A thick SAM layer, and the FeOOH film. The measured refractive index of the film depended on the thickness and was approximately 1.5 for a very thin FeOOH film and approached 2.3 for films thicker than 700 A. The literature value for the refractive index of goethite is 2.3. The thickness values reported have not been corrected for effects resulting from the multilayer nature of the film and, for the purpose of this work, should be considered a relative measure of film thickness. Further above 1500 A the apparent thickness began to decrease because of the inability of the ellipsometer to properly calculate film thickness. However, the ellipsometer proved a valuable and precise method of determining the induction period. Scanning electron microscopy of the samples was performed on an Electroscan environmental microscope, Model E30, or on a Jeol JSM-US microscope. X-ray photoelectron spectra were obtained on a Perkin-Elmer PHI 560 XPS/Auger/SIMS surface analysis system using a magnesium Ka photoline. The doublepass cylindrical mirror analyzer was calibrated to the 932.6 eV Cu 2pm and 75.14-eV Cu 3~312lines. Charge correction was calculated from the difference of the observed carbon 1s binding energy to the 284.8 eV value for the carbon 1s binding energy. A Phillips APD 3620 X-ray powder diffraction unit was used t o obtain powder diffraction data. A fixed Cu anode was used and operated at 40 kV and 25 d. Scans were taken in step mode at 0.02 deg/s from 10"to 70" for 20. The phases were identified by comparison with the JCPDSACDD, powder diffraction data base, CD-ROM version PDF-2, sets 1-40. Thin cross section samples for transmission electron microscopy were prepared by ultramicrotomy. The samples were cut into 10 x 10 mm pieces, embedded in epoxy (Bueler Epo-Kwick Resin), and cured for 24 h. The sample was then mechanically ground from the back side using #600 sandpaper to remove most of the silicon wafer. Residual substrate was then removed with a razor knife. Only the FeOOH film remained attached to the epoxy. After further trimming, the sample was microtomed (SorfalNT 6000Ultramicrotome)alonga direction perpendicular to the film plane. Imaging, selected area diffraction, and compositionanalysis were performed on a Jeoll200 analytical TEM and a Philips 400 TEM both at 120 keV. A solution speciation code, EQ3NR,30z31was used to solve the equilibrium equations iteratively and to output the concentration of each solution specieslisted in Table 1as well as the saturation (30)Jenne, A., Ed., Chemical Modeling in Aqueous Systems; ACS Symposium Series 93;American Chemical Society: Washington, DC, 1979.
(31)Wolery, T.J. EQ3NR-A Computer Program for Geochemical Aqueous Speciation-Solubility Calculations, UCRL-MA-110662pt.111,
Lawrence Livermore National Laboratory, 1992.
320 Langmuir, Vol. 11, No. 1, 1995
Rieke et al.
Table 1. Cumulative Formation and Solubility Constants for the Fe(NO&, HNOs System
equil const stoichiometricreaction 25 "C 70 "C -2.19 -1.14 1 Fe3++ H2O = FeOH2++ H+ logK11 = -5.67 -3.97 2 Fe3++ 2H20 = Fe(OH)Z++ 2H+ log K12 = -12.02 -12.02" 3 Fe3++ 3H20 = Fe(OH)3(,,) + 3H+ log K13 = -21.70 -18.7 4 Fe3++ 4H20 = Fe(OH)4- + 4H+ log K14 = -2.95 -1.55 5 2Fe3++ 2H20 = Fe2(OH)z4++ 2H+ log K22 = -6.31 -5.19 6 3Fe3++ 4H20 = Fe3(0H)d5++ 4H+ log K34 = 1.30 1.30" log K1L = 7 Fe3++ NOS- = FeN032+ 5.66 3.66 8 Fe(OH)3(am)+ 3H+= Fe3++ 3H20 log Ksp(am)= 0.53 -0.92 9 FeOOH(,,t) + 3H+ = Fe3++ 3H20 log Ksp(g0P.t)= 0.11 -3.01 10 FeOOH(hem)+ 3H+= Fe3++ 3H20 log Ksp(hem)= -14.00 -12.75 11 H20 = H+ + OHlog K, = 1.30 -0.85 12 HNO3 = NO3- + H+ log KHNo3= a Thermodynamic values for heats of reaction were not available for these reactions. The log K value at 25 "C was used at 70 "C. nm
with respect to the goethite, hematite, and amorphous FeOOH. While the equilibrium values have been written in the conventional manner, the actual reactions were modeled using Fe3+, H20, NO3-, and H+as the primary species from which all others wereformed. The input for mass balance assumed that solutions were prepared from Fe(N03)3and HN03. The actual formation of solid phases was not modeled and only supersaturationswith respectto a given solidwerecalculated. Thus, the results modeled the initial solution conditions before precipitation occurred. No attempt was made to model the progress of the precipitations, as this would have required unrealistic assumptions about the kinetics of solid phase precipitation.
Results and Discussion The kinetics of film deposition were determined by monitoring film thickness versus time. These data were analyzed in terms of classical nucleation theory to determine the interfacial free energy of nucleation (IFEN) and the preexponential factor and to estimate the critical nucleus size. This work is also confined to deposition on SAMs composed entirely of sulfonated vinyl monomers. In a future publication, the deposition of iron on mixed vinyVmethy1 monolayers will be reported.32 Prior to the kinetics analysis it is necessary to describe the structure and composition of the films deposited, describe the speciation in solution from which the film was deposited, determine the solubilities of the various mineral species, and describe the SAM substrates. These results provide necessary estimates of values required to complete the classical thermodynamic analysis, e.g. the mineral phase deposited, its values for supersaturation under specific conditions,and an estimate of the nucleation site density. Bri ht-field and dark-field TEM micrographs of a 2000- FeOOH film are shown in Figure 1. This film was grown in a 3.0 mM Fe(N03)3and 10.0 mM HN03 solution. These conditions afford very rapid growth and this thickness was achieved in about 90 min. In general, the films were fairly dense with uniform microstructure and thickness. The DF image revealed crystals with a columnar structure and 20 nm diameter, while the BF image revealed that each column consisted of lamellar features of about 20 %i in dimension. Shown in the inset is the diffraction pattern of the film and an authentic sample of goethite. The match of the film pattern was very close to that of the reference compound. The pattern was also compared with hematite, akaganeite, lepidocrocite, and six-line ferrihydrate; these reference materials did not match well with the film pattern. Table 1presents the known equilibrium coefficients and solubilityproducts relevant for a solution of Fe(NO& and HN03.33-37One nitrate and six hydroxide complexes with
1
(32)Rieke, P.C. Manuscript in preparation.
Figure 1. Bright field (BF) and dark field (DF) TEM images
of a FeOOH film approximately 2000 thick. In the inset is the electron diffraction pattern (right) compared with that of an authentic goethite sample (left).
iron are known. The minerals which can be formed from solution are the oxide hematite and the oxyhydroxides goethite, akaganeite, lepidocrocite, six-line femhydrate, two-line femhydrate, and the amorphous h y d r o ~ i d e . ~ ~ ? ~ ~ Goethite, akaganeite, and lepidocrocite are well-defined mineral species. The amorphous hydroxide and the femhydrates are not well-defined phases and will readily transform to the more stable species if heated or dehydrated. Akaganeite,while considered a iron oxyhydroxide, requires the presence of chloride to initiate formation and its formation was not expected. The Ksp values for hematite and amorphous FeOOH were taken from the data base of Johnson, Oelkers, and H e l g e ~ o n .These ~~ values are slightly different a t 25 "C from those in the other references but provide sufficient thermodynamic data to calculate aK,, a t 70 "C. As the mineral deposited is goethite, the choice of Kspvalues is not critical to our analysis. Femhydrate, goethite, and/or hematite were the expected products from precipitation induced by heating of acid solutions. The exact mineral formed is sensitive to (33)Shaw,W. H.R.; Walker,D. G. J.Am. Chem. SOC.1956,78,5769. (34)Baes, C. F.;Mesmer, R. E. The Hydrolysis ofCations; Robert E. Krieger PublishingCo.: Malabar, FL, 1986. (35)Flynn, C. M., Jr. Chem. Rev. 1984,84,31. (36)Johnson, J. W.; Oelkers, E. H.; Helgeson, H. C. Comput. G'eosci. 1992,18,899. (37)Schwertman,U.;Cornell, R. M. Iron Oxides in the Laboratory; VCH: New York, 1991.
FeOOH Deposition on SAMs
Langmuir, Vol.11, No. 1, 1995 321 I
-4
0.0125 0.0115 0.0105 0.0095 0.0085 0.0075
[HN03]
Figure 2. Solution speciation versus concentration of added HNo3 and at a constant Fe(N03)3concentrationof 2.0 mM and 70 "C. solution conditions. Schwertmann and CornelP' point out that ferrihydrate will form by thermal hydrolysis of acid Fe(N03)~solutions for a period of a few minutes. However, more extended hydrolysis leads to hematite and goethite. Mixing the Fe(NO3I3in a ambient temperature water favors goethite while mixing in preheated solutions favors hematite. Our procedure would by these measures favor goethite. X-ray diffraction of the precipitate collected from homogeneousprecipitation indicated a mix ofgoethite and hematite. As noted above the mineral formed on the films was identified as goethite. For the purposes of this work, the solubility was assumed identical to that of goethite. . The equations of Table 1were used as the data base in the EQ3NR computer code30*31 by which the solution speciation and mineral supersaturation ( S ) were calculated. S is the ratio of the solution ion activity product (IAP)and the solubility product (ICsp)for the mineral. The value of the equilibrium constants are given for both 25 and 70 "C.The later value was calculated from the heats and entropy of formation and were resident in the EQ3NR data base. Calculations were performed for added HN03 ranging from 7.5to 12.5 mM and added Fe(N03)~ ranging from 0.5 to 4.0 mM. The initial pH of these solutions varied from 1.96to 2.18 and the pH after deposition varied from the initial value by no more than 0.03 pH unit. It should be noted that outside this very narrow range of HN03and Fe(N03)3concentrations either no precipitate was formed or homogeneous precipitation was sufficiently rapid to obscure film formation. Figure 2 shows the solution speciation calculated with varied acid concentrations for a fixed Fe(N03)3concentration of 2.0 mM, while Figure 3 shows similar data for varied Fe(N03)s concentrations for a fixed HNO3 concentration of 11.5 mM. Under all conditions approximately 70% of the iron was as Fe(OHI2+,15% as free Fe3+,and 10%as Fe(OH)Z+. Other species, FeN0s2+and Fe2(0H)z4+, were also present in minor amounts. The species Fe3(OH)45+was not present in any significant amount. From these calculations the experimental solutions were expected to be quite simple in composition without significant amounts of complex or polymeric species. Further the relative distribution of species should be essentially identical for all amounts of added HN03 and Fe(NOd3. Figures 4 and 5 show the log(S) values for the mineral species as a function of added acid and added iron, respectively. Hematite is the least soluble mineral and has a S of lo6 to los. Goethite is also supersaturated
Figure 3. Solution speciation versus concentration of added Fe(NO& and at a constant €€NOSconcentration of 0.0115 M and 70 "C.
7
4
Hematite
0.0125 0.0115 0.0105 0.0095 0.0085
O.( 75
WO3I
Figure 4. Mineral supersaturation versus concentration of added HNOs and at a constant Fe(N03)s concentration of 2.0 mMand 70 "C. Amorphous FeOOH is not supersaturated under any conditions considered here. 9
3
'1 1
0 0.0005
0.0015
0.0025
0.0035
0
[Fe(N03)31
Figure 5. Mineral supersaturation versus concentration of added Fe(N03)sand at a constant €€NO3concentration of 0.0115 m M and 70 "C.Amorphous FeOOH is not supersaturated under any conditions considered here. under all the conditions considered here but at much lower supersaturations. Supersaturation ranged from about 50 to 600. The log@) values for goethite as taken from these plots were used to analyze the nucleation and growth data described below in detail. Amorphous Fe(OH13 is not shown in the figures as this mineral was not supersaturated under any important conditions in this work.
Rieke et al.
322 Langmuir, Vol. 11, No. 1, 1995 UV-vis absorbance spectra were taken of the solutions as a function of time in the water bath at a temperature of 70 "C. No change in the spectra was apparent until a precipitate began to form after about 12 h, well beyond the times used in film formation. Further, we were not able to detect any light scattering in solution during this period other than that due to unavoidable dust particles. Visual comparison of HeNe light scattering in sample and reference solutions was used to detect scattering by colloidal species. While qualitative, this method is quite sensitive but may not detect the presence of very small colloids present a t low concentrations. From these results we concluded that no significant amounts of polymeric or colloidal species were present during film growth and, that as predicted by the speciation calculations, mononuclear iron species were the primary species involved in film deposition. Occasionally a particular sample would undergo homogeneous precipitation prematurely during film deposition-presumably due to nucleation on adventitious particulate in solution. These samples were not used in the subsequent analysis. The sulfonic acid groups were introduced to the S A M substrate by exposure of the monolayers to gaseous SOs. According to Gilbert in his review on s u l f o n a t i ~ neither ,~~ the alkenesulfonate or the hydroxysulfonate may be formed with the later favored in the presence of excess water. Because fuming sulfuric acid was used to prepare most of the substrates, it is presumed that the alkenesulfonate has been formed. The advancing contact angle of water on the sulfonated substrates was between 8" and 15" with the majority of samples averaging about 10"13". The time of exposure to SOs did not influence the contact angle significantly, and from this we concludethat sulfonation was almost instantaneous. With care, excellent reproducibility of the contact angles could be obtained even between batches. The contact angle on methyl SAMs dropped from 109" to about 103" for short sulfonation times. This suggests only minor reaction with the alkane. For sulfonation times greater than 2.0 min, the contact angles further decreased. These long times were avoided to prevent excessive damage to the alkane chain. XPS spectra were taken of the sulfonated SAM after immersion for 60 min in a deposition solution containing 11.5 mM HN03 and 2.0 mM Fe(NO&. This solution was not heated and no precipitation was induced. After a brief water rinse, XPS analysis showedthat each iron was bound to approximately two sulfonic acid groups. Consequently we conclude that the iron was rapidly and probably irreversibly bound to the sulfonic acid groups. Film deposition can be viewed as having occurred not on free sulfonic acid groups but rather on a monolayer of iron bound to sulfonic acid groups. These films showed a slight increase in thickness of 10-15 A. In measuring the kinetics of film growth, we are interested predominately in the induction time and the rate of film growth after induction. Whether a n induction period was observed depended on the solution composition. Figure 6 shows a plot of UV-vis absorbance a t 290 nm for FeOOH films deposited on vinyl SAM/glass substrates. Fe(N0d3 (3.0 mM) was used in 10.0 mM HN03. Under these conditions film growth was very rapid and no detectable induction time was observed. The growth rate leveled off after about 200 min a t which time a precipitate began to appear in solution. Ifthe solution was exchanged with fresh solution, growth continued-apparently indefinitely, provided fresh solution was available. Eventually the films became several micrometers thick and (38)Gilbert, E. E. Sulfonation and Related Reactions;Robert E. Krieger Publishing Co.: New York, 1977.
8 1.5
2 1.0 0.5
. . .
.
I
1bO
.
I
.
260
..l.,.ll.,I 300
400
Minutes
Figure 6. UV-vis absorbance at 290 nm of FeOOH films versus time. The films were deposited on sulfonated SAMs formed on glass substrates.
6o l 500
I:[
200
100
0
d
100
200
360
Minutes
Figure 7. Thickness of FeOOH films versus time for four concentrations of Fe(N03)~. The concentration of HNOs was 0.0115M.Thelog(S)valuesare3.35at3.0mM, 3.23at2.5mM, 3.10 at 2.0 mM, and 2.94 at 1.5 mM. The data have not been corrected for the approximately 60 A optical thickness of the SiOdSAM substrate shown as a dashed line.
were powdery and loosely adhered. The following discussion will be limited to the initial linear growth region and the time required to induce this rate of growth. In all these results film growth was measured in the absence of a homogeneous precipitate. Shown in Figure 7 are plots of thickness on sulfonate SAMs using 11.5M HN03and various millimoles per liter concentrations of Fe(N03)s. Film thickness was negligible for some time depending on the Fe(N03)s concentration and then increased rapidly. The portion of the thickness curve with negligible film growth was taken as representative of the induction period. Linear least-squares lines were drawn through the data points for the growth portion of the curve. To distinguish this portion of the curve, only points with thicknesses greater than 100 A were considered. The induction times were obtained b the intersection of the lines a t a base thickness of 60 (The re orted thickness has not been corrected for the 50-60 SiOdSAM contribution. The dashed line in the figure a t 60 A should be taken as zero film thickness. As noted above absorption of iron resulted in a slight increase in film thickness to give a total thickness of 70-80 A prior to the onset of growth.) It can be seen that the growth rate was quite constant once nucleation occurred and did not depend distinctly on Fe(N0d3concentration. It is clear that induction times progress from nearly negligible in a 3.0 mM solution to about 170 min for a 1.5 mM solution. An exact value for the induction time was difficult to obtain a t the lowest
2
1,
FeOOH Deposition on SAMs
Langmuir, Vol.11,No.1, 1995 323
900 so0 700
600 .b.
5
500
0
2 e
400
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200 100
0 0
400
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600
Minutes
Figure 8. Thickness of FeOOH films versus time for six concentrations of HN03. The concentration of Fe(N03)3was 2.0mM. The log(S)values are 3.64at 7.5mM, 3.27at 10.0mM, 3.21at 10.5mM, 3.15at 11.0 mM, and 3.05 at 12.0 mM. The data have not been corrected for the approximately 60 A optical thickness of the Si02/SAM substrate shown as a dashed line. iron concentration because of scatter in the data. But, for example, the induction time for a 1.5 mM solution was between 140 and 210 min. More exact values could be obtained for the higher concentrations. The data in the figure represent a single experimental series using a series of surfaces from a single batch of SAMs and the same iron solution. Points were removed that showed signs of homogeneous precipitation. This experiment, as were those described below, was repeated several times. In all cases the results were similar in that a reproducible progression of measured induction times was observed with varied HN03 or Fe(N03)3 concentrations. The variability in measured induction time was about 60-40 min for long times and 10-20 min for short times. Shown in Figure 8 are similar plots on 100% sulfonate SAMs using 2.0 mM Fe(N03)~and various HNOs concentrations. Again the results are similar with increasing induction times as the solution becomes more acidic and a growth rate approximately independent of acid concentration. No growth was observed on the 15.0 mM sample. Again, a t high acid concentration, precipitation in solution resulted in many samples being eliminated from consideration. Still the results were reasonably reproducible between experimental runs. The induction times were used in classical nucleation theory to determine a value for the nucleation barrier height and subsequently the interfacial energy for nucleation, u. We briefly review nucleation theory for the convenience of the reader.14-20 The main results are
AG = -nkT ln(S) AG* =/3
1
(b(s))2
Figure 9. Analysis of induction time data using classical nucleation theory. Data shown were taken from Figures 7 and 8. Data points with the symbol 0 are for varied Fe(N03)~ concentration and constant HN03 concentration. Data points with the symbol are for varied HNO3 concentration and constant Fe(NO& concentration. Considering both data sets the value obtained for u from the slope is 148 mJ/m2 and the value obtained for Q/N* was 17 s-l. in terms of the supersaturation S and the surface energy term is obtained.
J = S2 exp(-&) -AG The induction time, tin& is taken as that point in time where growth of the nuclei dominates over formation of new nuclei and the number of nuclei formed remains constant a t a value The induction time is given by
t,, = N*IJ substituting in the above equations for J and AG* gives
F)
log(t,,,) = log -
+ ( ( k ~ ) ~ 2v2a3 . : 0log(SI2 3~
A = log(=) N*
+ oA
V 2 4
(kT ln(S))2
where AG is the free energy of a nucleus of size n and represents the tradeoff between creating new surface and bulk materials. Through entirely geometric arguments based on the square and cubic dependence of the surface area, A, and the bulk, respectively, a n expression for AG* can be obtained. Critical to this derivation is the assumption that u does not depend on the radius of the nucleus. AG* is the activation barrier to nucleation. Where S is the degree of supersaturation, p is a shape factor, v is the molecular volume, and kT has the usual meanin g. By use of the Arhennius rate theory for the rate of nucleus formation, J,an expression for the induction time
The result is a linear equation with a n intercept of A and a slope of Bd. As B can be calculated, a value for u can be found from a plot of log(tind)versus (log(S))-2. This was done for the induction time data obtained from Figures 7 and 8 and the calculated S values from Figures 4 and 5. The results are shown in Figure 9. Data obtained by variation of HN03 are shown as W; while data obtained by variation of Fe(N03)3 are shown as 0. The values obtained for u and QIN*are 148 mJlm2 and 17 s-l. This analysis is based on a log-log plot, which is notorious for ability to linearize any set of data. Consequently the validity of these data depends on the reasonableness of the values obtained for o and 8.Interfacial free energies for nucleation typically fall between 10 and 150 mJlm2 (39)Sohnel,0.; Mullin,J. W. J.CoZZoidZnterfaceSci. 1988,123,43. (40)Furedi-Milhofer,H.Pure Appl. Chem. 1981,53,2041.
324 Langmuir, Vol. 11, No.1, 1995
Rieke et al.
so that our measured value for u is within these b o ~ n d s . ~ ~Further, - ~ ~ s ~it~can , ~be ~ seen from Figure 9 that good agreement exists between data obtained by variation of either [H+l or [Fef31in comparison with the experimental error. The ability to produce similar results by two different methods, that is, by variation of both iron and acid concentration, provided strong evidence that the measured values were not coincidental. While it is possible that one set of experiments may have matched nucleation theory given the log-log analysis required, it is unlikely that both sets of experiments would have fortuitously given similar results. The very similar slopes and intercepts for these two analyses support the use of classical nucleation theory as a good descriptor of film deposition. The intercept from data of Figure 9 is equal to log(51/ N*), where N* is the number of critical nuclei formed a t the end ofthe induction period. As the end ofthe induction period represents the onset of growth at the expense of continued nucleus formation, the number of crystals present is taken to be equal to N*.39940An estimate ofthis can be obtained from Figure 1, the TEM micrograph of the film. The width of the crystals is about 20 nm, and 400 nmz is a crude estimate of the area per crystal. This gives a value for N* of 2.5 x 1015m-z and consequently an experimentally determined value for 51 of 4.2 x 10I6 m-z s-l In classical nucleation t h e ~ r y l ~ -the ~ O value for Q is given by
Q=N~A*(
E
)
112
9?tkT(n* ) 9 / 3 where N , is the number of nucleation sites per unit area, D is the collision frequency per unit area, A* is the area of a critical nucleus, n* is the number of molecules in the critical nucleus, and 5 is given by
$nw3= A o
oxyhydroxides. A crude estimate €or this value can be obtained from Figures 7 and 8 by using the postnucleation film growth rate, W d t % 0.015 f u s = 1.5 x d s .D is then given by
D = -d-L 1 dt v and has a value of 4.3 x 10I6m-z s-l. From these estimates the value for 51 can be calculated to be 4.0 x 10l6m-2 s-l. Considering the approximation involved in the theory and the crude estimates of some of the necessary values, this value is in very good agreement with the above experimentally determined value of 4.2 x 10l6 m-z s-l. The meaning of this calculation deserves some critical comment. First, the experimental data and the theoretical estimates used lead to a n uncertainty in these values of about 1 order of magnitude. Second, nucleation theory is fraught with problems concerning the preexponential f a ~ t o r . ~ -Differences ~' between theory and experiment of 10 orders of magnitude have been reported and closer agreements have been dismissed due to compensating errors. It may well be that the agreement between the values reported in this work is due to fortuitous compensating errors. This issue cannot be addressed until kinetic data can be evaluated for other metal oxyhydroxide systems. Alternatively the data of Figures 7 and 8 can be analyzed in terms of a conventional rate analysis. In this case the rate equation can be written in terms of reagent concentration
J = k[Fe3+l"[0H-l" The induction period can be viewed as the time required to build up the film to a very small but finite thickness, L, a t which the mechanism of film growth changes. The rate of deposition is given by
Using the relationshipsz0
A = @I3
equating these two and assuming a constant value for L gives
and
V = nv 51 can be rewritten as log(tind)= log(;) where K is a constant dependent on nucleus shape and Y is the molecular volume. A value for N , can be estimated from the number of monomeric iron species physisorbed to the surface prior to initiation of nucleation. XPS analysis showed that each iron was bound to approximately two sulfonic acid groups. The density of the sulfonic acid groups can be taken equal to the density of the hydrocarbon chains and this value has been estimated42to be 4.7 x 1 O l s m-2 to give a value for N , of 2.4 x 10l8 m-z. For simplicity, K is taken to be equal to the value for a spherical nucleus, (62/3n1'3). v is taken from the density ofgoethite and is equal to 3.44 x m3.The value for u is taken from the above analysis. D is normally taken to be the diffusion limited frequency of collision. While this may be appropriate for gas-phase nucleation or even nucleation of slightly soluble salts, it is clearly inappropriate for the very slow hydrolysis and condensation reaction involved in formation of iron (41)Good, R. J.;van Oss, C. J. In Modern Approach to Wettability; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1991. (42)Berg, J. C. In Wettability; Berg, J. C . , Ed.;Marcel Dekker: New York, 1993.
- n log[Fe3+l - m log[OH-l
plots of log(&) versus log[Fe3+lor log[OH-I will provide the values of n and m. The data of the figures were used to obtain [Fe3+land [OH-] values. This analysis is shown in Figure 10 and Figure 11, respectively. The slopes give values ofn = 4.7and m = 6.6. The overall reaction is then greater than tenth order. From a conventional rate perspective, this very high order is difficult to rationalize if only Fe3+ and OH- are considered. However the conventional rate equation can be written in terms of other species listed in Table 1. For example the rate equation might be written as
J = k'[Fe3(OH)~+1[Fe,(OH)~+l which reduces the overall order to 2 and still remains reasonably consistent with the above values of n and m. It does imply the formation of the species Fe5(OH)e9+which has an unreasonably high charge. Also, the trimer species (43)Tolman, R. C. J. Chem. Phys. 1949,17, 333. (44)Lothe, J.; Pound, G. M. J. Chem. Phys. 1962, 36, 2080. (45)Burton, J. J. Acta Metall. 1973,21, 1225. (46)Pound, G. M. Metal, Trans. A 1985, 16A, 487. (47)Chiang, P.; Donohue, M. D.; Katz, J. L. J. Colloid Interface Sci. 1988,122,251.
FeOOH Deposition on SAMs
Langmuir, Vol. 11, No. 1, 1995 325 very crude estimate of the size of the critical nucleus. In this case, the order of nucleation suggests a species such as Fe5(OH)e9+.We reiterate that this is a very simplistic model and should be interpreted to mean that the critical nucleus is still a small species. It is certainly larger than the FedOH)r5+species and probably contains less than 10 iron atoms. Alternatively the size of the critical nucleus can be determined from the assumption inherent in the derivation of AG*.14-20 The radius and number of FeOOH units are given by
r* =
n* = 4r*3 3v
Ig[Fe+*]
Figure 10. Analysis of inductiontime data using conventional rate theory. Data shown were taken from Figure 7 with varied Fe(N03)sconcentrationand constant HNO3 concentration.From the slope of this line the order of the reaction in Fe3+was calculated to be 4.7.
2.51 -112
, , , ,
,
,
-11.9
.
,
. , , . . .)
-11.8
,
-11.7
I
,
I
-11.6
Ig[OH-l
Figure 11. Analysis of inductiontime data using conventional rate theory. Data shown were taken from Figure 8 with varied HN03concentrationand constant Fe(NO&concentration.From the sloDe of this line the order the reaction in OH- was calculated to be 6.6. was calculated to be a t very low concentrations. Another problem with the conventional rate analysis is that it does not explain the sudden change in growth rate after the induction time. To do so would require a change in mechanism of growth to a reaction which is considerably faster. These features which are problematicin a conventional rate analysis are inherent features of the kinetic theory of nucleation. In part, the success of nucleation theory lies in its ability to explain kinetics of such high overall order while still utilizing a model based on realistic bimolecular reactions between small species. Further it is assumed that the forward rate of reaction becomes greater than the reverse rate of reaction for particles of size greater than the critical nucleus. By such a model it is possible to explain the transition from nucleation to growth and define the induction time that separates the two regimes. From this view point, the kinetic theory of nucleation is a conventional rate analysis in which a complex series of the elementary reactions are presumed. Christiansen and N i e l ~ o n l have ~ , ~ ~used the order of reaction obtained from a conventional rate analysis as a (48) Christansen, J. A,; Nielsen, A.
673.
E. Acta Chem. Scand. 1961,5,
2av 2.303kT log@)
The radius depends on the log(S) but in this work log(S) did not vary from 3.0 by any significant amount given the assumptions inherent in these equations. Solving these equations gave a value for n* of 3.7. This result also suggests a small species, in this case a tetrameric species, is the critical nucleus. Inherent in classical nucleation theory is the assumption that u is independent of radius. This is certainly not true for such small nuclei and consequently the value of n* can only be taken as approximate. Livage has argued that the building block of goethite is a planar tetramer and should be the primary species involved in goethite formation.49 Olation of these tetramers leads to double-chain polymers that in turn undergo interchain oxolation to form goethite. The above estimates for the size ofthe critical nucleus are consistent with this planar tetramer model. The speciation calculations showed that monomeric species dominate the solution composition, and it is likely that growth occurs by addition of monomeric species to the tetramer rather than by addition oftetramers. These results suggest that formation of a planar tetrameric critical nucleus is the rate-limiting step and that double-chainpolymer growth occurs rapidly by addition of monomeric species. A value for u of 148 mJ/m2 is on the high side of acceptable bounds. It would be useful to compare this value with that obtained in the absence of a surface. Unfortunately, we were not able to obtain sufficiently reproducible data for homogeneous or spontaneous nucleation to determine this value. As nucleation on the sulfonated S A M surfaces was much more rapid than in solution, it might be presumed that the u value for homogeneous nucleation is somewhat greater. This ignores any influence of the preexponential factor Q. As noted above, R is proportional to the value D that is related to the binuclear reaction rate for the primary steps in embryo growth and dissolution. It is possible that the influence of the surface is not in stabilization ofthe critical nucleus, i.e. reduction of u, but in catalysis ofthe primary reaction steps to form the tetrameric critical nucleus. The resolution to such a question will depend on studies of other metal oxyhydroxide systems or utilization of S A M substrates with other functional groups. Conclusions The kinetics of FeOOH film formation has been measured and compared with classical nucleation theory. The interfacial free energy for nucleation was found to be 148 mJ/m2. This value contains a contributionfor the interface with the substrate and the solution. The preexponential (49) Livage, J.; Henry, M.; Jolivet, P. P. In Chemical Processing of Materials:Hench. L. L..West., J. K.., Eds.., J Wilev & Sons: New York, 1992; 223pp. I
,
Rieke et al.
326 Langmuir, Vol. 11, No. 1, 1995 factor was found to be 4.2 x 10l6m-2 s-l which, considering the approximations made, compared well with the theoretical value of 4.0 x 10l6m-2 s-l. The critical nucleus was identified as a tetrameric iron hydroxide species by considering the order of nucleation. Great pains were taken to ensure that film formation was measured in the absence of bulk homogeneous nucleation. Classical nucleation theory was found to qualitatively describe the general form of the growth curves. A quantitative analysis gave values which fell within reasonable bounds. Two chemically distinct methods were used to vary supersaturation and this lent further quantitative support to the applicability of classical nucleation theory to this type of thin film deposition process. However, this work represents only one particular mineraysubstrate system; confirmation of the general applicability of classical nucleation theory will depend on comparison of similar experiments utilizing different minerals and substrates. In this work only a single type surface has been used and was formed by sulfonation of vinyl-terminated SAMs. We can only state that this surface is a good nucleator of FeOOH and the interfacial energy for nucleation on such a surface is about 148 mJ/m2. We cannot make any
conclusion about the role of sulfonic acid groups compared to surfaces with other functional groups or to surfaces with varied density of sulfonic acid groups. These are very important questions and can only be addressed by repeating the analysis reported here using a rationally designed series of substrates. By such a n approach, a quantitative comparison of surfaces can be achieved and consequently the role of various surface forces in nucleation can be elucidated.
Acknowledgment. This research was supported by the U S . Department of Energy (DOE), Office of Basic Energy Sciences. B.D.M. acknowledges support from the DOE, Office of Energy Research, as a Science and Engineering Research Semester program participant a t Pacific Northwest Laboratory. L.S. and L.L.W. acknowledge support by Associated Western Universities Incorporated (Washington State University) under Grant DEFG06-89ER-75522. Pacific Northwest Laboratory is operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. LA9402884
