Water in Contact with Magnetite Nanoparticles, as Seen from

pubs.acs.org/Langmuir © 2009 American Chemical Society

Water in Contact with Magnetite Nanoparticles, as Seen from Experiments and Computer Simulations Etelka Tomb
acz,* Ang
ela Hajd
u, and Erzs
ebet Ill
es Department of Colloid Chemistry, University of Szeged, H-6720 Szeged, Hungary

Krisztina L
aszl
o* Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economy, H-1521 Budapest, Hungary

Giovanni Garberoglio* CNISM and Dipartimento di Fisica, Universit a degli Studi di Trento, Via Sommarive 14, I-38050 Povo (Trento), Italy

P
al Jedlovszky* Laboratory of Interfaces and Nanosize Systems, Institute of Chemistry, E€ otv€ os Lor
and University, P
azm
any P. Stny 1/A, H-1117 Budapest, Hungary, and HAS Research Group of Technical Analytical Chemistry, Szt. Gell
ert t
er 4, H-1111 Budapest, Hungary Received May 26, 2009. Revised Manuscript Received July 3, 2009 The adsorption of water vapor at the surface of magnetite nanoparticles has been investigated both by experimental and by computer simulation methods. The water vapor adsorption/desorption isotherm has been measured on freshly prepared magnetite nanocrystals of the size below 10 nm. The change of the isosteric heat of adsorption with the surface coverage has been determined from the temperature dependence of this isotherm using the isosteric method. The adsorption isotherm has also been determined by performing a set of grand canonical Monte Carlo simulations at 300 K. X-ray photoelectron spectroscopy results as well as the temperature and coverage dependence of the isosteric heat of adsorption clearly indicates that dissociative chemisorption of the water molecules in the first adsorption layer occurs at the bare magnetite surface, resulting in a high density of surface hydroxyl groups. This dissociative chemisorption is followed by a multilayer physisorption of water at higher pressures. Computer simulation results can reproduce excellently both the adsorption isotherm and the isosteric heat of adsorption beyond the first chemisorbed layer of water. Results of the computer simulations reveal that physisorbed water forms several well-distinguished molecular layers on the magnetite surface; however, these layers are not built up sequentially. Instead, the building up of several molecular layers occurs simultaneously. The adsorption of the water molecules in this range appears to be a nucleation-like process, resulting in a rather rough external surface of the adsorption layer.

1. Introduction Metal oxide surfaces routinely come in contact with water in various environmental as well as industrial processes both as a result of simple hydration and also that of oxidation of pure metal surfaces in the presence of hydrating water. Such an oxidation of the iron surface to magnetite (Fe3O4) may cause severe problems in industrial cooling circles, including nuclear power plants. Furthermore, magnetic nanoparticles (e.g., magnetite) are presently the subject of intensive scientific investigations targeting their various potential biomedical applications, in which they are supposed to act in the aqueous environment of the human body. Clearly, hydrophobic surface of dispersed magnetite enhances the uptake of these nanoparticles by the liver, while hydrophilic coating and surface charges increase their retention period in the circulation and the chance to penetrate into interstitial spaces.1 *Correponding author. E-mail: [email protected] (E.T.); [email protected] (K.L.); [email protected] (G.G.); pali@ chem.elte.hu (P.J.). (1) Cheng, F. Y.; Su, C. H.; Yang, Y. S.; Yeh, C. S.; Tsai, C. Y.; Wu, C. L.; Wu, M. T.; Shieh, D. B. Biomaterials 2005, 26, 729.

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At metal oxide surfaces, water molecules can adsorb in different configurations, i.e., binding to the surface in various orientations to either the acidic or the basic centers, and dissociating (or not) into surface hydroxyl groups. Each of these processes can affect the behavior of the oxide particles in humid or aqueous environment. Such processes are of great interest in the case of nanosize metal oxide particles, in particular, for semiconducting (e.g., TiO2) or superparamagnetic (e.g., Fe3O4) properties. A detailed study of the full process of water adsorption at such surfaces from monolayer formation through multilayer adsorption to the presence of a metal oxide/liquid water interface is thus of key importance for a fundamental understanding the surface chemistry of these systems, and hence it is also essential for exploring the rich variety of potential applications of complex water-based fluids of metal oxide content, such as magnetic fluids. Different metal oxide surfaces were found to behave differently upon adsorbing water. Thus, studies on the contact of MgO surface with water molecules showed that dissociative chemisorption is energetically unfavorable, and that a simple physisorption

Published on Web 08/24/2009

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of intact water molecules is preferred.2 The main interaction here concerns the surface Mg atoms and the p pair of the water molecules. On TiO2, on the other hand, water molecules are strongly adsorbed by dissociating to generate surface hydroxyl groups. However, at saturation, adsorbate-adsorbate interactions favor the formation of an ice-like monolayer.2 Furthermore, the surface of iron oxide particles becomes completely covered by hydroxyl groups coordinated to the underlying Fe atoms upon H2O adsorption.3 It is followed by further adsorption of water molecules that are hydrogen bonded to the surface OH groups forming a highly ordered structure in the subsequent layers. Heat treatment is only able to remove the surface hydroxyl groups at high enough temperatures, leaving behind oxo bonds at the surface. However, any subsequent exposure of the surface to water vapor leads to the regeneration of these groups. Water plays a significant role in many processes occurring at iron oxide surfaces. In view of the importance of the water/iron oxide interface, it is not surprising that it has been the subject of intensive scientific investigation for a long time both by experimental and by computational methods. Thus, water vapor sorption capacity of synthetic magnetite was measured,4 and the adsorption of water on different iron oxide surfaces was also studied several years ago.5 It has been proved that the initial adsorption on epitaxial Fe3O4 (111) film is dissociative. At higher coverages, formation of water dimers occurs, which then probably act as nucleation centers for three-dimensional ice formation.5 Joseph et al. measured water adsorption isobars on magnetite in the temperature range of 140-352 K.6 By cooling the system, three different regions, referred to as γ (OH- + H+), β1 (physisorbed water monomers), and R (ice) states, were distinguished with increasing surface coverage. In these three regions, the isosteric heat of adsorption values of 60-73 kJ/mol, 48-52 kJ/mol, and 38 kJ/mol, respectively, were determined. They found that, although at low coverage the magnetite surface interacts predominantly with the dissociated derivatives of water (i.e., OH- and H+), this is not reflected in the isosteric heat of adsorption values. This was explained by the slow recombinative desorption of OH- + H+, which is of second order kinetics with a very small frequency factor, probably caused by a sterically demanding transition state. They claimed that it is well possible that water forms reactive oxygen or hydroxyl species upon adsorption at Fe3O4 surfaces, and these species may take part in various reactions.6 The water uptake measurement was found to be very sensitive to sample preparation. The water adsorption capacity was found to be reduced by a factor of about two when a 3 orders of magnitude weaker vacuum was applied. Similar, but temperature-related sensitivity was experienced earlier by Zettlemoyer et al., who evacuated R-Fe2O3 at 10-6 Torr in the temperature range of 298-648 K for 48 h. The increased uptake above 373 K was attributed to chemisorption. They found that, on average, 5.6 OH groups were adsorbed on a surface area of 1 nm2.7 Experimental studies were also complemented by computer modeling several times. Dissociative adsorption of water molecules was recently simulated at the surface of different iron oxide/ hydroxides (i.e., white rust Fe(OH)2, goethite FeO(OH), and (2) Ahdjoudj, J.; Minot, C. Surf. Sci. 1998, 402, 104. (3) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurence and Uses; VCH Publishers: Weinheim, Germany, 1996. (4) Shinkareva, E. V.; Lazareva, T. G.; Shchurevich, O. A.; Kurakevich, L. A. Russ. J. Appl. Chem. 2005, 78, 1596. (5) Leist, U.; Ranke, W.; Al-Shamery, K. Phys. Chem. Chem. Phys. 2003, 5, 2435. (6) Joseph, Y.; Ranke, W.; Weiss, W. J. Phys. Chem. B 2000, 104, 3224. (7) McCafferty, E.; Zettlemoyer, A. C. J. Colloid Interface Sci. 1970, 34, 452.

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hematite R-Fe2O3).8 Kundu et al. applied an empirical force-field based classical static energy minimization method to model the inverse spinel magnetite surfaces.9 On these surfaces, the relative adsorption energies of the molecular and dissociated forms of water were calculated. Further, water adsorption was also calculated on hydroxylated surfaces, however, without comparing them to experimental data.9 Chamritski and Burns simulated magnetite, maghemite (γ-Fe2O3) and hematite to explain the differences in their Raman spectra.10 Rustad et al. performed molecular dynamics simulations of the interface of magnetite both with neat liquid water and with the 2.3 M aqueous solution of sodium perchlorate. They analyzed the surface interactions as well as the distribution of the electrolyte in the solution, and found that the density of the ions exhibits an oscillatory behavior along the interface normal direction, reflecting a similar layering structure of the solvent itself.11 Nevertheless, to the best of our knowledge, no simulation study of the adsorption isotherm and of multilayer adsorption of water has been reported so far, neither at the surface of magnetite, nor at that of other iron oxides. In determining adsorption isotherms at gas/condensed interfaces by computer simulation, the grand canonical Monte Carlo (GCMC) method12,13 is a particularly suitable tool. In a GCMC simulation, the chemical potential rather than the number of the adsorbate molecules is controlled, while the temperature of the system is kept constant. Thus, by systematically varying the adsorbate chemical potential in a set of simulations, one can calculate the full adsorption isotherm. The GCMC method has successfully been applied for simulating the adsorption isotherm of different molecules at the surface of carbonaceous materials,14-19 MgO,20 silica,21,22 various covalent organic frameworks,23 and ice.24-27 In this paper we present a thorough investigation of the adsorption of water at the surface of magnetite both by experimental and computer simulation (i.e., GCMC) methods. This combined experimental and theoretical approach aims at furthering our understanding of magnetite-water interactions beyond what can be achieved by either experiments or theory alone. Indeed, simulation results can be validated by comparing them to (8) de Leeuw, N. H.; Cooper, T. G. Geochim. Cosmochim. Acta 2007, 71, 1655. (9) Kundu, T. K.; Rao, K. H; Parker, S. C. J. Colloid Interface Sci. 2006, 295, 364. (10) Chamritski, I.; Burns, G. J. Phys. Chem. B 2005, 109, 4965. (11) Rustad, J. R.; Felmy, A. R.; Bylaska, E. J. Geochim. Cosmochim. Acta 2003, 67, 1001. (12) Adams, D. J. Mol. Phys. 1975, 29, 307. (13) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon: Oxford, 1987. (14) Muller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189. (15) Muller, E. A.; Hung, F. R.; Gubbins, K. E. Langmuir 2000, 16, 5418. (16) Stirolo, A.; Chialvo, A. A.; Gubbins, K. E.; Cummings, P. T. J. Chem. Phys. 2005, 122, 234712. (17) Moulin, F.; Picaud, S.; Hoang, P. N. M.; P
artay, L. B.; Jedlovszky, P. Mol. Simul. 2006, 32, 487. (18) Moulin, F.; Picaud, S.; Hoang, P. N. M.; Jedlovszky, P. J. Chem. Phys. 2007, 127, 164719. (19) Moulin, F.; Hoang, P. N. M.; Picaud, S.; P
artay, L. B.; Jedlovszky, P. Comp. Lett. 2008, 4, 105. (20) Daub, C. D.; Patey, G. N.; Jack, D. B.; Sallabi, A. K. J. Chem. Phys. 2006, 124, 114706. (21) Puibasset, J.; Pellenq, R. J. M. J. Chem. Phys. 2003, 118, 5613. (22) Puibasset, J.; Pellenq, R. J. M. J. Chem. Phys. 2005, 122, 094704. (23) Garberoglio, G. Langmuir 2007, 23, 12154. (24) Jedlovszky, P.; P
artay, L.; Hoang, P. N. M.; Picaud, S.; von Hessberg, P.; Crowley, J. N. J. Am. Chem. Soc. 2006, 128, 15300. (25) Hantal, Gy.; Jedlovszky, P.; Hoang, P. N. M.; Picaud, S. J. Phys. Chem. C 2007, 111, 14170. (26) Jedlovszky, P.; Hantal, Gy.; Neur
ohr, K.; Picaud, S.; Hoang, P. N. M.; von Hessberg, P.; Crowley, J. N. J. Phys. Chem. C 2008, 112, 8976. (27) Hantal, Gy.; Jedlovszky, P.; Hoang, P. N. M.; Picaud, S. Phys. Chem. Chem. Phys. 2008, 10, 6369.

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real experimental data; good agreement between these data sets can confirm the validity of the model used in the simulation. On the other hand, atomistic simulations can provide a deeper insight into the molecular level structure of the system investigated than any experiment alone could do.

2. Methods 2.1. Synthesis and Fundamental Characterization of the Magnetite Nanoparticles. Superparamagnetic magnetite nanoparticles of particle size below 10 nm were prepared by the coprecipitation method. Details of the preparation and characterization procedure can be found in a previous publication.28 In order to characterize the magnetite nanoparticles, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) measurements were performed. The XRD pattern of freshly prepared magnetite was taken by using a Philips PW 1830/PW 1820 X-ray diffractometer, operating in the reflection mode with Cu KR radiation. The Joint Committee on Powder Diffraction Standards (JCPDS) database was used to identify the characteristic peaks in the diffraction pattern. The HRTEM image was taken using a JEOL-3010 transmission electron microscope by applying an accelerating voltage of 300 kV. XPS spectra were recorded using an ultrahigh vacuum system, with a Specs Phoibos 150 MCD9 X-ray photoelectron spectrometer apparatus. The excitation source was the nonmonochromatic KR radiation of the Mg anode (hν = 1253.6 eV). The gun was operated at 225 W power (15 kV, 15 mA). The pressure during the spectrum acquisition in the analyzing chamber was less than 5
10-9 mbar. The C 1s binding energy of adventitious carbon was used as energy reference: it was taken as 285.1 eV. For data acquisition, SpecsLab2 software was used. 2.2. Water Vapor Adsorption Measurement. Water vapor adsorption/desorption isotherm was measured in automatic mode on a volumetric Hydrosorb apparatus (Quantachrome) at constant temperature with vapor generated at 373 K. Samples were outgassed at 5
10-9 mbar and 473 K for 20 h prior the adsorption measurement. The mass loss during the preparation was 14.5%. The isotherms were evaluated according to the standard gas adsorption models.29 The apparent surface area was derived according to the Brunauer-Emmett-Teller (BET) model, using 0.125 nm2 as the cross-sectional area of water.30 Micropore analysis was performed using the Dubinin-Radushkevich (DR) method, taking the scaling factor β to be 0.204. The isosteric heat of adsorption was deduced from the adsorption isotherms, obtained at 293, 303, and 313 K on fresh batches of magnetite, using the isosteric method.31 This analysis has the advantage that no assumptions on the kinetics of adsorption or desorption need to be made. All these calculations were performed using the Quantachrome software. 2.3. Monte Carlo Simulations. In order to gain molecular level insight into the structure of the adsorbed water, Monte Carlo simulations were performed on the grand canonical (μ,V,T) ensemble12,13 at 300 K (V and T being the volume and temperature of the simulated system, respectively). In these runs, the chemical potential μ of water was systematically varied, and the adsorption isotherm was calculated by determining the average (28) Tomb
acz, E.; Ill
es, E.; Majzik, A.; Hajd
u, A.; Rideg, N.; Szekeres, M. Croat. Chem. Acta 2007, 80, 503. (29) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (30) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577. (31) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999; p 44.

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Figure 1. Two pictures of the (011) surface of the magnetite crystal. Left: HRTEM image; right: snapshot taken out from the simulations. Iron and oxygen atoms are marked by orange and red colors, respectively.

number of water molecules present in the basic simulation box ÆNæ as a function of μ. Two sets of simulations were performed, considering both the (001) and (011) surface of the magnetite crystal. The length of the Z edge of the basic simulation box, being perpendicular to the magnetite surface, was set to 10 nm, whereas the XY face of the box was 3.354 nm
3.354 and 3.354 nm
3.557 nm in the case of the (001) and (011) surfaces, respectively, in accordance with the geometry of the magnetite crystal. The basic box contained 16 layers of magnetite, consisting of 512 Fe3O4 units in the case of the (001), and 576 Fe3O4 units in the case of the (011) surface. A snapshot of the (011) magnetite surface is shown in Figure 1 as taken out from the simulations. In the simulations, the fugacity f of water, related to the chemical potential through the equation f ¼

kB T expð -μ=kB TÞ Λ3

ð1Þ

was fixed, and varied from 10-20 to 1, increasing its value by a factor of 10 in each step, i.e., in each new simulation. (In the above equation, Λ stands for the thermal de Broglie wavelength, and kB is the Boltzmann constant.) In this way, the full set of simulations covered the μ range from -152.1 to -37.3 kJ/mol. The f and μ values corresponding to the simulations performed are summarized in Table 1. In calculating the energy of the system, magnetite-water and water-water interactions were taken into account, whereas the internal energy of the magnetite crystal was disregarded, as magnetite was kept unchanged during the entire course of the simulations. Besides the charge-charge interactions, the atoms of magnetite interact with water through a Lennard-Jones potential, the parameters of which were taken from the universal force field (UFF).32 Water molecules were described by the four-site, rigid TIP4P potential model.33 In calculating the Lennard-Jones interaction between an atom of the magnetite crystal and a water molecule, the Lorentz-Berthelot combination rule13 was used. In order to reliably describe the screening of the charges in the crystal phase, we used two sets of charges for the atoms of the magnetite crystal, and tested the obtained adsorption isotherm against the experimental curve. The first set of charges was estimated by a Bader population analysis, performed using the ABINIT software package.34 The electron density was calculated (32) Rapp
e, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (33) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (34) Gonze, X.; Beuken, J. M.; Caracas, R.; Detraux, F.; Fuchs, M.; Rignanese, G. M.; Sindic, L.; Verstraete, M.; Zerah, G.; Jollet, F.; Torrent, M.; Roy, A.; Mikami, M.; Ghosez, P.; Raty, J. Y.; Allan, D. C. Comput. Mater. Sci., 2002, 25, 478; URL: http://www.abinit.org.

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Table 1. Data of the Adsorption Isotherms of Water at the (001) and (011) Surfaces of Magnetite, as Obtained from the Simulations (001) surface f/bar

-1

μ/kJ mol

ÆNæ

p/p0

1.00
10-20 -152.1 5.46
10-19 -146.4 5.46
10-18 1.00
10-19 -140.6 5.46
10-17 1.00
10-18 -134.9 5.46
10-16 1.00
10-17 -129.1 5.46
10-15 1.00
10-16 -123.4 5.46
10-14 1.00
10-15 -117.6 5.46
10-13 1.00
10-14 -111.9 5.46
10-12 1.00
10-13 -106.1 5.46
10-11 1.00
10-12 1.00
10-11 -100.4 5.46
10-10 1.00
10-10 -94.7 5.46
10-9 1.00
10-9 -88.9 5.46
10-8 -83.2 5.46
10-7 1.00
10-8 -77.45 5.46
10-6 1.00
10-7 -71.7 5.46
10-5 1.00
10-6 -66.0 5.46
10-4 1.00
10-5 -60.2 5.46
10-3 0.0001a 0.00057 -55.9 3.11
10-2 -54.5 5.46
10-2 0.001a 0.0014 -53.6 7.64
10-2 -48.7 5.46
10-1 0.01a 0.1 -43.0 1 -37.3 a Systems in which water density profile is calculated.

(35) Sasaki, S. Acta Cryst. B 1997, 53, 762. (36) Causa, M.; Dovesi, R.; Pisani, C.; Roetti, C. Surf. Sci. 1986, 175, 551. (37) Russo, S.; Noguera, C. Surf. Sci. 1992, 262, 245. (38) Marmier, A.; Hoang, P. N. M.; Picaud, S.; Girardet, C.; Lynden-Bell, R. M. J. Chem. Phys. 1998, 109, 3245. (39) Barker, J. A.; Watts, R. O. Mol. Phys. 1973, 26, 789. (40) Neumann, M. J. Chem. Phys. 1985, 82, 5663.

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Γ/μmol m

0 0 0 0 0 0 0 1.62
10-6 2.22
10-5 0.000326 0.00195 0.0257 0.251 1.42 8.48 27.2 58.4 126 169 201 569 2460 2693

by performing a self-consistent calculation with ion cores fixed at the crystallographic positions. The results of this calculation showed that Fe and O atoms carried charges of 1.5|e| and -1.125|e|, respectively, in good agreement with the experimentally reported values.35 In the second set of charges, only half of these values were used, since it is known that the charges resulting from any kind of population analysis tend to considerably overestimate the actual electrostatic interaction. In fact, it has been observed in simulations of MgO surfaces that halving the charges obtained by ab initio calculations36 results in a good reproduction of surface properties when a tight-binding approach is used.37 Moreover, the use of half the ab initio charges has been shown to satisfactorily model water adsorption on the same surfaces.38 As it is discussed later in this paper, the second (i.e., “half”) set of charges led to an excellent reproduction of the experimental isotherm also in our case, which made any further refinement of the charges needless. In the following, we always refer to the simulations performed with this set of “half” charges, unless otherwise indicated. The interaction parameters of the potential models used in the simulations are summarized in Table 2. In order to speed up the simulations, the magnetite-water potential energy was precalculated on a cubic grid with 0.02 nm spacing, and interpolated using cubic spline during the simulations. This interpolation scheme introduces an error in the evaluation of the solid-fluid potential generally lower than 0.1%. All interactions were truncated to zero beyond the center-center cutoff distance of 1.67 nm. The long-range part of the electrostatic interactions was taken into account using the reaction field correction method13,39,40 under conducting boundary conditions, using the atomic truncation scheme of H€unenberger and van Gunsteren. This method satisfactorily reproduces the

(011) surface -2

0 0 0 0 0 0 0 1.2
10-7 1.64
10-6 2.4
10-5 0.000144 0.00190 0.0185 0.105 0.626 2.00 4.31 9.28 12.4 14.8 42.0

ÆNæ

Γ/μmol m-2

0 0 0 0 0 0 0 0 4.88
10-6 3.07
10-5 0.000384 0.00399 0.0252 0.290 1.66 10.7 37.1 68.0 114 129 388 2257 2740

0 0 0 0 0 0 0 0 3.39
10-7 2.14
10-6 2.67
10-5 0.000277 0.00176 0.0202 0.115 0.746 2.58 4.73 7.92 8.98 27.0

Table 2. Interaction Parameters of the Potential Models Used (σ, ε and q Being the Lennard-Jones Distance and Energy Parameters and Fractional Charges, Respectively) atom magnetite

Fe O

ε/kJ mol-1

σ/nm a

0.2594 0.3118a

q/|e|

a

0.75b -0.5625b

0.0543 0.251a

waterc

O 0.3154 0.648 -1.04 Md H 0.52 a Reference 32. b Values corresponding to the “half” charge set. c Reference 33. d Nonatomic site along the H-O-H bisector.

electrostatic interactions while minimizing the artifacts due to the finite cutoff.41 In the simulations, trial displacement, insertion, and deletion moves were performed with probabilities of 0.2, 0.4, and 0.4, respectively. In a water displacement step, a randomly chosen water molecule was randomly translated by no more than 0.02 nm and randomly rotated around a randomly chosen space fixed principal inertia axis by no more than 15. The trial displacement moves were accepted with the probability Pdis given by the standard Metropolis recipe:13 Pdis ¼ min½1, expð -ΔU=kB TÞ

ð2Þ

We obtained an acceptance ratio of the displacement trials of about 0.5 at the highest densities investigated. In a trial insertion move, a water molecule was randomly inserted into the fluid phase of the system. According to the standard GCMC algorithm, such moves were accepted with the probability Pins, given as13  Pins ¼ min 1,

Vf expð -ΔU=kB TÞ kB TðN þ 1Þ

 ð3Þ

Finally, in a particle deletion step a randomly chosen water molecule was considered for removal from the system, and deleted (41) H€unenberger, P. H.; van Gunsteren, W. F. J. Chem. Phys. 1998, 108, 6117.

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Figure 3. Oxygen 1s peak on the XPS spectra of the Fe3O4 Figure 2. XRD pattern of our magnetite sample. The Miller indices of the magnetite crystal corresponding to the separate peaks of the spectrum are also indicated.

nanoparticles (open circles). The oxygen 1s peak is deconvoluted to the Gaussian functions corresponding to the O2- (dashed line) and OH- (dash-dot-dotted line) contributions after background (dotted line) subtraction. The function fitting the data, resulting from this procedure, is shown by a solid line.

with the probability Pdel, given as13   kB TN expð - ΔU=kB TÞ Pdel ¼ min 1, Vf

ð4Þ

In these equations, ΔU is the change of the potential energy of the system accompanying the attempted move, and f is the fugacity defined in eq 1. The systems were equilibrated by performing 5 million Monte Carlo steps at each state point. Then, in the production stage of the runs, the number of water molecules adsorbed at the magnetite surface was averaged over 10 million configurations. Finally, at selected chemical potential values, 100 sample configurations per system, separated by 100 000 Monte Carlo steps each, were saved for further analyses.

3. Results and Discussion 3.1. Characterization of the Crystalline Phase and Surface of Magnetite. The XRD pattern of freshly prepared, purified and freeze-dried magnetite sample, presented in Figure 2, shows that crystalline magnetite phase is present dominantly.28 In general, TEM images show rounded crystals in the size range of 4-10 nm with nonuniform size distribution of the nanoparticles, which accords well with the general feature of synthetic magnetite samples published in the literature.1,28 The HRTEM image, shown in Figure 1 (left), can be identified with the surface of magnetite particles in the (011) direction (the contrast is given simultaneously by two (111) and one (002) type sheets). The properties of our magnetite sample were further analyzed by carrying out XPS analysis. Besides the oxygen peak, on which the present study focuses, the definite iron band and also some trace carbons are detected in the XPS spectrum. Figure 3 displays the typical XPS spectrum of the O 1s region. After subtracting the background, the obtained data has been fitted by multiple Gaussian functions. In this way, the oxygen 1s peak has been deconvoluted into two spectral bands at 529.9 eV and at 531.5 eV. The most intense peak at 529.9 eV is attributed to the lattice oxygens (O2-) in the magnetite crystal. The binding energy value of 531.5 eV is due to the hydroxile (OH-) groups located in the surface layer. It should be noted that the obtained energies differ somewhat from recently published values.1 Since the XPS measurement probes our sample over a depth of about a few nanometers beneath the surface, the intensity ratio of the two oxygen spectral bands characterizes the composition of this Langmuir 2009, 25(22), 13007–13014

Figure 4. Water vapor adsorption/desorption isotherm measured at the surface of magnetite nanocrystals at 293 K. The adsorption and desorption branches of the hysteresis loop are marked by circles and squares, respectively. The lines connecting the symbols are just guides to the eye.

surface layer of the samples. All the samples analyzed show similar surface oxide ion contents of 83 ( 3%, whereas the average amount of hydroxyl groups at the surface is found to be 17 ( 3%. The latter value reflects that the common pretreatment procedure cannot fully remove the surface OH groups, and hence they can exist even in high vacuum, confirming the dissociative chemisorption of the first layer water molecules, and indicating the strength of the surface OH bonds formed in this dissociative chemisorption process.5 3.2. Experimental Adsorption Isotherm of Water. The measured adsorption/desorption isotherm of water vapor is shown in Figure 4 in the Γ versus prel form, where Γ is the surface density of water, and prel = p/p0 is the equilibrium pressure p of water relative to that of the saturated vapor p0. The obtained isotherm belongs to the group of irreversible, type IV isotherms according to the IUPAC classification. The hysteresis loop observed at low pressures is typical of adsorption at metal oxide surfaces. The apparent surface area of our sample, deduced from BET analysis, is found to be 42.6 m2/g, falling in the range (i.e., 4-130 m2/g) reported in the literature.28 The nominal monolayer capacity has been derived from the adsorption curve, and it results in 13.2 μmol/m2. The volume of the micropores is found to be 0.011 cm3/g, whereas the adsorption energy turns out to be 66 kJ/mol from the DR plot. DOI: 10.1021/la901875f

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Figure 5. Isosteric heat of adsorption of water at the surface of magnetite nanocrystals at 298 K (as determined from the adsorption isotherms measured and 293 and 303 K, solid curve) and at 308 K (as determined from the adsorption isotherms measured and 303 and 313 K, dashed curve) as a function of the surface coverage of the water molecules. Simulation data obtained at the (001) and (011) surfaces are also indicated (filled and open circles, respectively).

The isosteric heat of adsorption was calculated by the isosteric method31 from adsorption isotherms measured at different temperatures on fresh batches of the magnetite sample. Figure 5 shows the isosteric heat of adsorption Hads as a function of the surface density of adsorbed water Γ in two temperature ranges, i.e., 293-303 K and 303-313 K. (These temperature ranges are sometimes referred to by the average values of 298 and 308 K, respectively). Three different regions of the obtained Hads(Γ) curves can be distinguished. At low surface coverages (i.e., up to the Γ value of 8-9 μmol/m2), the heat of adsorption is nearly directly proportional to the surface coverage. The slope of this region is 104 ( 4 kJ/mol/layer and 110 ( 5 kJ/mol/layer, and the highest Hads value is 74 and 93 kJ/mol in the lower and higher temperature range considered, respectively. Although the positive temperature sensitivity of the Hads(Γ) curves was previously assumed to be the sign of H2O chemisorption,6,7 the latter values measure the gross heat of the formation of the surface OH groups. The obtained Hads(Γ) curves decrease monotonously in the water surface coverage range of about 9-20 μmol/m2, whereas at higher coverages they converge to the coverage-independent values of 43.0 and 44.4 kJ/mol in the lower and higher temperature interval, respectively. These values, as expected, are slightly less than the heat of evaporation value of water of 45.7 kJ/mol at 298 K,42 and are consistent with similar values reported in the literature.3,6 3.3. Adsorption Isotherm Resulted from the Simulations. The adsorption isotherms obtained from the simulations both at the (001) and (011) surface with both charge sets are shown in Figure 6 in the ÆNæ versus μ form. As is seen, the isotherms calculated at the two different surfaces are reasonably similar to each other in both cases, the (001) surface being slightly more attractive for water than the (011) one. In the case of the “full” charge set, water adsorption already occurs at very low chemical potential values, i.e., around -150 kJ/mol. Then, in a broad range of chemical potential values, the water molecules form a rather stable adsorption layer, as the isotherm exhibits a plateau up to the μ value of about -75 kJ/mol. A similar, although considerably shorter plateau is also seen on the isotherms corresponding to the “half” charge set (see the inset of Figure 6). In this case, adsorption only starts around μ = -65 kJ/mol, and the adsorp(42) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990-1991.

13012 DOI: 10.1021/la901875f

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Figure 6. Adsorption isotherm of water at the surface of magnetite, as obtained from the simulations. Solid lines: adsorption at the (011) surface, using the “half” charge set; dash-dot-dotted lines: adsorption at the (011) surface, using the “full” charge set; dashed lines: adsorption at the (001) surface, using the “half” charge set; dotted lines: adsorption at the (001) surface, using the “full” charge set. The inset shows the same curves on a magnified scale. The arrows indicate the state points at which sample configurations have been dumped for more detailed analyses.

tion layer remains stable in a roughly 15 kJ/mol wide range of chemical potentials. Further increase of the chemical potential leads to a progressive thickening of the adsorption layer, and, finally, to the condensation of water. From the behavior of the obtained curves, the chemical potential value corresponding to the point of condensation, μ0, is estimated to be -47.2 kJ/mol. On the basis of the behavior of the obtained isotherms, we have chosen three chemical potential values, i.e., -60.2, -54.5, and -48.7 kJ/mol for more detailed analyses in the case of the “half charge” systems. 3.4. Comparison of the Experimental and Simulated Isotherms. In order to compare the calculated isotherms directly to the experimental data we have converted them from the ÆNæ versus μ to the more conventional Γ versus prel form. The surface density of water is simply calculated as Γ ¼

ÆNæ 2XY

ð5Þ

whereas prel is related to the chemical potential through the equation20,24 prel ¼

p expðμ=kB TÞ ¼ p0 expðμ0 =kB TÞ

ð6Þ

where μ0 is the chemical potential value corresponding to the point of condensation, determined to be μ0 = -47.2 kJ/mol from the ÆNæ versus μ form of the calculated isotherm. (In eq 5, the factor 2 in the denominator accounts for the two magnetite surfaces present in the basic simulation box.) Obviously, eq 6 can only be used in the vapor phase, i.e., for prel e 1, and thus the full ÆNæ versus μ isotherm has only been converted to the Γ versus prel form up to the point of condensation. The simulated and experimental isotherms are compared in Figure 7. It is evident that the “full” set of charges strongly overestimates the amount of adsorbed water at the entire prel range. On the other hand, the use of the “half” set of charges leads to a rather accurate reproduction of the experimental isotherm. In particular, the results obtained at the (011) surface Langmuir 2009, 25(22), 13007–13014

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Figure 7. Comparison of the experimental and simulated isotherms, presented in the Γ vs prel form. The notations are the same as in Figures 4 and 6.

of magnetite match almost perfectly with the experimental adsorption isotherm. The data obtained at the (001) surface somewhat exceeds this curve; however, it remains close to the desorption branch of the experimental hysteresis loop in the full prel range studied. The agreement of the simulated and experimental curves corroborates the use of the “half” charge set, and provides some confidence in interpreting the results of these simulations. Besides the adsorption isotherm, we have also calculated the isosteric heat of adsorption of water at the simulated (001) and (011) magnetite surfaces using the “half” charge set, in order to compare these values to the experimental data. The isosteric heat of adsorption Hads has been calculated according to the fluctuation formula43 H ads ¼ RT -

ÆUNæ - ÆUæÆNæ ÆN 2 æ - ÆNæ2

ð7Þ

where the angled brackets Æ...æ denote ensemble averaging, U stands for the potential energy of the system, and R is the gas constant. The calculated Hads values, also shown in Figure 5, agree reasonably well with the experimental data beyond the first peak, i.e., at surface coverages larger than what corresponds to the first, chemisorbed monomolecular water layer, indicating that the simulation results represent a reliable model of the adsorbed water beyond this first molecular layer. 3.5. Characterization of the Adsorption Layer. As is seen from Figure 7, both the calculated and the experimental isotherms increase monotonously even at high prel values instead of reaching a constant plateau value. The observed non-Langmuir behavior of the obtained isotherms is related to the development of multilayers by strong, hydrogen bonding interaction of the adsorbates. The strong hydrogen bond formation ability of the water molecules indicates that their adsorption layer might be considerably thicker than the monomolecular water layer. This view is supported both by the steepness of the obtained isotherms (the surface density of water increases by a factor of 3-4 when prel increases from 0.05 to 0.5, see Table 1), and also by former results obtained for water adsorption at various other (e.g., soot) surfaces.18,19 In order to further investigate this point, we calculated the density profile of the adsorbed water molecules along the surface normal axis Z in the systems of the three selected chemical potential values. In these calculations, the basic simulation box was divided into 0.02 nm wide slabs along the Z axis, and (43) Nicholson, D.; Parsonage, N. G. Computer Simulation and the Statistical Mechanical Theory of Adsorption; Academic Press: London, 1982.

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Figure 8. Density profile of the adsorbed water molecules along the surface normal axis Z at the (001) (top panel) and (011) (bottom panel) surfaces of magnetite in the systems simulated at the chemical potential values of -60.2 kJ/mol (dotted lines), -54.5 kJ/mol (dashed lines), and -48.7 kJ/mol (solid lines). All profiles shown are averaged over the two surfaces present in the basic simulation box. The dashed vertical lines illustrate the boundary of the consecutive molecular layers in the adsorption layer. The magnetite crystal extends up to the Z value of about 1.7 nm.

the number of water O atoms was counted in each slab. The obtained density profiles, averaged over the two surfaces present in the basic box, are shown in Figure 8. As is seen, the water molecules indeed adsorb in several molecular layers. Thus, although at the chemical potential value of -60.2 kJ/mol (i.e., at prel ≈ 0.005) the adsorbed water molecules are exclusively located in the first molecular layer, at μ = -54.5 kJ/mol (i.e., at prel ≈ 0.05) the second molecular layer is already started to be built up, and in the case of the (001) surface, even traces of the third molecular layer appear. Finally, at the highest (i.e., lowest in magnitude) chemical potential considered (i.e., μ = -48.7 kJ/mol; prel ≈ 0.5), the thickness of the adsorbed water layer is about 1.0 nm, corresponding to about 4-5 molecular layers of water. This adsorption layer is rather highly ordered, as seen from the fact that peaks of the density profile corresponding to the consecutive molecular layers can be clearly distinguished, as is illustrated in Figure 8. Finally, it should be noted that, with increasing chemical potential, the amplitude of the density peaks that are already present also increases. Thus, upon increasing the chemical potential from -60.2 to -48.7 kJ/mol (and, hence, prel from about 0.005 to 0.05) the first density peak, located at about 1.9 nm becomes progressively higher. Similarly, the density peaks corresponding to the second and third molecular layers are clearly higher in the system of μ = -48.7 kJ/mol (prel ≈ 0.5) than at μ = -54.5 kJ/mol (prel ≈ 0.05). These findings indicate that the adsorption layer of the water molecules not only consists of several molecular layers, but the building up of the outer molecular layers clearly starts far before the inner layers are completed. DOI: 10.1021/la901875f

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4. Summary and Conclusions

Figure 9. Two instantaneous equilibrium snapshots of the water molecules that are adsorbed at the (001) surface of magnetite, as taken out from the simulation performed at the water chemical potential value of -48.7 kJ/mol. Iron, oxygen and hydrogen atoms are marked by orange, red, and white colors, respectively.

Furthermore, it is also clear from Figure 8 that the densities of the consecutive molecular layers are considerably lower than that of bulk liquid water (i.e., 1.00 g/cm3) even at relatively high pressures. Thus, even in the case of μ = -48.7 kJ/mol (prel ≈ 0.5) the density of the second molecular layer is only about half of that of bulk liquid water, in spite of the fact that traces of the fifth and sixth molecular layers of adsorbed water are already present at the magnetite surface. This finding is illustrated in Figure 9, showing two instantaneous equilibrium snapshot of the water layer adsorbed at the (001) surface of magnetite, as taken out from the μ = -48.7 kJ/mol (prel ≈ 0.5) simulation. Considering also the fact that the main driving force of the physisorption of water is the formation of new water-water hydrogen bonds (i.e., between the water molecules to be adsorbed and the ones that already belong to the adsorption layer), we can assume that, in this stage, the adsorption of water follows a nucleation-like mechanism. Thus, at those parts of the surface where the water density is already high, a new water molecule can find more possibilities for hydrogen bonding, and hence adsorbs with a higher probability than at positions of low water content. This procedure results in an increasingly rough outer surface of the adsorption layer, as it is clearly illustrated in Figure 9.

13014 DOI: 10.1021/la901875f

In this paper we presented a combined experimental and computer simulation study of the adsorption of water vapor at the surface of magnetite nanocrystals. This study clearly demonstrates the power of combining experimental methods with computer simulation investigations. Although the rigid, classical potential models used in the simulations are obviously unable to describe chemical reactions, such as the dissociative chemisorption in the first layer of water at the magnetite surface, we presented a model that reproduces very well the measured adsorption isotherm and also reasonably well the isosteric heat of adsorption beyond this first, monomolecular layer. Therefore, apart from the first water layer that is in direct contact with the magnetite surface, the simulation results can be regarded as a reliable model describing the adsorption process and the structure of the adsorption layer. Considering the properties of the first molecular layer, on the other hand, results of the various experiments performed can provide important information. Thus, the dissociative chemisorption of the water molecules at the bare magnetite surface is evidenced both by the XPS spectrum and by the surface coverage and temperature dependence of the isosteric heat of adsorption. The gross heat of this chemisorption resulted in about 74 - 93 kJ/mol in the temperature range of 293-313 K. The water adsorbed at the surface of magnetite is found to be arranged in a rather strict layering structure; in the simulations performed at relatively high pressures (prel ≈ 0.5), five separate molecular layers of the adsorbed water can be distinguished. However, in spite of this layered structure, the building up of the adsorption layer does not occur by a molecular layer-by-layer mechanism; instead, several molecular layers build up simultaneously. In this pressure range (i.e., above prel ≈ 0.01), the adsorption process is clearly governed by the formation of new water-water hydrogen bonds. This process seems to follow a mechanism similar to nucleation: new water molecules are attached to those parts of the surface that already accommodate a large amount of water, resulting thus in a rather rough outer surface of the adsorption layer. Acknowledgment. This project is supported by the Hungarian OTKA Foundation under Project Nos. 75328 and A7-69109, and by the bilateral collaboration program between MTA (Hungary) and CNR (Italy). P.J. is a Bolyai J
anos fellow of the Hungarian Academy of Sciences, which is gratefully acknowledged. The authors thank J
anos L
ab
ar for taking the HRTEM image. The Monte Carlo simulations were performed on the HPC facility WIGLAF at the Physics Department of the University of Trento.

Langmuir 2009, 25(22), 13007–13014