Design of Optimally Stable Molecular Coatings for Fe-Based

Aug 11, 2017 - Magnetic nanoparticles are widely used in biomedical and oil-well applications in aqueous, often harsh environments. The pursuit for hi...
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Design of Optimally Stable Molecular Coatings for Fe-Based Nanoparticles in Aqueous Environments Sebastian Zuluaga,*,† Priyanka Manchanda,† Yu-Yang Zhang,† and Sokrates T. Pantelides*,†,‡ †

Department of Physics and Astronomy and ‡Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *

ABSTRACT: Magnetic nanoparticles are widely used in biomedical and oil-well applications in aqueous, often harsh environments. The pursuit for high-saturation magnetization together with high stability of the molecular coating that prevents agglomeration and oxidation remains an active research area. Here, we report a detailed analysis of the criteria for the stability of molecular coatings in aqueous environments along with extensive first-principles calculations for magnetite, which has been widely used, and cementite, a promising emerging candidate. A key result is that the simple binding energies of molecules cannot be used as a definitive indicator of relative stability in a liquid environment. Instead, we find that H+ ions and water molecules facilitate the desorption of molecules from the surface. We further find that, because of differences in the geometry of crystal structures, molecules generally form stronger bonds on cementite surfaces than they do on magnetite surfaces. The net result is that molecular coatings of cementite nanoparticles are more stable. This feature, together with the better magnetic properties, makes cementite nanoparticles a promising candidate for biomedical and oil-well applications.

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nanoparticles. For example, Mikelonis et al.28 used the DLVO theory to study the aggregation of coated Ag nanoparticles on top of ceramic membranes. An issue related to colloidal stability is the bonding of the molecules to the surface of the nanoparticle. The colloidal stability of the nanoparticles may be compromised if the water itself or ions in the aqueous environment, for example, H+, Na+, and so forth, can cause the molecules to desorb over time. In this context, the strength of chemical bonds between the coating molecules and the nanoparticles has received considerable attention.29−32 However, investigations of the bond strength in aqueous environments are rather limited in scope.33 Magnetite (Fe3O4) nanoparticles, depending on the synthesis method, exhibit a saturation magnetization value in the range of 30 and 50 emu/g1,34,35 to 78 emu/g,36−39 which is 80% of the saturation magnetization in the bulk. On the other hand, experiments have shown that cementite (Fe3C) nanoparticles exhibit higher saturation magnetization as compared to that of magnetite nanoparticles, up to a value of 140 emu/g.40−46 This feature makes cementite nanoparticles a promising alternative to magnetite for the mentioned applications. Even though the electronic and magnetic properties of cementite have been widely studied by the theory community,47−52 the theoretical

INTRODUCTION Magnetic nanoparticles have recently attracted significant attention in scientific and industrial communities due to their use in the fields of catalysis,1,2 spintronics,3−6 and biomedical applications.4,7−11 More recently, such nanoparticles have also been investigated for oil and gas applications, such as enhanced oil recovery and reservoir characterization.12 Nanoparticles can flow through micron-size pores across long distances in the reservoir rocks, and their nonzero magnetic permeability acts as a contrast agent. Therefore, tracing these contrast agents using electromagnetic (EM) tomography technology is a promising technique to characterize the reservoir and enhance oil recovery.12,13 The magnetic properties of these nanoparticles, such as superparamagnetic behavior and high magnetic susceptibility, depend on their size, shape, and crystallinity.14−16 Nanoparticles that are used in the fields mentioned earlier need to be highly water-dispersible, monodisperse, have a large saturation magnetization, remain stable, that is, retain their properties in pertinent harsh environments. Accordingly, the synthesis of high-quality nanoparticles that are dispersible in water17−23 has been studied widely, finding that the coating ligands play a crucial role in the colloidal stability of nanoparticles. For example, Milowska et al.24 demonstrated that ligand−ligand interactions govern nanoparticle agglomeration (Au nanoparticles). At the core of the colloidal stability, the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory offers a classical explanation of colloids in suspension and has been used to predict and explain25−27 the interaction among © 2017 American Chemical Society

Received: June 9, 2017 Accepted: July 28, 2017 Published: August 11, 2017 4480

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Figure 1. (a−d) Molecules under consideration. (e−h) Lowest energy configuration adsorption geometries on cementite (001) and magnetite (001) surfaces (cementite on the left and magnetite on the right). Upon adsorption, some molecules lose all or some H atoms, depending on the surface on which they are adsorbed. (i) Top views of the cementite (001) surface (left) and the magnetite (001) surface (right). Primary and secondary adsorption sites are marked with letters p and s, respectively. In green, we show the distance between atoms or adsorption sites. In panels (a)−(i) the red, green, black, white, purple, and blue spheres represent the O, B, C, H, P, and Si atoms, respectively.

adsorbed molecules: the H+ ion attaches to the departing molecule, whereas a H2O molecule attaches to the surface. Furthermore, our results show a direct relation between the O− Fe bond strength (the bond between the adsorbed molecule and the surface) and the electronegativity of certain atoms in the molecule.

aspects of molecule desorption from cementite surfaces in aqueous environments, which are critical for the stability of nanoparticles, have not been investigated. In this article, we focus on the issue of the stability of the chemical bonds between the coating molecules and the nanoparticles in an acidic aqueous environment, which is common in oil wells. We will use quantum mechanical calculations to investigate possible desorption processes of several molecular species adsorbed on magnetite and cementite nanoparticles in aqueous environments and examine both the binding and desorption mechanisms. The results indicate that molecules bonded to cementite surfaces have a higher stability than that of the same molecules bonded to magnetite in aqueous environments, making cementite a preferred material over magnetite, especially for oil-well applications. More specifically, we investigated the effects of H+ and H2O on the desorption of four molecules (acetic acid, boronic acid, ethyl phosphate, and ethyl trihydroxy silane) from cementite and magnetite (001) surfaces. It is important to mention that we are not interested in the hydrophobic or hydrophilic properties of the molecules under consideration. We are only interested in the nature of the bond (molecule−nanoparticle) and how this bond is affected by the aqueous environment. Our results indicate that the molecules bind more strongly to cementite, mainly because the interatomic distances allow more bonds to form between the molecule and the surface. We also find that whereas water molecules by themselves compete weakly for the adsorption sites occupied by coating molecules, H+ ions and H2O molecules together enhance the desorption of the

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RESULTS This work is based on the following three questions: What are the key elements ruling the bond between the molecules and the nanoparticles? What role do the H2O molecules and H+ ions play in the desorption of the molecules from the surface of the nanoparticle? What type of molecule exhibits the largest stability on the surface of the nanoparticles under liquid conditions? We start by pointing out that the molecules used to coat the nanoparticles usually have long tails that create a “shell” that protects the nanoparticle. However, according to test calculations, the length of the tail has a negligible role in the binding of the molecule to the surface of the nanoparticle. For example, as the tail of the acetic acid increases by one, two, and three C atoms, the bond strength changes by only 0.07 eV. Therefore, we can conclude that the bond strength is not sensitive to the length of the tail and we can use molecules with short tails. In this way, we are able to lower the computational cost, without compromising the accuracy. Nevertheless, there might be other effects arising from long tails, having to do with coiling, that we will not discus as they fall outside the scope of this article. The four molecules considered in this work are 4481

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higher bond strength over that of the acetic and boronic acid molecules on the magnetite surface, as seen in Table 1. The reason behind the difference in the binding geometry lies in the crystal structure of the two surfaces, as shown in Figure 1i. In the case of cementite, the distance between two neighboring primary adsorption sites (atoms labeled with letter “p”) is 2.5 Å, and the next primary or secondary (atoms labeled with letter “s”) available adsorption sites are located 2.5 or 4.2 Å away (see the left hand side of Figure 1i). Because the distance between the atoms in the ethyl phosphate and ethyl trihydroxy silane molecules is 2.6 and 2.8 Å, both molecules are able to stretch and bind in a tridentate fashion on the cementite (001) surface. In contrast, for the magnetite surface, the distance between two neighboring primary adsorption sites is 2.9 Å, but the next available adsorption site is located 5.9 Å away (see the right hand side of Figure 1i). Neither the ethyl phosphate nor the ethyl trihydroxy silane molecules are able to stretch that much. Therefore, these two molecules can only bind to the magnetite surface in a bidentate fashion. A similar situation is observed in other surface terminations of cementite and magnetite; see the Supporting Information. On the other hand, it is surprising to notice that even though the acetic and boronic acid molecules are very similar (differing only in one atom) the bond strengths of these two molecules are very different. For example, when the molecules are adsorbed on magnetite, the bond strength differs by 1.40 eV, whereas when they are adsorbed on cementite, the bond strength differs by 2.37 eV (see Table 1). A similar feature is seen between ethyl phosphate and ethyl trihydroxy silane. These results lead us to believe that it is possible to increase or decrease the O−Fe bond strength by simply changing one atom in the molecule that does not play a direct role in the bonding with the surface. With this idea in mind, we took the acetic acid molecule and constructed two more toy systems where C is replaced by N and Al. In the same way, the P atom in the ethyl phosphate molecule was replaced by Si, S, and Al. Then, the bond strengths, to the cementite (001) surface, were calculated using eq 1. The results indicate that there is a clear correlation between the bond strength (the O−Fe bond) and the electronegativity (by Pauling scale) of the substituted atom, as shown in Figure 2a: as the electronegativity of the atom increases, the bond strength decreases. Analogous results were found for the magnetite surface. This is an important result because it allows us to identify molecules that would establish the strongest bond with the surfaces by simply looking at the properties of the atoms. Similar results have been obtained for the adsorption of benzene rings on graphene.53 However, in that particular case, the binding is mainly due to van der Waals interactions and the differences in binding energies are very small. Desorption of Molecules under Liquid Conditions. Having examined the nature of the bond between the molecules and the nanoparticle surfaces, we focus on how a liquid environment affects the desorption of the molecules. The first thing to take into account is that the nanoparticles are floating in liquid water that contains H+ ions, typically in the form of H3O+ ions, which affect the desorption process. The desorption of the molecules assisted by water and hydrogen takes place in the following way: when an H+ ion arrives at an adsorbed molecule, it may bind to the molecule, which weakens the bonding to the surface. Water molecules are always plentiful and therefore handy to assist by taking the molecule’s place on the surface. Thus, in a cooperative fashion, the desorption of an

shown in the upper panels of Figure 1. It is important to note that we have chosen molecules that bind to the surfaces either in a bidentate (acetic acid and boronic acid) or a tridentate (ethyl phosphate and ethyl trihydroxy silane) fashion. We first identify the most stable adsorption configuration for each of the molecules, and these are shown in the middle panels of Figure 1. Even though in all cases it is energetically favorable to donate the hydrogen atoms to the surface (dissociative adsorption), the figure shows that in the cases of ethyl phosphate and ethyl trihidroxy silane adsorption on magnetite, the molecules retain one hydrogen. The reason behind it is that these two molecules are not able to bind in a tridentate fashion to the magnetite surface. We will discuss this point further later in the text. Bond Strength. Once we have found the most stable configurations, we can move to the first goal, which is to understand the key elements ruling the bond strength between the molecule and the surface of the nanoparticle. To do this, we focus on how much energy is needed to desorb the molecule from the surface in a vacuum. This can be achieved using the following equation E B = Esurf + Emol − Emol − surf

(1)

where Esurf, Emol−surf, and Emol denote the energies of the relaxed surface, the molecule adsorbed on the surface (as shown in Figure 1e−h), and the molecule in vacuum, respectively. We will refer to this energy as the bond strength. Using eq 1, we tabulated the bond strength between the molecules and the surfaces (cementite and magnetite) in Table 1. This table shows that whereas water molecules establish weak Table 1. Bond Strength for the Four Molecules and the (001) Surfaces of the Two Nanoparticles under Considerationa bond strength

a

molecule

Fe3C

Fe3O4

acetic acid boronic acid ethyl phosphate ethyl trihydroxy silane water molecule

3.75 6.12 6.65 9.29 0.58

3.00 4.40 3.72 5.20 0.72

All energies are in eV.

bonds (0.58 and 0.72 eV) with the cementite and magnetite surfaces, the other molecules establish strong bonds (between 3.00 and 9.29 eV). Therefore, we can conclude that water molecules are not able to displace the adsorbed organic molecules from the surface of either nanoparticle. Table 1 also shows that for all cases, except water, the bond strength of molecules is larger in cementite than that of their counterparts in magnetite. However, more interesting is the fact that in the case of cementite the bond strength of ethyl phosphate and ethyl trihydroxy silane is considerably higher than the bond strength of acetic acid and boronic acid. This trend can be easily explained by looking at Figure 1e−h and noticing that these two molecules (ethyl phosphate and ethyl trihydroxy silane) bind to the cementite surface in a tridentate fashion, whereas the other molecules (acetic and boronic acid) bind to the surface in a bidentate fashion. In contrast, Figure 1 also shows that all of the molecules bind in a bidentate fashion on the magnetite surface. As a result, the ethyl phosphate and ethyl trihydroxy silane molecules do not exhibit a considerably 4482

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Figure 2. (a) Bond strength vs electronegativity. The figure shows the bond strength of two types of molecules where one atom is being replaced (black and red curves), as a function of electronegativity (using the Pauling scale). The figure shows the two molecules and the atom being replaced. (b) Desorption energies, of the four molecules under consideration, as a function of μH (and pH) for cementite and magnetite surfaces.

adsorbed organic molecule can be mediated by H+ ions and H2O molecules. The process is thermodynamic in nature: the initial state is an adsorbed organic molecule and a reservoir of water molecules and H+ ions that defines their chemical potentials. The final state is a desorbed hydrogenated organic molecule, floating in water, and H2O molecules adsorbed in the molecule’s place. Defining the problem in this fashion, we can calculate the total energies of the initial and final states, using appropriate chemical potentials to ensure that the two states have precisely the same number of atoms. In this way, we get a lower bound for the desorption energy because the kinetic details may entail energy barriers that raise the net energy cost for desorption. The number of water molecules involved in the reaction described above depends on the binding configuration of the molecules. For molecules that bind in a bidentate or a tridentate fashion, two and three water molecules are involved in the desorption process, respectively. On the other hand, the number of hydrogen atoms involved in the reaction depends on the pH of the system. In a system of pH 6 (slightly acid environment), there is one H3O+ ion per 16.65 nm3 (roughly a cube of side 100 nm, which is roughly of the same order of magnitude as the diameter of a nanoparticle). Under these circumstances, the chance of having two hydrogen atoms assisting the reaction is very low (second-order reaction). However, as the pH of the system decreases, the probabilities of having more than one hydrogen assisting the desorption process increases. For now, we will consider only the case where one hydrogen assists the desorption of the molecules (first-order reaction). In the desorption process under liquid conditions, there will be an exchange of water and hydrogen atoms, as explained earlier. Therefore, we need to know the energy required to withdraw water and hydrogen atoms from the water reservoir. These energies are chemical potentials μH and μH2O. The desorption energy of the molecules under liquid conditions is then defined by

This equation assumes that, because the body of the molecule is surrounded by water molecules both before and after desorption, we only need to account for the changes that occur: the head of the molecule is initially attached to the surface, but after desorption, the head of the molecule interacts with a layer of water molecules, whereas water molecules take the place of the molecule on the surface. In the equation, m denotes the number of water molecules that are adsorbed at the newly available adsorption sites left behind by the desorbed molecule. Emol−surf, Esurf−mH2O, and Emol−H−lH2O denote the energy of the surface with the molecule adsorbed on it, the energy of the surface with m H2O molecules adsorbed on it, and the energy of the desorbed molecule hydrogenated with one extra hydrogen atom and l water molecules interacting with the molecule’s head, respectively. In the case of acetic and boronic acids, l = 2, whereas in the case of ethyl phosphate and ethyl trihydroxy silane, l = 3. It is important to note that under this convention, a positive desorption energy indicates the energy that one needs to provide to the system to desorb the molecule from the surface with the help of water molecules and one hydrogen atom coming from the water reservoir. It is important to point out that under our sign convention, the μH and μH2O values are negative. Their absolute values denote the energy needed to withdraw a hydrogen and a water molecule from the reservoir. Now we need to address the issue of how to calculate μH and μH2O. μH2O is calculated by constructing a system with 32 H2O molecules in a box of 990 Å3 (the density of water is 1 g/cm3) and dividing the total energy by 32. We find μH2O = −14.63 eV. For μH, we note that this is the chemical potential of a neutral hydrogen atom, not of a H+ ion. For our purposes, it would be perfectly fine to present our results as a function of μH, which we in fact do in Figure 2b. We will discuss these results shortly, but because the reservoir contains H+ ions, it is useful first to discuss how we can relate μH to μH+ because the latter is related to the pH of the reservoir by54 μH+ − μH0+ = −pH × 2.303kBT

E D = Esurf − mH2O + Emol − H − l H2O − (Emol − surf + μH + (m + l ) × μ H O ) 2

(3)

Here, μ0H+ is the proton chemical potential at standard state (T = 25 °C and a H+ concentration of 1 mol/L), kB is the Boltzmann constant, and T is the temperature. We can

(2) 4483

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Figure 3. Electronic charge, Q(R), as a function of the radius, R, of concentric spheres about the nucleus of H atoms (a) and oxygen atoms (b) calculated using the self-consistent valence pseudo-charge density of different species. Note that for H2 in vacuum the total valence charge in a sphere of 0.30 Å is only 0.15 electrons, which suggests that the bulk of the two electrons in the molecule resides in the bond region, namely, the region at the mid-point of the interatomic separation of 0.74 Å. That bond charge is outside the spheres we employ here to compare H in different environments. The same is true for O−H bonds. For these bonds, the charge in the bond regions cannot be uniquely divided into the H and O charge. Such a division of the total charge is only possible by phenomenological schemes, which, by construction, assign a positive charge to H and a negative charge to O atoms.

shown that the self-consistent charge on “ions” in so-called ionic crystals is also essentially zero,55,56 even though phenomenological charges are by construction nonzero; the “oxidation state” reflects only orbital occupancies, not physical charge). We emphasize that the connection between μH and pH has no bearing on the principal results of the present article, namely, the relative desorption energies of the molecules under study, which we shall now describe. We will present the hydrogen- and water-assisted desorption energies as functions of μH and use the connection we just established between μH and pH to express the desorption energies also as functions of pH. Hydrogen- and water-assisted desorption energies of the four molecules under consideration, calculated using eq 2, are shown in Figure 2b. Comparing with the values of bond strengths given in Table 1, it is clear that the hydrogen- and waterassisted desorption energies are significantly smaller. Nevertheless, the general trend found in the bond strength is maintained, that is, the acetic acid is the molecule that requires the least amount of energy to be desorbed, whereas ethyl trihydroxy silane requires the largest amount of energy to be desorbed. Furthermore, the trend is maintained regardless of the pH value. It is also important to point out that the desorption energy of acetic acid reaches and maintains a value of 0 for μH values above −3.23 eV (below pH = 10) for cementite, whereas for magnetite, it maintains a value of 0 eV for the range shown in Figure 2b, that is, the desorption of the molecules does not require energy. This is an important result because it tells us that the key element governing the desorption energy is the bond strength, which we have shown to be tunable in principle by exchanging specific atoms in the molecule, as shown in Figure 2a. The results presented here allow us to conclude that the molecules coating the cementite nanoparticles are more stable, under liquid conditions, than their counterparts coating the magnetite nanoparticles. Furthermore, we have found that the ethyl trihydroxy silane molecule is the most stable one. This conclusion is based on two results: (1) the bond strength and desorption energies are larger for the cementite surface than for the magnetite surface and (2) the geometry of the cementite

evidently write μH = μH+ + μe, where μe is an effective average chemical potential of electrons in the reservoir. Instead of attempting a calculation of μe, we proceed as follows. We added an extra water molecule in our 32-molecule supercell and dissociated it into an H+ ion and a OH− ion, placed as far apart as possible. The system is then relaxed, whereby H+ binds to a nearby water molecule, producing H3O+. We note that the plus and minus superscripts are nominal. A self-consistent calculation was performed to obtain the electronic density ρ(x) in the entire supercell. This electronic density is obtained from pseudo-wave-functions, but it is adequate for our purposes. We used this electronic density to investigate the magnitude of the physical charge on each species by integrating ρ(x) in concentric spheres of radius R about each nucleus and plotting the resulting charge Q(R) as a function of R. For comparison, we also calculated Q(R) for H in an H2 molecule in vacuum and for O in an O2 molecule in vacuum because there is no doubt that H and O in these molecules are electrically neutral. The results are shown in Figure 3. One surprising result in Figure 3b is that the physical charge in the inner core of the oxygen species in OH, H2O, and H2O3 in the liquid environment are essentially identical to the physical charge in the inner core of oxygen in the O2 molecule in vacuum, that is, the O species in a liquid environment are essentially electrically neutral. At the same time, the physical charge on H in OH matches the physical charge on H in the H2 molecule in vacuum (Figure 3a), that is, H in OH in a liquid environment is essentially neutral. It follows that the entire OH is essentially electrically neutral. We might then conclude that the compensating H3O species is also electrically neutral. The curves in Figure 3a, however, indicate that the charge on hydrogen atoms on water molecules and on H3O in a liquid environment is ∼0.02 electrons smaller than that of a neutral H in a sphere of radius 0.30 Å (at longer distances, the charge is strongly influenced by the neighboring atoms). For our purposes, we can infer that the nominally H+ ions (H3O+ ions) are essentially neutral, whereby the effective μe that we need to get μH is essentially zero, that is, μH ≃ μH+. Apparently, only a minute residual charge on the ions is responsible for the observed ionic conductivity even in pure water (it has been 4484

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molecule assisted by more than one hydrogen atom, which in turn reduces the desorption energy. These results give us one more evidence that the coated cementite nanoparticles exhibit a higher stability than that of the magnetite counterparts under liquid conditions.

surface allows the molecules to bind to the surface in a bidentate and tridentate fashion, whereas the geometry of magnetite allows the molecules to bind only in a bidentate fashion. These results were obtained using the (001) surface termination of cementite and magnetite, which is the most stable surface termination in both cases. However, this is also true for other surface terminations (see also Supporting Information). Role of Excess Hydrogen in the Desorption Process. Until now, we have shown that molecules bonded to cementite exhibit larger desorption energies than those of their counterparts adsorbed on magnetite. Also, we have shown that as the pH decreases, the desorption energy decreases. Now, we will take a closer look at how the bond strength is affected by hydrogenation of the adsorbed molecules. We considered the case where the molecules have been hydrogenated by one hydrogen atom. We then used an equation analogous to 1, taking into account the extra hydrogen atoms in the initial and final states, to find the bond strength between the hydrogenated molecules and each of the two surfaces (cementite and magnetite (001)). These numbers can be found in Table 2,

a

hydrogen atom binding energy

molecule

Fe3C

Fe3O4

Fe3C

Fe3O4

acetic acid boronic acid ethyl phosphate ethyl trihydroxy silane

0.58 3.59 4.19 6.81

0.90 3.49 2.37 4.12

0.00 0.56 0.53 0.74

1.30 1.20 1.72 1.53

CONCLUSIONS

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COMPUTATIONAL DETAILS

In this work, we have analyzed the desorption of four molecules from the cementite and magnetite surfaces under liquid conditions. The geometry of the cementite structure allows for the tridentate binding of the molecules on all of the studied surface terminations. In contrast, there are several surface terminations for the magnetite structure that do not allow the binding of molecules in a tridentate fashion. As a result, the desorption energies of molecules found for the cementite surfaces are higher than the ones found for magnetite. This allows us to conclude that the coated cementite nanoparticles exhibit a higher stability than that of the magnetite counterparts under liquid conditions. Furthermore, our results indicate that ethyl trihydroxy silane molecules are the best candidate to coat the cementite nanoparticles because they exhibit the largest desorption energies under liquid conditions. The results of the calculations indicate that the hydrogen atoms and water molecules play a key role assisting the desorption process. Although both of them reduce the desorption energy of the adsorbed molecules, H also weakens the bond strength between the molecules and the surfaces. Finally, the results indicate that the strength of the bond, between the adsorbed molecule and the magnetic surface, depends on the electronegativity of certain atoms in the molecule, which are not directly involved in the bond. This feature allows for additional tuning of the desorption energies to get the highest possible stability.

Table 2. Bond Strength between the Hydrogenated Molecules and the Cementite and Magnetite (001) Surfaces (Second Column); Binding energy of the hydrogen atom to the adsorbed molecules (third column)a hydrogenated molecule bond strength

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Units are in eV.

where we show that the bond strengths of the hydrogenated molecules are significantly lower than the ones of the nonhydrogenated molecules; see Table 1. In conclusion, hydrogen atoms weaken the bonds between the molecules and the magnetic surface. The results shown above lead us to the following question: is it possible that in a slightly acidic environment, say pH = 6 (μH = −2.97), an adsorbed molecule gains hydrogen atoms over time and then desorbs from the surface hydrogenated by two or three hydrogens? To answer this question, we assume that the adsorbed molecules have been hydrogenated by one hydrogen atom, and we calculate the energy required to detach the hydrogen atom, from the molecule, and return it to the water reservoir. The results are tabulated in the third column of Table 2, where we can see that in the case where the molecules are bonded to the cementite surface the binding energies of the hydrogen atoms are small, less than 0.74 eV. This means that it is highly unlikely that the hydrogen molecules stay bonded to the adsorbed molecules for a long time: If the entire hydrogenated molecule is not desorbed from the surface quickly, the hydrogen atoms are transferred back to the water reservoir. However, in the case where the molecules are adsorbed on the magnetite surface, the binding energies are not negligible, up to 1.72 eV in the case of boronic acid. This result means that it is likely to have a hydrogen atom adsorbed on the molecule for long enough to have the desorption of the

Quantum mechanical calculations were performed at the density-functional-theory (DFT) level using the VASP code.57 We used projector-augmented-wave (PAW) pseudopotentials58 and a plane wave expansion with a kinetic energy cutoff of 500 eV, while a k-point grid of 4 × 4 × 1 was used. To avoid interaction between slabs, we used a 20 Å vacuum between slabs. We have employed generalized gradient approximation according to Perdew−Burke−Ernzerhof (PBE).59 We took into account the Hubbard “+U” corrections60 to improve the description of the Fe3O4 system, using a U−J value of 3.61 eV for Fe d-electrons. To correctly account for the van der Waals interactions in the system, we used the vdW-DF exchangecorrelation functional.61−64 The typical nanoparticles of interest have diameters above 10 nm, where flat facets dominate the surface of the nanoparticle. In cementite and magnetite, the (001) termination is known to be the most stable.49,65 Therefore, we will use this surface to model the surface of the nanoparticles. On the other hand, it is important to mention that test calculations indicate that the bond strength of the molecules on other surface terminations differs from the ones obtained in the (001) surface termination by less than 0.3 eV. The cementite and magnetite slabs contain 32 and 68 atoms, respectively. All atoms in the supercell were allowed to relax until reaching a force of 0.01 eV/Å. 4485

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00762. Geometry of cementite and magnetite surfaces (001), (010), (100), (110), and (111) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Z.). *E-mail: [email protected]. Phone: +1 (615) 3434321 (S.T.P.). ORCID

Sebastian Zuluaga: 0000-0002-4462-6633 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Abu Dhabi National Oil Company (ADNOC) by a contract through the Petroleum Institute, Abu Dhabi and the McMinn endowment. Computations were carried out at the Extreme Science and Engineering Discovery Environment (XSEDE),66 which is supported by National Science Foundation grant number ACI-1053575, and the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We would like to thank Saeed Alhassan, Georgios Papavassiliou, Dimitris Gournis, Michael Karakassides, Hae Jin Kim, and Mohammed Subrati for a valuable collaboration that led to this project and for useful discussions.

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ABBREVIATIONS EM, electromagnetic; NP, nanoparticle; DFT, density functional theory; PAW, projector-augmented wave; PBE, Perdew− Burke−Ernzerhof

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REFERENCES

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DOI: 10.1021/acsomega.7b00762 ACS Omega 2017, 2, 4480−4487