Remarks on the Interpretation of IR-Absorption Studies Applied to the

Article pubs.acs.org/JPCC

Remarks on the Interpretation of IR-Absorption Studies Applied to the Surfaces of Silica Nanoparticles Reinhard Hentschke,* Christina Ballnus, and Jan Meyer Fachbereich Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany ABSTRACT: Using a force field based molecular modeling approach, we calculate the IR absorption intensity between 3000 and 4000 cm−1 for small silica particles and their model aggregates covered by different amounts of water. We find that the IR absorption in this range of wavenumbers is dominated by surface and bulk-like water concentrated in pores. In addition, we compare our results to experimental spectra in this regime and the interpretation thereof. Our conclusion is that the presence of only small amounts of adsorbed water blots out the different types of surface silanol groups.

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INTRODUCTION Nanoparticles, solid particles whose diameter typically is significantly less than 100 nm, are often distinguished by a large surface area to weight ratio. They do have many applications related to their surface structure, which in turn depends on the method of synthesis. In general, knowledge of the surface structure makes it easier to adjust and possibly optimize certain properties of interest. We focus on silica nanoparticles. An important application of silica nanoparticles, to name just one example from many, is in the rubber industry,1,2 in particular in automobile tires, where the rolling resistance and wet grip are significantly improved when silica is used instead of or partially replacing carbon black.3 However, the silica surface must be chemically compatibilized with the hydrophobic polymer. The necessary chemistry depends on factors like surface density of silanol groups, their type, and the presence of physisorbed water.4 An experimental method, easy to apply and thus frequently used, to characterize the surfaces of silicas is infrared (IR) spectroscopy. In particular this method is employed to distinguish the types of silanol groups on surfaces of differently synthesized silica nanoparticle (e.g., refs 5−19). However, this particular interpretation is made difficult by water physisorbed in variable amounts on the silica surface. It is therefore useful to study the effect of variable amounts of water on the observed IR spectra, in particular in the range between 3000 and 4000 cm−1, using molecular modeling, which here is force field based normalmode analysis. The appeal of this method is the direct observability of the relation between molecular dynamics and the attendant IR modes on simplified surfaces. A second motivation is the modeling of silica nanoparticles itself. Even though the structure of silica particles is amorphous, some modeling studies of silica−polymer interaction do use particles based on crystal structures (e.g., refs 20 and 21), whereas others do use amorphous particles or surfaces in such studies (e.g., refs 22−24). In recent Raman studies on the structural properties of silica nanoparticles,25,26 the authors use a © XXXX American Chemical Society

shell model, distinguishing the core from the surface shell, to interpret their data. This model in turn is supported by the molecular dynamics simulation study of the structure of amorphous silica nanoparticles in ref 24. Nevertheless, in general it is difficult to decide which approach is to be preferred. In particular this is true for surface properties. This is due to the lack of explicit comparison of these models to the results of experimental techniques probing the surface structure and energetics of nanoparticles. Examples are the distribution of adsorption site energies, as obtained for instance from inverse gas chromatography27 or IR spectra, which we focus on in this study. Thus, we also try to answer the question whether or not IR spectra (obtained for silicas) can be used to rule out possible inconsistencies due to an inappropriate construction of nanoparticles in modeling studies. In the following we use a force field based normal-mode analysis to calculate the IR absorption intensity in the range from 3000 to 4000 cm−1 for model silica nanoparticles and their model aggregates without and with additional surface water. We find that the IR absorption in this range of wavenumbers is governed by heterogeneously distributed surface water, concentrated in pores of different topology. In particular, the presence of only small amounts of adsorbed water blots out the different types of surface silanol groups. We relate our results to a number of experimental IR studies and their molecular interpretation of the characteristic IR bands in the above range of wavenumbers.

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METHOD The silica particles studied here are based on spherical cutouts from α- and β-cristobalite crystals using the crystal structures from refs 28 and 29 (for more details see ref 30). Cut bonds are saturated with either −OH or −H as shown in Figure 1. The possible types of surface groups present after completion of the Received: January 25, 2016 Revised: April 14, 2016

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DOI: 10.1021/acs.jpcc.6b00807 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

contacts, close to the particle surface. In the case of aggregates, the water molecules are inserted preferentially between the silica particles. All subsequent calculations are carried out using the Merck molecular force field (MMFF) as implemented in the Spartan molecular modeling package35 We note that the MMFF is based on a rather broad training set and thus possess a better transferability than some other force fields. Even the early MMFF parametrization included the atoms in the present study.36,37 Another subtle point is electrostatics. The attendant parameters, e.g. partial charges, often are not part of the force field, which reduces reproducibility in applications. In addition, MMFF always had a strong hydrogen bond parametrization, which is of particular importance in the present study. For every particle an initial structural relaxation (energy minimization) is performed. An example is the primary particle shown in the top panel of Figure 3. The particles in the other two

Figure 1. Surface atoms are shown in black. Groups saturating the free valences are shown in gray.

above procedure are shown in Figure 2. We note that the average surface density of hydroxyl groups, close to 6/nm 2, is in accord

Figure 2. Possible types of surface groups.

with numbers for precipitated silica in the experimental literature.31 The number is only slightly higher than the density of 5/nm 2 discussed by Zhuralev.32 Silanol densities in accord with this work were obtained in molecular dynamics simulations of the structure of silica glass surfaces.33 The actual range of values, however, depends on the processes from which the silica is derived. Wet processes result in silanol numbers close to 6/nm 2 , whereas in the case of fumed silicas this number can be between 2/nm2 and 3/nm2.34 The typical size of our primary silica particles is between 330 to 400 atoms with attendant radii close to 1 nm. This particle size is comparable to particle sizes in previous simulations (e.g., ref 24). It is roughly 1 order of magnitude smaller than typical experimental primary particle sizes. However, in the following we want to be able to compare IR spectra from isolated particles to those obtained for cluster models based on the same particles, which is computationally expensive. Therefore, we have omitted a variation of the particle size, even though experimental studies have shown that the primary particle diameter, possibly indirect via changes in surface morphology, can affect the line intensity (e.g., ref 14). Additional particles and model aggregates thereof are produced containing surface water molecules in respective amounts corresponding to 6, 12, and 24 wt % of the total mass. In the case of isolated primary particles, the randomly oriented water molecules are placed, also randomly but avoiding close

Figure 3. Top: Space filling model of one of the relaxed silica particles studied here. Middle: Same particle, before relaxation, shown as balland-stick model with additional water molecules, in space filling representation, randomly distributed on the surface. In this case the water coverage is about 6 wt %. Bottom: Same unrelaxed particle as in the previous panel when the water coverage is about 24 wt %. B


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quantities are ϵo, the electric constant, h = 2 π ℏ, Planck‘s constant, n(ν̃) the index of refraction of the sample, c, the velocity of light. We expand the cluster’s dipole moment in terms of its normal mode coordinates qν; i.e., ν is the mode index:

panels are shown before relaxation. Thus, even though the core structure of our particles is crystalline, the surface (shell) is not, due to the relaxation. The structural relaxation is followed by the calculations of the normal-mode frequencies ων of the particle. If physisorbed water is present, then the water is included as part of the particle or aggregate. We note that the frequency range of interest here allows to include the physisorbed water in the standard normal-mode analysis. We also note that force fields, which are not especially optimized for normal-mode frequency calculations, for instance by including additional off-diagonal terms and suitable molecular training sets for the parametrization, usually are not very precise;38 i.e., typical peak shifts are between 10 to 20 cm−1 and sometimes even larger. However, in the present study we are interested in rather broad spectral features, which are separated by 100 cm−1 or more.

⎯→ M (t ) = M⃗ o +

ν

⎛ ∂M⃗ ⎞2 α(ν)̃ ∝ ν⎜⎜̃ |0 ⎟⎟ ⎝ ∂qν ⎠

ν

(e βℏων − 1)2

(β −1 = kBT )

(4)

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COMPARISON TO EXPERIMENTS Guy et al.17 have performed IR absorption experiments on various silicas, used predominantly in the tire industry, in order to obtain information on the type and interrelation of the silanol groups. Even though we focus on this particular work, it is important to note that the characteristic modes displayed in these spectra are present also in most of the works listed in the introduction. The relative peak heights are variable of course, depending on silica type and treatment, but the positions of the discernible peaks are quite universal. Figure 5 shows absorption spectra of two industrial silicas. Here Sil_Exp2 is an experimental silica, whereas Z195HR, i.e., Zeosil195HR, is a commercial product. Their respective specific surface areas are 162 m2/g (BET)/161 m2/g (CTAB) and 175 m2/g (BET)/154 m2/g (CTAB). The moisture content is 7.5 wt % in the case of Sil_Exp2 and 6.1 wt % in the case of Z195HR. Additional properties are listed in Table 1 of the above reference. The two curves reproduced here bracket spectra obtained for three additional silica types possessing intermediate moisture contents. The authors identify the different peaks with different silanols according to type and local configuration. The small but rather sharp peaks at around 3750 cm−1 indicate free or isolated hydroxyl groups (cf. the discussion by Unger of IR spectra obtained from Aerosil and Cabosil in ref 5; included in the figure

(1)

The quantity kB is Boltzmann‘s constant. For solids CV differs little from CP, which here is the experimental reference. Notice that increasing the particle size leads to an approach of the particle’s heat capacity toward the bulk value. However, in the following we are interested particularly in the high frequency or large wavenumber OH-stretch modes, which do not contribute to CV. At these large wavenumbers, the exponent βℏων becomes large. The attendant terms in the sum of eq 1 are small and thus do not contribute to the heat capacity. This holds true unless T exceeds room temperature by a significant factor. We calculate the IR absorption coefficient α(ν̃) (in MKSA units) as it appears in the Lambert−Beer law for the intensity I as a function of sample thickness x (I(x) /I(0) = exp[−α(ν̃) x]): ∞ π dt α(ν)̃ = ν(1 ̃ − exp[ −βhcν]) ̃ −∞ 3ϵoℏVMn(ν)̃

∫

exp[− 2πicνt̃ ]⟨M⃗ (0) ·M⃗ (t )⟩o

(3)

assuming independent modes (as intrinsic in ordinary normalmode analysis). The derivative factors, i.e. (∂M⃗ /∂qν|0)2, are obtained as part of the above normal-mode analysis. The shape of the particular adsorption line at ν̃ is given by the Fourier transform of ⟨qν (0) qν (t)⟩o. Here we merely assume that the result is a Lorentzian of width Δν = 30 cm−1 for all modes. This roughly corresponds to a lifetime of 1 ps, which is rather characteristic for the dynamics of the surface water (the same lifetimes were used in the related study on water films by Buch41). A theoretical spectrum thus obtained is the sum over Lorentzians of width Δν and height α(ν̃) according to relation 4. It includes all wavenumbers in the range between 3000 to 4000 cm−1 obtained by normal-mode analysis. In addition, every final theoretical spectrum, as shown in the following figures, is an average over several independent theoretical spectra, i.e. each is calculated from independently generated and relaxed configurations. The number of spectra in the aforementioned averages is three for single primary particles and four for aggregates.

As an aside Figure 4 shows the heat capacity at constant volume for two ‘dry‘ silica particles calculated via the standard textbook expression40 (ℏων)2 e βℏων

∂M⃗ |0 q̲ (t ) + .... ∂qν ν

Here cluster means particle or aggregate possibly including surface water. The index o refers to the undeformed particle, for which qν = 0 (for all ν), possessing the constant dipole moment M⃗ o. In addition, at room temperature we have β ℏων ≫1 for molecular vibrations in the range of frequencies of interest here. Thus

Figure 4. Specific heat capacity at constant volume, cV, vs temperature, T. Solid line: small particle (radius about 0.5 nm). Dashed line: larger particle (radius about 1.25 nm). Dots: experimental cP-data for bulk SiO2 taken from ref 39.

C V / kB = β 2 ∑

∑

(2)

A detailed derivation can be found in ref 40 (section 21-1). Here ν̃ is the wavenumber related to the frequency via ω = 2 πcν̃. The quantity ⟨M⃗ (0)·M⃗ (t)⟩o is the dipole autocorrelation function in the unperturbed sample, where the dipole moment M⃗ (t) is the dipole moment of the entire sample volume VM at time t. Other C


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scaled identically and are thus directly comparable, i.e. we fix the scaling of the peak heights in the theoretical spectra in this figure and maintain the same scaling throughout for all theoretical spectra. However, no direct comparison of peak heights is possible with the experimental data or even between experimental data obtained independently, i.e., our scaling of the spectrum from ref 8 relative to the two other experimental spectra is arbitrary. Figure 6 shows the experimental spectra from ref 17 in comparison to four theoretical spectra, one of which is the

Figure 5. Experimental and theoretical IR absorption intensities vs wavenumber. Black curves: two commercial silicas, denoted as silExp2 (dashed line: moisture 7.8 wt %) and Z195HR (solid line: moisture 6.1 wt %) from Figure 1 in ref 17; experimental spectrum taken from Figure 5 in ref 8 obtained for Cabosil at 500 oC (dotted line). Green curve: αcristobalite particle. Red curves: β-cristobalite particles distinguished by different fractions of geminal silanol groups as explained in the text. The arrows indicate the position of the two stretch modes of the isolated water molecule.

are two arrows indicating the positions of the stretch modes of the isolated water molecule). We note that this peak can be much more pronounced in the case of for instance pyrogenated silica as shown in ref 16. Figure 5 also includes our theoretical results for dry α- and βcristobalite particles together with another experimental spectrum. This spectrum is for a different silica obtained at high temperature intended to remove the surface water.8 The peak widths are narrower due to the longer lifetime in comparison to our above assumption, which is more appropriate for hydrogenated particles. The position of the pronounced peak in the theoretical spectra corresponds rather well to the above peak attributed to isolated silanol groups. Overall there is little difference between our particles based on α- and β-cristobalite. The two β-cristobalite results, solid and dotted red curves, included in the figure are distinguished by their different proportion of geminal silanol groups. In the case of the dotted red curve the number of geminal silanol groups is reduced approximately by a factor of 2 to 37% of the surface hydroxyl groups (counting each OH-group separately). The position of this feature correlates rather well with the position of the second broader peak exhibited by the experimental curves in the range between 3650 to 3700 cm−1, which Unger identifies with silanol groups involved in H-bonding and internal silanol groups. Interestingly a similar shoulder is present also in the high temperature spectrum. Visual inspection of the normal mode vibrations for the 37%β-cristobalite particle reveals the following: (i) The peak at around 3670 cm−1 is due to geminal OH-groups. Generally one of the geminal OH-groups displays a coupling to another excited OH-group in close proximity (O···O distance about 0.28 nm). (ii) The pronounced peak at around 3750 cm−1 is mainly contributed by isolated but coupled OH-groups (partially involving geminal OH-groups as well). In the case of the two other particles, possessing significantly larger fractions of geminal groups, this distinction is much less clear. Usually patches or strings involving several OH-groups, isolated and geminal, exhibit coupled stretch vibrations. It is important to note that all theoretical α-results shown in this and in the following figures, unless stated otherwise, are

Figure 6. Comparison of the experimental IR spectra shown in Figure 5 with theoretical results obtained for one dry and three hydrated particles. Green curve: result for dry α-cristobalite. Blue curves: αcristobalite particle with variable amounts of physisorbed water (solid line, 6 wt %; dashed line, 12 wt %; dotted line, 24 wt %).

spectrum for dry α-cristobalite also included in the previous figure. The three additional theoretical spectra are from αcristobalite particles when different amounts of water molecules, corresponding to 6, 12, and 24 wt % of the total mass (water plus particle), are distributed on their surfaces as described in the previous section. Notice that the theoretical spectra become broader when the amount of physisorbed water increases. There is a noticeable downshift toward the sharp onset of absorption at about 3750 cm−1 exhibited by the experimental spectra. The general accord with the experimental spectra above 3500 cm−1 improves significantly. In addition we notice a distinct shoulder between 3450 to 3500 cm−1, in the 24 wt % case, where experimental silica IR spectra do exhibit a thus far unexplained broad peak. Overall we believe that this supports the conclusion that the experimental spectra in this range are dominated by the absorbed water (except for the narrow asymmetric stretch-mode peak). Closer inspection of the modes between 3450 to 3650 cm−1, in the cases of 6 and 12 wt % surface water, reveals the following picture: (i) modes around 3450 cm−1 are localized, i.e., they involve OH-stretch deformations of a central water molecule surrounded by on average three neighboring OH-groups, which also can be contributed by silanols. (ii) around 3650 cm−1 the vibrational modes have become less localized collective modes involving 10 or more coupled OH-groups, mostly contributed by physisorbed water. It is worth noting that the broad band around 3650 cm−1 in the literature is attributed to H-bonded silanols that are perturbed by inter particle contacts,7,9,11 which of course here are absent. (iii) upon approaching 3750 cm−1 the stretch modes become localized again, i.e. the absorption close to 3750 cm−1 is due to isolated stretch modes of the silanol groups. Inspection of the modes contributing to the shoulder between 3450 to 3500 cm−1 in the 24 wt %-case reveals mainly small two-dimensional patches of H-bonded water molecules exhibiting slightly D


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We explore this concept based on simple model aggregates as depicted in Figure 8. The overall water content again is 6 wt %

distorted symmetric stretch vibrations with little coupling to silanol groups. Figure 7 is a comparison of the three types of model particles considered in this study. The spectra of their dry versions are

Figure 7. Comparison of the three types of model particles already considered as dry particles in Figure 6. In all three cases the water coverage is 6 wt % of the total mass. The solid blue curve is the same as in the previous figure. The two red curves correspond to the two red curves in Figure 5 for β-cristobalitenow including surface water.

shown in Figure 6. Here the spectra are for the particles including surface water (6 wt %). In order to be able to better distinguish any differences, the curves are scaled to the same number of modes contributing in this range of wavenumbers. Overall the differences are small, and possibly within the scatter of the method. It is worth noting that the significantly different numbers of geminal OH-groups in the case of the two βcristobalite particles do not lead to correspondingly different spectra. Buch41 has used molecular dynamics simulations to investigate the bulk and surface coordination of molecules in liquid water. He finds that both local environments, the bulk and the free surface, yield a broad peak centered at about 3400 cm−1. If this is the case, then one might expect the following. The interaction between the bulk-like water with the physisorbed water in the first shell at the particle‘s surface is responsible for the peak at 3500 cm−1. This idea has been discussed recently by Hamamoto et al.18 The type of hydrogen bonding, which causes the upshift of the bulk water peak, may be inferred from the work of Lenz and Ojama.42 These authors model the frequency distributions associated with different donor−acceptor configurations. A peak in the frequency range of interest can be produced by 3-fold coordinated water molecules donating both their hydrogens. We return to this point below. Figure 6 shows that the increase of the water coverage extends IR absorption toward the lower wavenumbers, thereby improving the general accord with the experimental spectra. However, the required water coverage becomes unrealistic owing to its homogeneous distribution on the surface of a single model particle. Depending on processing conditions one can expect that the energy hypersurface of realistic silica is much more complex. In previous work we have studied the energy site distribution of similar silica model particles using various small probe molecules.30 Comparison with experimental measurements for water indicate that certain high energy adsorption sites present on experimental silica are absent on the model particles. In particular the formation of aggregates (cf. the small-angle X-ray study in ref 44) due to the strong hydrogen bonding between (moist) silica surfaces should lead to a rough surface topology and strong adsorption sites.

Figure 8. Top: Model aggregate consisting of three α-cristobalite particles with 6 wt % water concentrated between them. Bottom: Model aggregate consisting of four α-cristobalite particles in a tetrahedral arrangement with 6 wt % water concentrated between them. The yellow dotted lines indicate hydrogen bonds in both panels.

(referring to the mass of the entire cluster). However, the water now is concentrated between the silica particles. The respective spectra obtained for two types of aggregates, formed by three and four primary particles, are shown in Figure 9. Both cluster spectra are scaled to match the height of the peak at 3750 cm −1 obtained for the isolated primary particle. Notice that the overall bandwidth is the same as in Figure 6 for the case of 24 wt % water. In fact the line shapes of the 6 wt % three particle-cluster spectrum in comparison to the 24 wt % single particle spectrum in the range from 3300 to 3550 cm −1 are almost identical. Also included in Figure 9 are experimental data obtained on bulk water. The height of this experimental spectrum is freely adjusted, however. Overall it appears plausible that the experimental IR spectrum in the range considered here, i.e., from 3000 to 4000 cm −1, is due to physisorbed water molecules rather than being due to the direct observation of silanol groups. Above 3450 cm −1 the major contribution to the absorption coefficient is from water in small pores, i.e., pores less wide than one or two layers of water. Below 3450 cm −1 the major contribution to the absorption coefficient appears to be bulk-like water adsorbed in larger pores allowing for more than two water layers. In general, the absorption in the E


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modeling approach developed here can therefore help to elucidate the relation between the mechanical properties of filled elastomers and the filler‘s surface chemistry.

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AUTHOR INFORMATION

Corresponding Author

*(R.H.) E-mail: [email protected]. Telephone: +49 202 439 2628. Fax: +49 202 439 3122. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Per Jensen for an insightful discussion of this subject. Financial support of this work through the Bergische Universität Wuppertal is gratefully acknowledged.

Figure 9. Experimental and theoretical IR-absorption spectra. Black curve: Z195HR (solid line: moisture 6.1 wt %) as in Figure 5. Blue curves from bottom to top: α-cristobalite particle with 6 wt % water homogeneously distributed on its surface (solid line), cluster consisting of three (dashed line) and four (dotted line) α-cristobalite particles with 6 wt % water heterogeneously distributed as shown in Figure 8. Red dashed curve: Experimental data, suitably scaled, for bulk water taken from Figure 8 in ref 42 based on ref 43.

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REFERENCES

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range between 3400 to 3700 cm −1 does depend strongly on the formation of hydrogen bonded water network “patches” exhibiting correlated vibrational excitations. In the case of homogeneously distributed water in small amounts, i.e., 6 wt %, on a single particle, this is strongly reduced. Only when the water content is high, either homogeneously, i.e., 24 wt %, or locally in pores, is network formation sufficiently promoted. In addition Figure 9 indicates that the peak structure of the absorption spectrum does strongly depend on the type/shape of the pore, and consequently on the silica production process, most likely due to frustration of excitation correlation lengths by the surface morphology.

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CONCLUSION Using a force field based normal-mode analysis, we have calculated the IR absorption coefficient between 3000 and 4000 cm−1 for silica primary particle models (radius ∼1 nm). The model particles possess a crystalline core enclosed in a relaxed surface structure. We have studied isolated primary particles as well as selected model aggregates formed by three or four primary particles. Particles and aggregates were covered homogeneously and heterogeneously by variable amounts of water. Our conclusion is that the experimental spectra in this range are dominated by the heterogeneously adsorbed water (except for the narrow asymmetric stretch-mode peak at about 3750 cm−1). We also conclude that experimental IR spectra in this range do not a priori preclude the modeling of surface properties of silica particles generated on the basis of crystalline core structures as described in this work. The overall accord between our spectra and the experimental spectra, to us, indicates that the surfaces of our particles, including the physisorbed water, are reasonable representations of realityfrom the point of view of those who are interested in the particles surface chemistry. The model is inappropriate if a structural profile from surface to center is attempted. Modeling studies, focusing on the surfaces of silica particles or their interfaces (e.g., silica−polymer), thus far neglect the presence of physisorbed water. However, in certain important applications, like polymer reinforcement, physisorbed water significantly influences the surface chemistry (e.g., silinization reactions10,11) and may even influence the filler−filler interaction and thus the mechanical properties of the filler network. The F


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Remarks on the Interpretation of IR-Absorption Studies Applied to the

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