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Computational Modeling and Inelastic Neutron Scattering Contributions to the Study of Methyl-Silica Xerogels: A Combined Theoretical and Experimental Analysis Isaura Ospino, Asunción Luquin, Monica Jimenez-Ruiz, Jose Ignacio Pérez-Landazábal, Vicente Recarte, Jesus Carmelo Echeverria, Mariano Laguna, Ana Isabel Aliende Urtasun, and Julian J. Garrido J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07310 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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The Journal of Physical Chemistry

Computational Modeling and Inelastic Neutron Scattering Contributions to the Study of MethylSilica Xerogels: A Combined Theoretical and Experimental Analysis Isaura Ospino †, Asunción Luquin †, Mónica Jiménez-Ruiz ‡*, José Ignacio Pérez-Landazábal †, Vicente Recarte †, Jesús C. Echeverría †, Mariano Laguna §, Ana Aliende Urtasun †, Julián J. Garrido †*. AUTHOR ADDRESS. Public University of Navarre - Institute for Advanced Materials (InaMat), Pamplona, Campus Arrosadia-31006 Pamplona, Spain. Institut Laue-Langevin, 71 avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France. Instituto de Síntesis Química y Catálisis Homogénea (ISQCH)-Universidad de Zaragoza, Pedro Cerbuna-50009, Zaragoza, Spain

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ABSTRACT. In amorphous materials, such as xerogels, the properties are the key to most of their applications. Therefore, it is of great importance to get insight into the xerogel structure. In order to achieve a better understanding of structure formation in methyl-silica xerogels and thus be able to better influence the properties of these materials, we have chosen the molecular approach to model the hybrid-xerogel structure using the density functional theory. A theoretical cage model for hybrid xerogels was performed from PBEPBE/6-31G(d,p) method. The model could explain the presence of ordered domains in the structure. A comprehensive listing of all IR and 29Si NMR assignments is provided. The effects in the structure of the organic functional groups incorporated on the host structure network are investigated using inelastic neutron scattering spectroscopy (INS). The combination of experiments and modeling permitted an analysis of the INS studies, including unique INS assignments, contributing to the understanding of the skeletal SiO2 network of silica xerogels and so to obtain information about the structure of this material.


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INTRODUCTION Silica xerogels (SiO2 xerogels) have applications in adsorption, separation media, sensors, drug-delivery, and oil-spill clean-up.1 These materials are obtained by the sol-gel method, which is the most used process for the synthesis of a wide range of silica materials (sols, gels, porous glasses, films, and fibers).2 In sol-gel chemistry, RxSi(OR’)4−x alkoxysilanes are often employed, where R is any organic group covalently bonded to the polysiloxane network. By mixing two precursors and changing the molar ratio RSi(OR’)3/ Si(OR’)4, the control of the amount of organic groups in the inorganic matrix can be achieved. In RSi(OR’)3 (R= Me, Et, Ph) precursors one of the four condensation sites is blocked. The most commonly encountered picture about the sol-gel process is that of an SN2 like mechanism with a pentacoordinated silicon atom in the transition state. The presence of organic groups in the hydrolyzed species may induce cyclation reactions in addition to the formation of an amorphous matrix. When tetraethoxysilane (TEOS) is the silicon precursor, tetrahedral silica forms an amorphous network. In addition, the most stable species in the following cascade reaction are probably the cyclic tetramer, the hexagonal prism, and the cubic octamer, which are assumed to be the predominant starting point of particle growth.3,4 They could present ordered domains in an amorphous matrix. We have previously reported the synthesis of hybrids xerogels by sol-gel process from mixtures of methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) at pH 4.5 and 10, and 333 K. We have characterized these materials using FTIR,

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Si-NMR, XRD (structures), FE-TEM

(morphology), and gas adsorption (N2 at 77 K and CO2 at 273 K) (porosity).5,6 In this study, the cluster evolution and the effects of the organic functional groups incorporated on the network of


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the host structure are investigated combining neutron vibrational spectroscopy and the density functional theory (DFT) calculations. Among vibrational spectroscopies, Inelastic Neutron Scattering (INS) spectroscopy presents very specific features. First, this technique is mainly sensitive to vibrations involving hydrogen atoms, which have much higher incoherent cross-section (σinc) than other elements. In addition, INS permits the measurement at all the vibrational modes without selection rules.7,8 For these reasons, INS is a highly powerful technique for studying the lattice dynamics and atomic and molecular vibrations in materials, especially those containing hydrogen. In this study, we restrict ourselves to energy transfers equivalent to that of the mid infrared region, 200-4000 cm-1.9 INS intensities can be computed with good reliability using known atomic cross-sections. Modern quantum chemical methods provide accurate intensities and so spectral assignments are very reliable.10 Recently, quantum chemistry studies have helped to advance significantly in the understanding of the properties of amorphous materials. Although using the periodic supercell models is considered to be the most relevant approach for the computational modeling of amorphous materials,11-13 isolated molecular models provide similar results to those obtained by the supercell models and perform good enough simulations and nearly identical results to the experimental characteristic data.14,15 In order to achieve better information on the formation of hybrids-xerogels structure and thus, to be able to influence better the properties of these materials, in the present work, we begin by performing a hybrid xerogels surface molecular model (MTEOS/TEOS) starting from a cyclic tetramer, which agree with different structural and spectroscopic experimental data. To validate this model, the different optimized structures will be compared with experimental chemical shift values and vibrational frequencies of hybrid xerogels (MTEOS/TEOS) previously reported by


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some of us.5 The other part of this study involves the INS spectroscopy studies, with the aim to understand the hybrid-xerogel structure, starting from the molecular model in which the final structure also depends on the TEOS/MTEOS proportions. The key point to most applications of this amorphous material are its properties. It is therefore of great importance to obtain information on the xerogel structure. EXPERIMENTAL SECTION. The powdered material samples for INS study were synthesized from MTEOS/TEOS mixtures where MTEOS percentages varied from 0% to 100% (0%: MX00; 30%: MX30; 70%: MX70 and 100%: MX100) according to our previous works.5 The neutron scattering experiments were performed on the IN1-Lagrange (large analyzer for genuine excitations), which is installed at the hot source of the high flux reactor at the ILL (Institute Laue- Langevin).16 The secondary spectrometer setup is placed on the space focusing of neutrons scattered by the sample, which are all then recorded with He gas detector. The focusing reflecting surface is built around the vertical sample−detector axis to reflect neutrons with the fixed energy of 36 cm-1. The INS spectra were recorded at 10 K over the wavenumber range 200–4000 cm-1 using the Cu220 monochromator, which has an energy resolution of ∆E/E ≈ 2%. As the final neutrons energy is much smaller than the incident, the observed intensity is directly proportional to the generalized phonon density of states, which allows a direct comparison of the calculated neutron spectra with the experimental data. THEORETICAL SECTION In this study, we have chosen the molecular approach to model the hybrid-xerogel surface structure using DFT calculations on a well-studied silica-xerogel previously reported by some of


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us.5 The ground state was optimized for each model by using the hybrid-type Perdew–Burke– Ernzerhof exchange correlation functional (PBEPBE)17 and the 6-31G(d,p)18 basis set was used for all atoms. Vibrational frequencies were calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. It should be mentioned here that the absence of negative calculated frequencies confirms the stability of optimized geometry. To bring the theoretical values closer to experimental measurements of the infrared spectra, we used the single scale factor: 0.986 (NIST Standard Reference Database). Shielding was calculated using the Gauge-Independent Atomic Orbital (GIAO) method.19 Single-point calculations with basis set 6-311+g(d,p) were performed on optimized geometries for the

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Si

NMR signals. Chemical shifts of 29Si (δ(29Si)) were calculated using eqn: δ(29Si) = σ(29SiTMS) σ(29Si). Where σ(29SiTMS) is the chemical shielding for the reference compound tetramethylsilane (Si(CH3)4, TMS) and σ(29Si) is the shielding of the

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Si nucleus in the

compound under investigation. All calculations were done with the help of the Gaussian 03 program package.20 This output was used as a model for the normal modes calculation using DMol3 simulation package in Materials Studio software.21 The double numerical plus-d-function basis set (DND) all-electrons was used for all the calculations. The DND basis set includes one numerical function for each occupied atomic orbital and a second set of functions for valence atomic orbitals, plus a polarization d-function on all atoms. Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation (PBE-GGA) was used in all the calculations in order to achieve the fine geometrical optimization of the systems after which the residual forces were converged to zero. Once the equilibrium structures of the systems were found, lattice dynamics theory was used to obtain the phonon frequencies by diagonalization of the dynamical matrices. Then, the corresponding DFT quantum chemical calculations provide


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information on the frequencies of the normal modes and the amplitudes of the atomic displacements from the equilibrium positions. The Vibrational Density of States (VDOS) is calculated from the partial density of states, gi, of the different atomic species: VDOS(ω) ~ ∑igi. Hydrogen atoms have the largest and mostly incoherent scattering cross section. Accordingly, when performing INS experiments on hydrogenated structures, the neutron spectra will be dominated by the contributions of the H atoms, thas is gH(ω). Therefore, the calculated neutron scattering spectra were obtained from the normal modes considering the hydrogen atoms. The LAMP program was used for gH(ω) visualization.22 Cartesian coordinates for all optimized structures are given in the Supporting Information. RESULTS AND DISCUSSION Theoretical calculations were carried out based upon the previous results of hybrid xerogels MTEOS/TEOS prepared at pH 4.5 and 333 K with ethanol as solvent. MTEOS percentages in the siliceous mixture, varied from 0 to 100%. These results showed that increasing the MTEOS content caused structural changes in the skeletal SiO2 network. Furthermore, the IR data showed that xerogels contain a significant amount of T8-like polycyclic species, in addition to linear and branched oligomeric structures. X-ray diffraction (XRD) data, at lower angles (2θ≤10°), may indicate the appearance of ordered domains.5 In the past, we also proposed some cage-like structures from the information on the fragments atomic composition provided by the mass spectra.6 Models for hybrid xerogels surface and spectroscopic analysis at theoretical level. A model that can represent the hybrid TEOS:MTEOS xerogels was performed from a cyclic tetramer. From this cyclic tetramer, other authors reached a similar cage-structure23,24 (Figure 1a). This


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cage-structure is similar to the silsesquioxanes compounds often experimentally considered as models for silica surfaces.25 However, this structure can not represent all MTEOS molar ratios, according to the

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Si NMR spectra data obtained for xerogels with different amounts of TEOS

and MTEOS (0% to 100% molar ratio MTEOS) in a previous work.5 In the

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Si NMR studies,

the Qn species corresponds to the TEOS in the xerogels matrix and the MTEOS signals were related to the T2 and T3 species (the Tn notations is used for silicon atoms bonded to three oxygen atoms and Qn for silicon atoms with four oxygen-bridging atoms26,27). Beside, as the amount of MTEOS increases, the signal due to the T3 units increases. For that reason, some bonds were changed and four-cage models were obtained (J1, J2, J3, J4) (Figure 1b). For the xerogel synthesized from TEOS (MX00), the conformational equilibrium geometry of the four cage models (J1-0, J2-0, J3-0, J4-0) (Figure 1b) were researched using DFT with PBEPBE/6-31G(d,p) level of theory in the gas phase, which has been proven to be particularly efficient and accurate for the calculation of inorganics and organic systems.17 The positive value of each calculated frequency verify the stability of optimized geometry. According to this results, the J4-0 cage model has the lowest relative energy (-5812.36 a.u.) (Table S1 of Supporting Information). From the J4-0 cage model (0% of MTEOS), the methyl groups bonded to silica atoms were included in the xerogel model to obtain the structure for 30% of MTEOS (J4-30) and 70% of MTEOS (J4-70) Figure 2a. In the case of 100% of MTEOS cage model (J4-100), which had not Q species, some bond from J4 were changed Figure 2b. In these models (J4-0, J4-30, J4-70, J4100), the final structure also depends on the proportions of the organic functional groups incorporated to the network (TEOS:MTEOS). The final structure generated by this procedure provides a sufficiently realistic model of xerogel surface. The resulting models were optimized in


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the gas phase. In addition, the frequency calculation was performed to determine if the optimized structure was a minimum. The relative energy calculated and structural characteristics are given in the Supporting Information. We observed that methyl groups, incorporated on the cage model network give structures energetically less stable (the highest energy value) (Table S2 of Supporting Information). This is probably related to the experimental observations showing that the gelation time exponentially increases with the initial concentration of the hybrid precursor5 perhaps due to blockage of a bonding positions (methyl group of the MTEOS).28 Furthermore, the polycondensation of the oligomer mixture was presumed to be responsible for gelation. This oligomerization process evolved towards a mixture of silicate species with short chains, up to 4 Si atoms, and cyclosilicates of 4 Si atoms (cyclic tetramer), both containing side chains of 2−4 Si atoms.29 The selected bond lengths and bond angles are shown in Table S3 of the Supporting Information. The bond distance between silicon atoms connected by an oxygen bridge (≡Si-OSi≡) increases with increasing the MTEOS molar ratio, 3.286 Å (J4-0), 3.295 Å (J4-30), 3.294 Å (J4-70) and 3.368 Å (J4-100). These values agree with experimental XRD data, which show that xerogels have a large peak angle characteristic of amorphous silica at 2θ~23° that corresponds to a distance range from 3.8 Å, in xerogel 0% MTEOS to 4.0 Å in 100% MTEOS that also increases with increasing the MTEOS molar ratio and this peak is associated with the spacing between silicon atoms connected by an oxygen bridge (Figure 3).5 Furthermore XRD experimental data showed a second peak at 2θ

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