Importance of the embedding environment on the strain within small rings in siliceous materials

Bromley, Stefan T.; Moreira, Ibério de P. R.; Illas, Francesc; Wojdeł, Jacek C.

doi:10.1103/physrevb.73.134202

Cited by 19 publications

(28 citation statements)

References 44 publications

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“…All NPs in this low hydroxylation sub-set, as for the full set of all (SiO 2 ) 16 (H 2 O) N isomers considered, do not exhibit terminal oxygen defects. However, at this low degree of hydroxylation, many NPs possess rather strained threemembered (SiO) 3 rings 47 indicating that further ring-opening via hydroxylation would be energetically favourable. 41 The N = 8 sub-set of NPs can be regarded as having a high 50% degree of hydroxylation and often exhibit between one and three geminal silanol groups (i.e., two hydroxyls bound to one silicon atom).…”

Section: Methodsmentioning

confidence: 99%

“…In silica, the O-Si-O angle is largely constrained by the rigid tetrahedrality of the SiO 4 sub-units. In pure silica, these tetrahedra can be distorted by highly strained small (SiO) n rings 47,49 or terminating defects 50,51 typically found in anhydrous silica. As low barrier reactions with water are generally favoured at such sites, quickly leading to ring opening or healing of terminating defects, [52][53][54][55][56] it is expected that the O-Si-O angle distribution in nanosilica should be relatively narrow and invariant above a certain degree of hydroxylation.…”

Section: O-si-o Anglesmentioning

confidence: 99%

“…For low degrees of hydroxylation, NPs commonly display (SiO) 3 three-membered rings that have fairly high internal strain energies. 47 If these rings are not accurately described, this could potentially disrupt the total energies of the NPs specifically for low degrees of hydroxylation. Thus far, however, our analysis could not find any link between the presence of three-membered rings and the ability of ReaxFF to reproduce DFT-derived total energies.…”

Section: Relative Total Energiesmentioning

confidence: 99%

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Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields

Escatllar

Ugliengo

Bromley

2017

The Journal of Chemical Physics

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We analyze the performance of the FFSiOH force field and two parameterisations of the ReaxFF force field for modeling hydroxylated nanoscale silica (SiO). Such nanosystems are fundamental in numerous aspects of geochemistry and astrochemistry and also play a key role during the hydrothermal synthesis of technologically important nanoporous silicas (e.g., catalysts, absorbents, and coatings). We consider four aspects: structure, relative energies, vibrational spectra, and hydroxylation energies, and compare the results with those from density functional calculations employing a newly defined dataset (HND: Hydroxylated Nanosilica Dataset). The HND consists of three sets of (SiO)(HO) nanoparticles (NPs), each with a different degree of hydroxylation and each containing between 23 and 26 distinct isomers and conformers. We also make all HND reference data openly available. We further consider hydroxylated silica NPs of composition (SiO)(HO) with M = 4, 8, 16, and 24 and infinite surface slabs of amorphous silica, both with variable hydroxylation. For energetics, both ReaxFF and FFSiOH perform well for NPs with an intermediate degree of hydroxylation. For increased hydroxylation, the performance of FFSiOH begins to significantly decline. Conversely, for the lower degree of hydroxylation both parameterisations of ReaxFF do not perform well. For vibrational frequencies, FFSiOH performs particularly well and significantly better than ReaxFF. This feature also opens the door to inexpensively calculating Gibbs free energies of the hydroxylated nanosilica systems in order to efficiently correct density functional theory calculated electronic energies. We also show how some small changes to FFSiOH could improve its performance for higher degrees of hydroxylation.

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Section: Methodsmentioning

confidence: 99%

Section: O-si-o Anglesmentioning

confidence: 99%

Section: Relative Total Energiesmentioning

confidence: 99%

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Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields

Escatllar

Ugliengo

Bromley

2017

The Journal of Chemical Physics

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“…Finally, we note that the exclusively silanone-terminated clusters ͑A, B, F͒ show an interesting trend between ͑SiO͒ n n-membered-ring strain and absorption onset. Previous work 68 has demonstrated that the ring size and the associated strain energy in silica decreases strongly in the order twomembered ringϾ three-membered ringϾ four-membered ring, where the latter is found to be almost ring-strain free. When moving from an inherently highly strained system ͑cluster A, where the silicon centre of the silanone group is part of a two-membered ring͒ to lesser ͑cluster B with the silicon of the silanone groups forming part of either a two or three-membered ring͒ and an even lesser strained systems ͑cluster F where the silicon of the silanone groups forms part of a four-membered ring͒ the absorption onset consistently lowers.…”

Section: Resultsmentioning

confidence: 91%

Optical excitations of defects in realistic nanoscale silica clusters: Comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods

Zwijnenburg

Sousa

Sokol

et al. 2008

The Journal of Chemical Physics

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Optical excitations of low energy silica (SiO(2))(4) clusters obtained by global optimization, as opposed to constructed by hand, are studied using a range of theoretical methods. By focusing on the lowest energy silica clusters we hope to capture at least some of the characteristic ways by which the dry surfaces of silica nanosystems preferentially terminate. Employing the six lowest energy (SiO(2))(4) cluster isomers, we show that they exhibit a surprisingly wide range of geometries, defects, and associated optical excitations. Some of the clusters show excitations localized on isolated defects, which are known from previous studies using hydrogen-terminated versions of the defect in question. Other clusters, however, exhibit novel charge-transfer excitations in which an electron transfers between two spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum is found to be very different from that of the same defects in isolation. Excitation spectra of all clusters were calculated using time-dependent density functional theory (TD-DFT) and delta-SCF DFT (DeltaDFT) methods employing two different hybrid density functionals (B3LYP and BB1K) differing essentially in the amount of incorporated Hartree-Fock-like exchange (HFLE). In all cases the results were compared with CASPT2 calculated values which are taken as a benchmark standard. In line with previous work, the spatially localized excitations are found to be well described by TD-DFT/B3LYP but which gives excitation energies that are significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination in contrast is found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. Finally, our calculations suggest that the increased quality of the predicted excitation spectra by adding larger amounts of HFLE is mainly due to an increased localization of the excited state associated with the elimination of spurious self-interaction inherent to (semi-)local DFT functionals.

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“…Generally, structures made mainly with rings containing more than three -(Si-O)-units are found to have low tetrahedral distortion. [39] Interestingly, several of the structures shown in Fig. 3, can be viewed as being generated from condensation of building blocks of smaller sized isomers.…”

Section: Resultsmentioning

confidence: 99%

Global optimisation of hydroxylated silica clusters: A cascade Monte Carlo Basin Hopping approach

Cuko

Macià

Calatayud

et al. 2017

Computational and Theoretical Chemistry

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We report on a global optimisation study of hydroxylated silica nanoclusters (SiO 2 ) M ·(H 2 O) N with sizes M = 6, 8, 10 12, and for each size with a variable number of incorporated water molecules (N = 1, 2, 3…). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we propose a "cascade" global optimisation approach.Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a few-term simple but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO 2 ) 8 ·(H 2 O) N case, and then applied to the M = 6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO 4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to -2/3, further confirming work in previous studies for other cluster sizes.

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Importance of the embedding environment on the strain within small rings in siliceous materials

Cited by 19 publications

References 44 publications

Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields

Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields

Optical excitations of defects in realistic nanoscale silica clusters: Comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods

Global optimisation of hydroxylated silica clusters: A cascade Monte Carlo Basin Hopping approach

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