Bulk-terminated or reconstructed Fe 3 O 4 (001) surface: water makes a difference

Valentin

2019

Phys. Rev. Lett.

Self Cite

Iron oxide magnetic nanoparticles (NPs) are stimuli-responsive materials at the forefront of nanomedicine. Their realistic finite temperature simulations are a formidable challenge for first-principles methods. Here, we use density functional tight binding to open up the required time and length scales and obtain global minimum structures of Fe 3 O 4 NPs of realistic size (1400 atoms, 2.5 nm) and of different shapes, which we then refine with hybrid density functional theory methods to accomplish proper electronic and magnetic properties, which have never been accurately described in simulations. On this basis, we develop a general empirical formula and prove its predictive power for the evaluation of the total magnetic moment of Fe 3 O 4 NPs. By converting the total magnetic moment into the macroscopic saturation magnetization, we rationalize the experimentally observed dependence with shape. We also reveal interesting reconstruction mechanisms and unexpected patterns of charge ordering.

Section: +supporting

confidence: 75%

Shaping Magnetite Nanoparticles from First Principles

Valentin

2019

Phys. Rev. Lett.

Self Cite

“…These observations agree well with previous results based on DFT+U and HSE calculations. 20,34 For both DBT and SCV surfaces, the distance between the first two layers is largely compressed with respect 11 to the bulk value (0.828 and 0.812 Å vs. 1.061 Å, respectively), while the distance between the second and the third layers is only slightly changed. The relaxation in the surface layers is also in accordance with that obtained by DFT+U and HSE methods.…”

Section: Resultsmentioning

confidence: 99%

“…For all the details on the DFT+U and HSE calculations on magnetite bulk and surface systems, the reader is referred to our previous studies where they are reported. 34,44 III.…”

Section: Methodsmentioning

confidence: 99%

An efficient way to model complex magnetite: Assessment of SCC-DFTB against DFT

The Journal of Chemical Physics

Seifert

Valentin

2019

Self Cite

Magnetite has attracted increasing attention in recent years due to its promising and diverse applications in biomedicine. Theoretical modelling can play an important role in understanding magnetite-based nanomaterials at the atomic scale for a deeper insight into the experimental observations. However, calculations based on density functional theory (DFT) are too costly for realistically large models of magnetite nanoparticles. Classical force field methods are very fast but lack of precision and of the description of electronic effects. Therefore, a cheap and efficient quantum mechanical simulation method with comparable accuracy than DFT is highly desired. Here, a less computational demanding DFT-based method, i.e. self-consistent charge density functional tight-binding (SCC-DFTB), is adopted to investigate magnetite bulk and low-index (001) surface with newly proposed parameters for Fe-O interactions. We report that SCC-DFTB with on-site Coulomb correction provides results in quantitatively comparable agreement with those obtained by DFT+U and hybrid functional methods. Therefore, SCC-DFTB is valued as an efficient and reliable method for the description magnetite. This assessment will promote SCC-DFTB computational studies on magnetite-based nanostructures that attract increasing attention for medical applications.

“…properly deal with the strong correlation effects among Fe 3d electrons, 22 DFTB+U with an effective U-J value of 3.5 eV was adopted according to our previous work on magnetite bulk and (001) surface. 11,23 The convergence criterion of 10 -4 a.u. for force was used during geometry optimization, and the convergence threshold on the self-consistent charge (SCC) procedure was set to be 10 -5 a.u.…”

Section: Formentioning

confidence: 99%

“…Since SCV was discovered, two computational works have concomitantly appeared in the literature where water adsorption on such reconstructed surface was modeled by density functional theory calculations. In one of these studies by some of us, 11 adsorption of one to four water molecules on both the bulkterminated and the SCV Fe3O4 (001) surface unit cell models has been investigated by hybrid density functional calculations (HSE06). 12 We have shown that in certain experimental conditions of water partial pressure and temperature, the hydrated bulk-terminated surface may become more stable than the SCV one, that, however, is the most stable in a wide range of experimental conditions.…”

Section: Introductionmentioning

confidence: 99%

Insight into the interface between Fe3O4 (001) surface and water overlayers through multiscale molecular dynamics simulations

The Journal of Chemical Physics

Bianchetti

Siani

et al. 2020

Self Cite

In this work we investigate the Fe3O4 (001) surface/water interface by combining several theoretical approaches, ranging from a hybrid functional method (HSE06) to density-functional tight-binding (DFTB) to molecular mechanics (MM). First, we assess the accuracy of the DFTB method to reproduce correctly HSE06 results on structural details and energetics and available experimental data for the adsorption of isolated water, dimers, trimers, etc. up to a water monolayer. Secondly, we build two possible configurations of a second and a third overlayer and perform molecular dynamics simulations with DFTB, monitoring the water orientation, the H-bond network, and ordered water structures formation. To make our models more realistic, we then build a 12-nm thick water multilayer on top of the Fe3O4 (001) surface slab model, which we investigate through MM molecular dynamics. The water layers structuring, revealed by the analysis of the atomic positions from a long MM-MD run for this large MM model, extends up to about 6-7 Å and nicely compares with that observed for a water trilayer model. However, MM and DFTB MD simulations show some discrepancy due to the poor description of the Fe---OH2 distance in MM that calls for further work in the parametrization of the model. * Corresponding author: cristiana.divalentin@unimib.it SUPPLEMENTARY MATERIALSee supplementary material for further computational details, tables with comparative analysis, figures of the structures for three and six water molecules adsorbed on the top Fe3O4(001) surface, tables with additional structural information, electron density profile of bulk water on the Fe3O4 (001) surface (against liquid) and comparative linear number density profiles of all the systems under investigation.