Properties across dimensions: an atomistic computational study of bismuth ferrite surfaces and nanocrystals

Project Information

BFO-Surf

Grant agreement ID: 744027

DOI

10.3030/744027

Project closed

EC signature date 17 March 2017

Start date 1 October 2017

End date 1 August 2020

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 187 419,60

EU contribution

€ 187 419,60

Coordinated by

EIDGENOESSISCHE TECHNISCHE HOCHSCHULE ZUERICH
Switzerland

Periodic Reporting for period 1 - BFO-Surf (Properties across dimensions: an atomistic computational study of bismuth ferrite surfaces and nanocrystals)

Reporting period: 2017-10-01 to 2019-09-30

Summary of the context and overall objectives of the project

With an ever-increasing societal demand for energy and in the face of the current climate issues, the need for low-energy-consuming electronics and novel modes of energy production has never been greater. Ferroelectrics, materials which posses a spontaneous polarization which can be switched by an electric field, are promising energy-efficient device components for digital information storage, with the functionality relying on the manipulation of their polarization in ultrathin films. They are also promising catalysts, in particular for the splitting of water for hydrogen generation, for carbon sequestration and for pollutants removal. These reactions are thought to be triggered by changes in the chemical environment of the surface by switching the ferroelectric polarization.
These applications require the use of thin films and nanoparticles, nanometer-sized materials where the role of the surface and the interface is of paramount importance in determining the resulting properties. However, the intrinsic polarization in a ferroelectric material creates polar discontinuities at the interfaces and surfaces which can cause loss of polarization and thus functionality. Thus understanding the stabilization of ferroelectricity at the nanoscale is vital for their use in the technologies of the future.

The first objective of the project is to show how the interface and surface in ferroelectric materials of technological importance influence the overall polarization of the thin films and nanoparticles, and how they can stabilize it. The second aim is to understand the atomistic mechanisms underlying the application of ferroelectric to the splitting of water.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

"We used a modelling technique based on quantum mechanics, called density functional theory, to study the properties of model ferroelectric thin films and nanoparticles.

1st objective: role of surface and interface in ferroelectric thin films.

In this section of the project we looked at two different ferroelectrics: lead titanate (PbTiO3) and bismuth ferrite (BiFeO3). The reason for studying these two materials is that they present different properties which could affect the surface and interfacial behavior.

We first examined PbTiO3. We used density functional theory to disentangle the role played by the surface, interface and electrostatic properties in determining the direction and strength of the polarization in thin film. We observed that the direction of the polarization depends only on electrostatic properties and not on the nature of the chemical bonding at the surface and interface. We also show that the structure of the interface and surface can be tailored toward a specific polarization direction and strength, and that great control in the engineering of ferroelectrics thin films can be achieved. This can be done by engineering specific defects or adsorbates at the surface or at the interface. An example of this is an O adatom (with a -2 formal charge) screening the surface charge of a positively charged surface (occurring if the polarization is pointing towards the surface). Conversely, depositing an O adatom (with a -2 formal charge) on a negatively charged surface (occurring if the polarization is pointing away from the surface) would lead to the opposite effect: the large negative surface charge would be unfavorable and the polarization would flip direction. This work has been recently published [1,2].

In BiFeO3 we instead studied the interaction between the ferroelectric polarization and the charged layers. Indeed, In the (001) direction bismuth ferrite has a structure of layers with alternating positive and negative charge. The surface charge due to the ferroelectric polarization, and the amount of charge needed to screen the charged layers is the same in magnitude in bismuth ferrite. The sign instead depends to the direction of the ferroelectric polarization and the surface termination. Thus a system where the two contributions cancel out can be constructed, and they have non-charged surfaces. On the contrary, when the sign of the two contributions is the same, heavily charged surfaces occur. We showed that such charged systems are unstable, and in our simulations the polarization direction spontaneously reverses, leading to the system with uncharged surfaces. We also showed that, as in the case of PbTiO3, defects can effectively screen this surface charge and lead to a stabilization of this unfavorable system. This work is under review, and a preprint is available [3].

2nd objective: water splitting using ferroelectric materials.

We investigated how the difference in surface charge in the self-compensating (""happy"") and charged (""unhappy"") surfaces of bismuth ferrite (001) affects the behavior of adsorbed water. We observed that water adsorbs molecularly on happy surfaces and dissociatively on unhappy ones. Moreover, we found that the dissociation products of water can stabilize the unhappy surface, H+ ions compensating the charges on the negative surface and OH- ones those on the positive one. We used these results to build a water splitting cycle which goes as follows (see image Summary.jpg):
1) Water adsorbs molecularly on a happy system;
2) If the ferroelectric polarization is externally switched using, for example an electric field, the system becomes unhappy and water dissociates
3) Only the dissociation products which stabilize the surface charges are left on the surface
4) By switching the polarization a second time, the surfaces are happy again. More dissociation products desorb and molelcular water takes their place on the surface.

This work is under review, and a preprint is available [3].

References:
1. Chiara Gattinoni, Nives Strkalj, Rea Hardi, Manfred Fiebig, Morgan Trassin, and Nicola A. Spaldin,, Proc. Nat. Acad. Sci., vol 117 (2020)
2 Nives Strkalj, Chiara Gattinoni, Alexander Vogel, Marco Campanini, Rea Haerdi, AntonellaRossi, Marta D. Rossell, Nicola A. Spaldin, Manfred Fiebig and Morgan Trassin, Nat. Comm. accepted (2020)
3 Ipek Efe, Nicola A Spaldin, Chiara Gattinoni, https://arxiv.org/abs/2010.14895"

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

"We have uncovered a number of properties of ferroelectric nanoscale materials, which are of great interest to the broad ferroelectric community.

In our work on lead titanate we have disentangled the role of surface and interface bonding, and electrostatics in ferroelectric thin films, showing that electrostatic effects are dominant and that they can be controlled by manipulating the defect and adsorbate chemistry in a thin film. This is something which can be done with modern thin film growth techniques. While these effects had been previously observed and investigated in other ferroelectrics, in our work we have provided a ""ranking"" on which effect is dominant and thus more important to address. This work is a step forward in the creation of untra-thin films which can be integrated in low energy consuming electronic devices.

In our work on bismuth ferrite we have shown that the special properties of its (001) surfaces can be utilized for the creation of a water splitting cycle for energy production. The field of catalysis using ferroelectric is growing and it is showing a lot of promise. Our proof of principle is now being tested by experimental groups to create a much needed efficient eater splitting device for hydrogen production."

Water splitting cycle with bismuth ferrite