Elsevier

Surface Science

Volume 632, February 2015, Pages L22-L25
Surface Science

Surface Science Letters
Wulff shape of strontium titanate nanocuboids

https://doi.org/10.1016/j.susc.2014.10.014Get rights and content

Abstract

Here we describe the Wulff shape of strontium titanate nanocuboids prepared by a hydrothermal method and annealed at high temperature. Transmission electron microscopy was used to measure the faceting ratios d(110):d(100) which are compared with surface energy ratios γ(110):γ(100) from first-principles calculations. Internal voids attributed to the Kirkendall effect were also measured and show agreement with the external faceting. Experiment and theory are shown to agree strongly within statistical and density functional theory error.

Introduction

Oxide materials have been developed for a broad array of applications ranging from catalysis [1], to dielectrics [2], ferroelectrics [3], and transparent conductors [4], [5]. One material which has been studied in detail is strontium titanate (SrTiO3 or STO) due to its prototypical cubic perovskite crystal structure [6] and its widespread use as a substrate for growth of thin films. There have been several works published regarding synthesis of STO in various nanoscale morphologies [7], [8], [9]. In spite of the considerable volume of literature on the subject, there is limited understanding of the properties of STO in nanoparticle form. It is also important to consider that surfaces are distinct from the bulk due to the loss of coordination going from an “infinite” periodic structure to an abrupt termination of the said periodicity [10]. Given that the surface-to-volume ratio increases as particle size decreases, the properties of STO nanoparticles could be quite different from bulk STO.

It is well-known that the nanoparticle shape is thermodynamically controlled by the thermodynamic Wulff construction [11]. This is the surface that minimizes the total surface free energy of a crystal, and is found by taking the inner envelope of tangents of the surface energy as a function of crystallographic orientation. As such, the coverage of different facets will be fixed for a particular material system in thermodynamic equilibrium [12].

In this note we report the Wulff shape of STO annealed in air using electron microscopy to measure both the external shape as well as that of the internal Kirkendall voids (Fig. 1).

Section snippets

Methods

Strontium titanate nanocuboids were prepared by hydrothermal synthesis as described elsewhere [13], [14], [15], [16]. The samples were dispersed on SiN TEM grids and subsequently annealed at different temperatures (700 °C to 950 °C in steps of 50 °C) for various times in a fused silica tube within a tube furnace. A JEOL JEM-2100 FasTEM was used for TEM imaging and electron diffraction (TED) measurements. The TED measurements were used to determine the crystallographic orientation and characterize

Results

The results of TEM imaging reveal a general cubic morphology with the (100) facets dominating, but with additional significant coverage of (110) faces. The nanocuboids were single crystals as evidenced by the nanodiffraction measurements (see Fig. 3). HREM demonstrates that faces which appear flat at low magnifications have many defects and step edges, which are a combination of the (100) and (110) faces. There were also defects present within the nanocuboids with the same shape and faceting as

Discussion

The fact that the nanocuboids largely maintained their shape before and after annealing as evidenced by the d(110):d(100) ratios remaining statistically unchanged indicates that the shape we have observed is the thermodynamic limit and is representative of the Wulff construction. Other shapes have been observed such as the cubic morphology in which the (100) surfaces dominate, but the synthesis surfactant is likely a limiting reagent for crystallization on the (110) surfaces, and as such a

Acknowledgments

We acknowledge the funding from the Northwestern University Institute for Catalysis in Energy Processes (ICEP) on grant number DOE DE-FG02-03-ER15457 (JAE, FR) and the National Science Foundation on grant number DMR-1206320 (LC and LDM). LC also acknowledges support from a National Science Foundation Graduate Research Fellowship.

References (29)

  • J. Chen et al.

    Electrochim. Acta

    (2009)
  • A.R.J.H. Voorhoeve et al.

    Science

    (1977)
  • G. Arlt et al.

    J. Appl. Phys.

    (1985)
  • K. Kiss et al.

    J. Am. Ceram. Soc.

    (1966)
  • L. Goris et al.

    J. Electron. Mater.

    (2009)
  • A. Klein et al.

    Transparent conducting oxides for photovoltaics: manipulation of fermi level, work function and energy band alignment, materials

    (2010)
  • A. Bhalla et al.

    Mater. Res. Innov.

    (2000)
  • K. Fujinami et al.

    Nanoscale

    (2010)
  • U.a. Joshi et al.

    Small (Weinheim an der Bergstrasse, Germany)

    (2005)
  • J. Urban et al.

    J. Am. Chem. Soc.

    (2002)
  • G.A. Somorjai

    Principles of Surface Chemistry

    (1972)
  • G. Wulff

    Zeitschrift fur Krystallographie und Mineralogie

    (1901)
  • S. Miracle-Sole

    Wulff shape of equilibrium crystals

    arXiv, preprint arXiv:1307.5180

    (2013)
  • F.A. Rabuffetti et al.

    Chem. Mater.

    (2008)
  • Cited by (16)

    • Shape, thermodynamics and kinetics of nanoparticles

      2023, Encyclopedia of Nanomaterials
    • Charged domain wall modulation of resistive switching with large ON/OFF ratios in high density BiFeO<inf>3</inf> nano-islands

      2020, Acta Materialia
      Citation Excerpt :

      The dark contrast in the core is considered to be the metal cation vacancy induced “Kirkendall voids” in the film. These vacancies are likely caused by the atomic inter-diffusion during the deposition and the annealing procedure due to the spontaneously formed dipole disclinations in the cores of the center-type domains [35,36]. Fig. 4(b) and (e) are atomically resloved HAADF images corresponding to the blue rectangle in (a) and red square area in (d), respectively.

    • Morphology and oxidation state of ALD-grown Pd nanoparticles on TiO<inf>2</inf>- and SrO-terminated SrTiO<inf>3</inf> nanocuboids

      2016, Surface Science
      Citation Excerpt :

      Our approach to minimize the “materials gap” is using a controlled nanoscale support material with well-defined surfaces. SrTiO3 (STO) nanoparticles can be synthesized with distinct faceting [2–5]. Our approach follows previously established work [6–9] utilizing SrTiO3 nanocuboids with low size dispersion and well-defined (001) surfaces as the support material of the catalytic nanoparticles.

    View all citing articles on Scopus
    View full text