Surface Science LettersWulff shape of strontium titanate nanocuboids
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)
- et al.
Electrochim. Acta
(2009) - et al.
Science
(1977) - et al.
J. Appl. Phys.
(1985) - et al.
J. Am. Ceram. Soc.
(1966) - et al.
J. Electron. Mater.
(2009) - et al.
Transparent conducting oxides for photovoltaics: manipulation of fermi level, work function and energy band alignment, materials
(2010) - et al.
Mater. Res. Innov.
(2000) - et al.
Nanoscale
(2010) - et al.
Small (Weinheim an der Bergstrasse, Germany)
(2005) - et al.
J. Am. Chem. Soc.
(2002)
Principles of Surface Chemistry
Zeitschrift fur Krystallographie und Mineralogie
Wulff shape of equilibrium crystals
arXiv, preprint arXiv:1307.5180
Chem. Mater.
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